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Qualitative Tear Film Disease
Small Animal Ophthalmology
0195-5616/90 $0.00 + .20
Qualitative Tear Film Disease Cecil P. Moore, DVM, MS*
Deficiency of aqueous tear, or keratoconjunctivitis sicca, is a widely recognized disease or group of eye diseases of small animals, particularly dogs. It is characterized by desiccation of the ocular surface with accompanying inflammation, pain, progressive corneal disease, and reduced vision. Problematic cases of canine keratoconjunctivitis occur when aqueous tear volume is adequate and when other recognized causes of surface disease-that is, infection, frictional irritation, and ineffectual blinkingare excluded as primary causes. In such cases, qualitative tear abnormalities may be primary or contributory causes of the surface disease.
TEAR FILM PHYSIOLOGY Source and Components of the Preocular Tear Film Preocular tear is a complex trilaminar fluid film consisting of lipid, aqueous, and mucin layers. 35 The outer lipid layer is relatively thin at 0.1 J.Lm, the intermediate aqueous layer is the thickest at approximately 7 J.Lm, and the innermost mucous layer has variable thickness ranging from 1 J.Lm over the cornea to 2 J.Lm or greater thickness over the conjunctiva. 35• 43 The superficial lipid layer, secreted by the tarsal (meibomian) glands, provides a thin oily covering over the aqueous tear, retards evaporation, and promotes stable, even spread of tear fluid over the cornea. 35 The lipid tear layer is composed predominantly of cholesterol and waxy lipids with some polar lipids. 2 The molecular weight of meibomian lipids (meibum) is higher and its polarity is lower than those of sebum. 2 Meibomian secretions in humans contain lower levels of triglycerides and free fatty acids and more cholesterol than sebaceous secretions. 2 Meibum is· fluid at eyelid temperature. 57 The meibomian glands are holocrine glands arranged linearly within *Diplomate, American College of Veterinary Ophthalmologists; Associate Professor, Department of Veterinary Medicine and Surgery, College of Veterinary Medicine, and Head, Ophthalmology Section, Veterinary Teaching Hospital, University of Missouri-Columbia, Columbia, Missouri Veterinary Clinics of North America: Small Animal Practice-Vol. 20, No. 3, May 1990
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the dense connective tissues of the eyelid margin (tarsal plate). These modified sebaceous glands are more highly developed in the upper eyelid. 46 Typically, 20 to 40 glands are located in each eyelid. 46 Within the tarsal plate, the meibomian glandular acini form beige grape-like clusters that open into central ductules. These ductules, which lie at right angles to the eyelid margin, deliver meibum to the surface of the eyelid through small orifices on the lid margins (Fig. 1). 35• 46 Each meibomian gland orifice is surrounded by a cuff of epithelial cells. The openings are normally even with the lid margin and form a trough or line, sometimes referred to as the "gray line," just anterior to the mucocutaneous junction (see Fig. 1). Gentle pressure applied to the eyelid margin will normally allow a clear fluid lipid to be expressed from the glandular openings. Although compression of the eyelids during normal blinking may contribute to release of meibum, the precise physiologic mechanism controlling secretion of meibomian lipid is not well understood. The aqueous tear component is the intermediate tear layer, which, in small animals, is produced by the orbital and third eyelid lacrimal glands. 46 Aqueous tear accounts for most of the total tear fluid volume and consists of water, inorganic salts, glucose, urea, surface active polymers, glycoproteins, and some serum proteins. 58 These components have not been quantified for canine or feline tears. Aqueous tear contains proteins secreted by the lacrimal gland-that is, lactoferrin, tear-specific prealbumin, and secretory immunoglobulin A. 15· 28 · 35 Varying amounts of lysozyme are also produced by the lacrimal glands of mammals. 7· 35• 48 The total protein content
Figure l. Normal canine eye. Note row of meibomian gland openings (gray line), glistening corneal surface, tear meniscus along the ve ntral eyelid margin, and mucous thread at the medial canthus. These features indicate the presence of normal tear components.
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of tear is slightly less than l %, which is considerably less than the percentage of protein present in plasma. 35 Aqueous tear serves the metabolic needs of the avascular cornea by supplying glucose, electrolytes, oxygen, and water to the superficial corneal layers. 35 Aqueous tear lubricates and cleanses the ocular surface. It flushes away metabolites, such as C0 2 and lactic acid, in addition to particulate debris and bacteria that would otherwise accumulate on the ocular surface. Antibodies (secretory lgA) 51 and lysozyme (in some species?' 48 present in aqueous fluid protect against microbial invasion. Lysozyme is present in very low levels in canine tear. 48 Third eyelid (TE) and orbital (or main) lacrimal glands are tubuloacinar and histologically similar in dogs . 30 Ductules from these glands deliver aqueous tear secretions into the conjunctival fornices .46 Three to five ductules from the orbital lacrimal glands open into the dorsal lateral conjunctival fornix . 36 The TE gland delivers aqueous tears into the ventral fornices near the base of the TE. The exact number and location of the ductular openings of the TE gland have not been determined. The relative contribution of each of the lacrimal glands to total reflex tear secretion was investigated in dogs by surgically removing one or both glands and then measuring tears with the Schirmer tear test. 24 It was concluded that the contribution of each gland in reflex tear production varies from dog to dog-that is, although the orbital lacrimal gland is the primary source of reflex aqueous tear in some dogs, the TE gland may be the main source in others. When either gland was removed separately, a compensatory increase in production by the remaining gland was apparent. The role of each gland in the production of basal secretions (versus reflex tears) was not determined. Removal of both glands resulted in a total absence of secretions, indicating that accessory conjunctival glands play no appreciable role in aqueous secretions. The deepest tear layer consists of mucin, a hydrated glycoprotein produced by conjunctival goblet cells. Mucin forms an interface between the aqueous tear and the hydrophobic corneal epithelium. 44 In dogs, goblet cells are found in highest density in the conjunctival fornices (Fig. 2). 38 Mature goblet cells are filled with mucin glycoprotein in small droplets that are packaged in the supranuclear Golgi complex. These droplets coalesce into larger droplets as the secretions move toward the cell's apex. Because droplets of mucin are delivered to the epithelial surface through the apices of globlet cells, they are classified as apocrine secretory cells. Goblet cells eject mucin droplets onto the conjunctival surface as the result of normal cell maturation and ocular motion. Follow,ing deposition of secretions onto the conjunctival surface, goblet cells lose their connections to the epithelial basement membrane and are desquamated. The exact life span of conjunctival goblet cells is not known . As glycoproteins are released onto the epithelial surface, secretions of adjacent goblet cells coalesce into mucous threads that consist of a network of interconnecting strands of intracellular and surface mucin . 1 Microfilaments present in the surface microvilli of conjunctival epithelium are hypothesized to facilitate microvillar contraction and thereby aid in spread of the surface mucin. 18• 44 Superficial epithelial cells synthesize a mucopro-
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Figure 2. Histologic section of normal canine conjunctiva (PAS/H&E stain; X 100). Goblet cell mucoprotein stains intensely (PAS positive) and reveals a high density of secretory cells in the ventral conjunctival fornix .
tein packaged as subsurface vesicles that fuse with the cell membrane, become part of the microvilli, and bond to the overlying mucous layer. 17 The role of the subsurface vesicle mucus, therefore, is to anchor goblet cell mucin to the surface me mbranes of the epithelial cells. 18 A relatively small amount of soluble mucin is also a component of aqueous tear. The soluble mucin is believed to be a secretory product of mucous cells of lacrimal glands and accounts for the glycoprotein portion of the aqueous layer. This dissolved mucin decreases the surface tension of tear fluid and enhances spread , stability, and coherence of the aqueous layer to the cornea. 26 Goblet cell-derived preocular mucin fills in irregularities of the corneal surface, resulting in an optically smooth ocular surface. 43• 44 Bacteria and foreign particles are trapped by mucoproteins. Mucin harbors immunoglobulins (lgA) and lysozyme and aids in lubrication and hydration of the conjunctiva and cornea. 61 Dynamics of Tear Film Formation In addition to the presence of normal secretory components, formation of the tri-stratified composite tear film is dependent on the integrity of the eyelid margins, normal ocular movements, and an intact blink mechanism. As the eyelids close, the superficial lipid accumulates on the lid margins where it is compressed into a relatively thick layer between the upper and lower eyelid margins. Basal levels of aqueous tear are secreted into the conjunctival cul-de-ces and bathe the globe. Most of the tear volume-that is, basal secretion-remains under the eyelids. As the lids open, an aqueous
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tear surface is formed over the globe, upon which the compressed lipids rapidly spread. The aqueous layer therefore should normally be coveted by a lipid monolayer. 27 In addition, as movement of the eyelids occurs with concurrent movements of the globe, contact between palpebral conjunctiva and corneal and bulbar conjunctival surfaces results in spread of goblet cell mucin directly over the glycocalyces of surface cells. The continuous distribution of this mucous glycoprotein over the ocular surface is essential for providing a compatible interface between aqueous tear and the hydrophobic corneal and conjunctival epithelial cell membranes. 1• 4 • 27 • 40
PATHOPHYSIOLOGY OF TEAR FILM DISEASE Because of the complex interactions among the lipid, aqueous, and mucin tear components, abnormalities in the quantity or quality of any of these three primary components may alter the fluid dynamics and thus compromise the functions of the tear fluid. Hypertonicity and dehydration of conjunctival and corneal epithelia are the most common events associated with deficiency of one or more of the tear components. 31 Hypoxia of the corneal epithelium and subepithelial corneal stroma may then occur. Tear-deficient eyes have an increased susceptibility to ocular surface infections caused by colonization of either opportunistic or pathogenic microorganisms. Lack of efficient moisturization may result in frictional irritation to surface cells by the eyelids or TE. Potentially toxic tissue metabolites-that is, lactic acid, desquamated cells, denatured mucus, and other organic debris-may accumulate on the ocular surface. Because an overwhelmingly' high percentage of the total tear volume is aqueous tear, quantitative tear deficiency is considered synonymous with insufficient production of aqueous fluid . Abnormalities or deficiencies of tear components other than aqueous fluid are considered to be qualitative disorders. Diseases causing decreased secretion of aqueous tears are diseases of the lacrimal glands or the nerve supply to these glands. Dry-eye states associated with lacrimal gland hyposecretion are collectively referred to as keratoconjunctivitis sicca (see separate article). Distributional abnormalities resulting from lagophthalmos, buphthalmos, eyelid paresis, corneal anesthesia, TE deformities, or frictional irritants may result in an abnormal tear covering or rupture of the tear film with surface drying. These disorders therefore must be distinguished from primary quantitative or qualitative tear deficiencies.
QUALITATIVE ABNORMALITIES Lipid Abnormalities Disturbances of the tarsal or meibomian glands.may result in abnormalities in the superficial lipid layer. Inflammation of the mucocutaneous junction, often referred to as marginal blepharitis, frequently involves the
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meibomian glands. Marginal blepharitis, blepharoconjunctivitis, and meibomianitis are most commonly caused by suppurative bacteria-for example, Staphylococcus sp.-that result in swelling of the eyelid margin, accumulation of exudates, and abnormal lipid secretions. 14· 50 In humans, yeasts are also common infectious causes of marginal blepharitis. 60 In the author's experience, inflammatory diseases of the eyelids of dogs and cats may also be caused by gram-positive aerobic bacteria and yeasts. Diseases of the meibomian glands have been associated with chronic epithelial keratopathy (cystic epithelial keratopathy) and epithelial keratitis in humans. 8 The corneal pathology observed was presumably a manifestation of a deficient lipid layer and an inability to conserve tear by preventing aqueous evaporation. Meibomian gland infections also produce abnormal lipid degradation products that may be toxic to ·surface cells, further contributing to the pathogenesis of the disease. 33 Abnormalities in preocular lipid occur when the tear is contaminated with highly polar lipids from abnormal skin secretions or infected meibomian glands. These abnormal polar lipids are disruptive to the normal nonpolar lipids and may result in premature dispersion of the aqueous layer. 27 Dogs and cats with acute meibomianitis typically have swollen eyelid margins (Fig. 3) with slight "pointing" of the meibomian gland openings. Affected openings may be plugged with dried and discolored meibum. Chronic meibomianitis may result in rupture of glandular acini and release of lipid secretions into the periacinar tissues. The formation of multiple chalazia and lipid granulomas3 may frictionally irritate the eye and complicate any surfacing abnormality that is present (Figs. 4 and 5).
Figure 3. Acute meibomianitis and conjunctivitis in a dog (right upper eyelid). The swollen, rounded eyelid margin and "pointing" of the gland openings (arrows) characterize diffuse meibomian gland inflammation.
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Figure 4. Chalazion of right upper eyelid. The focal enlargement and beige discoloration of meibomian gland, as seen through the tarsal conjunctiva, indicate chronic granulomatous inflammation, with probable rupture of the meibomian gland. .
In cases of chronic meibomianitis, superficial keratitis may be noted. The ke ratopathy may be characterized by somewhat subtle clinical findings, including faint diffuse epithelial edema (see Fig. 5), small multifocal punctate areas of roughe ned epithelium (which may or may not retain fluorescein stain), and a fine superficial perilimbal vascular infiltrate. The degree to which the corneal disease is directly attributable to deficient lipid and unstable tear film is unclear.
Figure 5. Multiple chalazia of right upper eyelid. Note several adjacent meibomian glands with inspissated contents (black arrows). Mild superficial corneal edema may be a manifestation of chronic meibomianitis (white arrows).
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It is probable that surface disease results from a combination of insults, including poor surfacing of the tear and frictional or toxic effects from inflamed meibomian glands. It should be noted that some patients have serious meibomian disease with relatively little or no apparent corneal changes. Most of these patients have an abundant production of aqueous tear, suggesting that reflex tearing may compensate for the lack of tear conservation. It is also possible that toxic inflammatory products and abnormal lipids are simply diluted or flushed away by the aqueous tears. Deficiency of meibomian secretion resulting from maldevelopment of meibomian glands, although rare, has been reported in humans. 9 Eyelid agenesis occurs as a congenital condition of small companion animals where affected areas of the eyelid are devoid of normal eyelid tissue, including meibomian glands. Although relatively uncommon in small animals, eyelid agenesis appears to occur more commonly in cats than dogs. 45 Epiphora, conjunctivitis, and superficial keratitis are the predominant clinical signs. Surface pathology presumably results from a combination of insults, including an absence of meibomian secretions, exposure, trichiasis, and, in some cases, spastic entropion. Diagnosis. Diagnosis of lipid tear abnormalities depends on abnormal findings from a detailed examination. To be most informative, a thorough examination of the eyelid margins should be performed with a focused light and a magnifying source. Although a slit lamp biomicroscope is recommended for this purpose, binocular magnifying loupes and a separate focal light source, such as a Finoff transilluminator, are adequate. During the course of the ocular examination, particular attention is focused on the appearance of the eyelid margins and meibomian glands (see Fig. 1). With the eyelid everted, the examiner will normally note (with magnification) the serial arrangement of multiple meibomian glands just beneath the tarsal conjunctiva. Although each gland appears parallel to the adjacent glands and perpendicular to the eyelid margin, when considering the multitude of glands, the alignment is essentially radial with respect to the axis of the eye. Visualization of the glands may be enhanced by transillumination of the eyelid. An infrared technique of photographing meibomian gland profiles (termed meibomoscopy) recently has been developed. 49 Swollen, rounded eyelid margins indicate acute or subacute marginal blepharitis (see Fig. 2). Hyperemia of the mucocutaneous junction with dry, crusty porphyrin-stained exudates on the lid margins are also indicative of marginal blepharitis. During the detailed examination, the examiner should note the occurrence of any elevated, focal , beige subconjunctival masses typical of chalazia (see Fig. 3 and Fig. 4). When p~esent, chalazia indicate chronic meibomianitis of affected glands with periglandular granuloma formation . Following the administration of topical anesthetic solution, gentle manipulation of the eyelid margin with blunt-tipped forceps (with shallow serrations) will allow inspection of secretions expressed from the meibomian glands. Normal meibomian lipid appears as a clear _viscous oil similar in appearance to clear vegetable oil. Abnormal meibomian secretions are typically thick and opaque or may appear inspissated with a cheesy
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consistency. Expression of coiled semisolid strands of abnormal lipid is not uncommon with chronic meibomian disease. The technique of polarized light biomicroscopy recently was used to characterize surface lipid morphology from normal dogs and cats and to study surface abnormalities of dogs with confirmed or suspected tear film disease. 11- 13 Patterns of lipid distribution were described and the overall thicknesses of surface lipid were determined in normal dogs (between 0.013 and 0.581 j.Lm) and cats (0.089 j.Lm). This technique has been proposed as a practical means of examining the functional integrity of the tear film. In addition to enhancing our understanding of tear film diseases, it has been suggested that polarized biomicroscopy may provide a more effective means of monitoring patients undergoing treatment for tear film abnormalities. 12 Treatment. Treatment of lipid tear abnormalities depends on the specific meibomian disorder present. Bacterial meibomianitis should be treated with both topical and systemic antibiotics. Topical broad spectrum antibiotic ointment-that is, gentamicin or triple antibiotic combinationshould be applied three times daily for a minimum of 3 weeks. Ideally, antibiotic selection is based on antibacterial susceptibility testing. Because opportunistic or pathogenic bacteria may be involved in cases of acute meibomianitis, aerobic bacterial cultures are necessary for definitive diagnosis. It is most rewarding to culture secretions expressed directly from the affected meibomian glands. Systemic and topical antibiotics are also indicated in cases of granulomatous blepharitis secondary to ruptured or abscessed meibomian glands. 3 Oral cloxacillin or trimethaprim-sulfadiazine may be prescribed initially for 2-3 weeks. If the latter is used, the owner should be alerted to the possibility of drug-induced lacrimal hyposecretion. As with acute meibomianitis, topical bacteriocidal agents such as gentamicin or triple antibiotic ointment should be applied topically three times daily. Concurrent administration of systemic corticosteroids may be necessary to resolve the extensive, diffuse eyelid inflammation. 29 Immunostimulant therapy has also been advocated for cases of chronic bacterial blepharitis. 14 Surgical curettage of chalazia and removal of sequestered secretions may be necessary to permit resolution of focal granulomas. Bacterial meibomianitis is regrettably often recurrent and requires intermittent intensive treatments or continuous maintenance therapy. In cases of chronic meibomianitis, periodic manual expression of meibomian material from the glands with blunt-tipped or mildly serrated thumb forceps may be helpful in evacuating inspissated secretions. Treatment of chronic or recurrent infections should be based on antibiotic selection following culture and susceptibility testing. ' Exudates on the eyelid margins may be softened and removed by the use of warm moist compresses to the eyelids for several minutes two or three times daily. This is particularly helpful during the initial treatment period. Application of warm moist compresses results in local vasodilation and improved blood flow to affected areas and, therefore, presumably augments healing. . In treating diffuse meibomian diseases, topical ointments provide the benefit of serving as a lipid substitute. Ophthalmic ointments (or emollients) are usually petrolatum-based preparations but may also contain vegetable
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oil, mineral oil, liquid lanolin, or some combination of these ingredients. In addition to serving as lipid substitutes, emollients lubricate the ocular surface, which is especially beneficial in cases where irregular conjunctiva, rough eyelid margins, or conjunctival granulomas are associated with chalazia. Congenital absence of meibomian glands associated with eyelid agenesis may require both medical and surgical therapy. Grafting of periocular skin, using either a pedicle or sliding skin flap, into the defeCtive area will generally restore eyelid function. 45 In addition to surgical reconstruction, topical application of lubricant ointments will provide a desired lipid-like effect to enhance wetting of the ocular surface. Mucin Deficiency Insufficient production of mucous glycoprotein by conjunctival goblet cells results in deficiency of preocular mucin in humans and dogs . 37• 39• 41 • 61 Human diseases resulting in preocular mucin deficiency include vitamin A deficiency, alkali burns, cicatricial pemphigoid, Stevens-Johnson syndrome, trachoma, and radiation injury. 31 Practolol and echothiophate have been reported as drug-induced causes of mucin insufficiency. 31 These abnormalities result in loss of tear film stability with resultant corneal desiccation. In the dog, the pathogenesis of spontaneously occurring mucin-deficient dry eye disease may vary among cases. 37• 39 Diffuse infiltration of chronic inflammatory cells into the conjunctival mucosa and submucosa may markedly reduce or eliminate goblet cells. Although causes for these infiltrates are often not determined, infectious and immune-mediated causes have been hypothesized. 39 Conjunctival hypoplasia was suspected in one of the reported cases. 39 Squamous metaplasia of the conjunctiva, which may be produced experimentally by vitamin A deficiency, results in abnormal keratinization of the secretory epithelium and loss of conjunctival goblet cells. 52• 53 Severe cicatrization following diffuse ulcerative conjunctival disease is another possible cause of abnormal epithelium and goblet cell loss. Furthermore, it has been hypothesized that the relative avascularity of the conjunctiva that may occur with scarring might result in local deficiency of an essential nutrient, such as vitamin A, to the conjunctival mucosa. 55 Clinical features of canine mucin deficiency are chronic keratoconjunctivitis, corneal ulceration, absence of ocular discharge, and adequate aqueous tear secretions (Fig. 6). 39 The notable absence of appreciable ocular discharge is presumably related to lack of mucin production. Histopathologic studies of conjunctival specimens from affected eyes reveal s,parse numbers or a total absence of goblet cells (Fig. 7). Diagnosis. The clinical diagnosis of canine ocular mucin deficiency is supported by performing tear film breakup time (BUT) tests and confirmed by biopsying conjunctiva and quantifying epithelial goblet cells. 39 Tear film BUT tests are performed by instilling one or two drops of fluorescein stain into the eye and then manually holding the eyelids open. The time is recorded from the last blink to the appearance of the. first dry spot, which appears as a dark area in the yellow-green fluorescent film. A cobalt blue
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Figure 6. Chronic keratitis associated with mucin deficie ncy. Absence of appreciable ocular discharge, chronic inflammation of the cornea and conjunctiva, and nonresponse to symptomatic topical therapy characterize mucin deficiencv. Although not present in this e"xample, nonresponsive or recurrent corneal ulceration may also occur.
filter should be used when viewing the cornea. Normal BUT in dogs is approximately 20 seconds (19 ± 5 sees). 38 In affected animals, tear breakup occurs in usually less than 5 seconds. 39 Although BUT is regarded as a clinically useful test, the reliability of this test has been questioned. Erroneously rapid BUTs may be recorded when corneal surface irregularities are present. Corneal exposure, corneal anesthesia, or surface frictional irritants may result in evaporation or abnormal distribution of tear fluid and rapid tear BUT. 59 Preservatives in ophthalmic irrigating solutions and fluorescein preparations may artificially lower the BUT. 32 These possible sources of variability have limited the usefulness of tear BUT as a diagnostic and research tool. Determination of goblet cell frequency from a conjunctival biopsy is an indirect measure of mucin production. Collecting a sample of conjunctiva for histopathology involves instillation of topical anesthetic solution, grasp-
Figure 7. Conjunctival biopsy from the eye shown in Figure 6. Note absence of goblet cells and infiltration of mononuclear inflammatory cells. Biopsy was taken from ventral conjunctival fornix just anterior to the third eyelid, whe re goblet cells are normally present at high densities (see Fig. 2).
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ing and tenting (with Bishop-Harmon forceps) conjunctiva from the ventral fornix just anterior to the base of the TE, and removing a 3 mm X 4 mm specimen with conjunctival scissors. Because a heterogenous density and distribution of conjunctival goblet cells has been demonstrated in dogs, conjunctival sites with the .highest and most predictable density of goblet cells are most appropriate for sampling (see Fig. 2). 38 The optimal area for biopsy in the dog is the lower fornix conjunctiva between the TE and the lower eyelid. 38 Conjunctival biopsy specimens are gently stretched over a flat surface, such as a section of tongue depressor, and fixed with iO% buffered formalin . Sections of conjunctival epithelium are stained with periodic acid Schiff and hematoxylin reagents. Goblet cells and nongoblet epithelial cells are counted and an index of goblet cell density (the ratio of goblet cells to all epithelial cells) is determined. The highest index has been demonstrated to occur in lower conjunctival fornix (anterior to the TE) where the value is 0. 29-0.3. 38 In severely affected animals the index at this site may be as low as 0.05 or lessY· 39 Treatment. Treatment for mucin-deficient keratoconjunctivitis consists of topical mucin replacements (mucinomimetics), symptomatic treatment of corneal ulcers, if present, and topical anti-inflammatory therapy in selected cases. 37 • 39 The use of topical mucinomimetics applied at 4-6 hour intervals is the mainstay of therapy. Commercially available artificial tears with a patented polymer base (e.g., Adsorbobase, Alcon Laboratories, Ft. Worth, TX) have mucinomimetic properties. Viscoelastic substances containing a hyaluronate component42 or 2% methylcellulose (Hydroxypropyl Methylcellulose 2% in BSS, Department of Pharmaceutics, University of Tennessee, Memphis, TN) rnay also be effective wetting agents. These lubricants have extended contact time with the ocular surface and, therefore, provide enhanced wetting of the ocular surface. Given that the presence of conjunctival inflammatory cells appears to be inhibitory to the existence of normal goblet cell populations, 37 • 39 topical corticosteroids may be indicated when marked mucosal and submucosal mononuclear inflammatory cell infiltrates are noted histologically. When ulcerative keratitis is present, corticosteroids should not be administered until ulcerations have healed, as evidenced by negative fluorescein stain retention. Any concurrent infections must be treated simultaneously with appropriate antimicrobials. The use of topical retinoic acid in the treatment of mucin-deficient ocular surface disease. is controversial. In experimentally induced vitamin A deficiency that is characterized by squamous metaplasia and absence of conjunctival goblet cells, topical 0.1% retinoic acid was demonstrated to be beneficial in reversing the conjunctival metaplasia and rejuvenation of a secretory mucosa. 53 The benefits of topical retinoic acid in treating mucindeficient disease not specifically caused by vitamin A deficiency is debatable, however. Because naturally occurring vitamin A deficiency is extremely uncommon in small animals and the availability of retinoic acid products suitable for topical application has been limited to experimental protocols, topical vitamin A cannot be recommended at pre.sent for routine use in treating mucin-deficient surface disease.
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Lacrimomimetics are usually not indicated when treating mucindeficient keratoconjunctivitis because, in most cases, aqueous tear production is adequate. 39 In instances where both aqueous and mucin deficiencies are present, combination therapy aimed at treating both deficiencies is appropriate. Effects of lacrimostimulant drugs, such as pilocarpine or cyclosporine, on conjunctival mucous secretions are unknown, but their potential usefulness in treating ocular mucin deficiency merits investigation. Other Potential Qualitative Disorders In problematic cases of canine keratoconjunctivitis where recognized causes of surface disease have been excluded, other possible qualitative abnormalities of the tear fluid must be considered. It is probable that specific tear components other than lipid and mucin, when present in abnormal quantities, may cause overt ocular disease or contribute substantially to the pathogenesis of presently enigmatic eye diseases of small animals. Additional important components of tear fluid essential to its multiple functions are tear proteins, a number of miscellaneous organic constituents, and electrolytes. These components influence osmolality and tear pH, which are also discussed briefly here. Specific levels of tear proteins vary considerably among species. 35 • 48 • 56 Tear proteins in most mammals consist of three main componentsglobulins, albumin (tear-specific prealbumin), and lysozyme. 35 • 48 • 56 Globulins and lysozyme are protective (antimicrobial), while tear prealbumin is believed to be a retinol-binding protein that allows delivery of vitamin A to the cornea. 15 The role of several proteins present in smaller quantitiesthat is, lactoferrin, transferrin, cendoplasmin, and soluble glycoproteinsis unclear. Separation, identification, and· quantification of tear proteins are possible with sodium dodecylsulfate-polyacrylamide gel electrophoresis and high performance liquid chromatography (HPLC) systems. 6 • 20• 21 • 34 In the future, detection of abnormal distributions and/or quantities of tear proteins may prove to be quite useful in the accurate diagnosis and prognosis of ocular surface diseases. · Additional organic components of tears of possible clinical significance are vitamin A, cholesterol, and glucose. The role of vitamin A in the regulation and differentiation of epithelial tissues is well documented. 23 Deficiency of vitamin A in humans and animals leads to reduction or loss of normal secretions with subsequent keratinization of the conjunctiva and cornea (xerophthalmia). 23• 52• 53 Desquamation of cells with ulceration and progressive loss of tissue followed by rupture of the eye may also occur. Repletion of vitamin A to deficient animals restores conjunctival and corneal health and cells return to their normal nonkeratinized states. 53 Using HPLC methods, tear all-trans retinal quantitation is possible from microvolumes of tear fluid. 54 Normal levels of all-trans retinal have been determined for tear fluids of companion patients. Elevated tear cholesterol levels may occur with systemic diseases of local metabolic disturbances and could partially account for deposition of cholesterol in the cornea, resulting in substantial corneal opacities. In human tears, cholesterol is normally present in considerably lower levels
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than those found in serum. The role of tear cholesterol in the development of degenerative keratopathies (lipodystrophies) with stromal crystalline deposits is unknown. Similarly, it is not known whether elevated tear cholesterol is involved in crystalline keratopathy observed in dogs treated long-term with topical corticosteroids. Sampling tear glucose has been pursued mainly as a means of detecting or monitoring a patient's diabetic status. Of equal or greater interest may be quantifYing tear glucose as a measure of surface normalcy. Chemical, osmotic, or mechanical insults may shift tear glucose levels dramaticallythat is, the shift is usually to lower values in reflex tearing and to higher values in states of surface dehydration. 25 A spectrophotometer may be used for tear glucose measurements by transferring tear from a collecting capillary tube into a measuring pipette. 25 Regrettably, spectrophotometer units capable of managing microvolumes are relatively expensive. Several ions found in tears are essential to the osmotic regulation of the extracellular and intracellular spaces. The primary cations of tear are sodium and potassium, and the predominant anions are chloride and bicarbonate. 35 Although sodium in tears is about equal to plasma, potassium is considerably higher than corresponding plasma concentrations. 58 This indicates an active secretion of potassium into the tear. Calcium and magnesium are other cations present in tear, although they occur at relatively low concentrations. 47 These cations play an important role in regulating membrane permeability and are presumably associated with cofactor and enzyme functions of ocular surface cells. In humans, tear calcium is normally lower than the free fraction in plasma. 58 Increases in tear calcium may occur with diminution of tear volume or with changes in the ocular surface environment. The author has encountered cases in which elevated tear pH was associated with calcium deposits on the cornea. Corneal calcification has been noted in primary canine corneal dystrophies and in keratopathies secondary to chronic inflammation. 5 Excess calcium may precipitate preocular mucus and reduce its effectiveness as a wetting agent (Robinson J: Personal communication, 1989). By influencing the osmotic pressure of tears, the particles within tears (especially the ions) affect the turgescence (i.e., state of hydration) of the cornea. 35 Because of evaporation, the open-eye tear osmotic pressure is equivalent to 0. 97% NaCl solution. 25 This is slightly higher than the 0. 9% reference level for many ophthalmic solutions. When tear osmolality falls below 0. 9%, the cornea may become edematous. Methods are available for measuring the osmolality of nanoliter volumes of tear; the equipment is expensive, however. . Tear pH is an important factor in determining patient comfort, the tolerance of various topical eye medications, and susceptibility of eyes to certain infections. 25 Clinical methods for measuring tear pH have generally not been reliable. Tear pH shifts may normally occur to such an extent in a given patient as to make interpretation of findings very difficult. 16 In one group of normal human subjects, the pH of unstimulated tears was 7.45, with most variability among subjects and in a given subject occurring at different times of the day. 10 A more acidic pH was noted after prolonged lid closure-that is, after sleeping.
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Accurate measurement of tear pH involves use of micromethods with temperature regulation and isolation of the sample from the air. 19· 25 Tears can be measured and monitored to within 0.02 of a pH unit. The cost and availability of suitable equipment and available technical support have limited the use of tear pH measurements in clinical settings.
IMPLICATIONS FOR FUTURE RESEARCH The need for basic research into components of tear in healthy and diseased eyes is apparent. It is unfortunate that more is not known about the biochemical composition of canine and feline tear fluid . Until additional basic information becomes available, it will not be possible to precisely define cause and effect relationships between ocular surface diseases and deficiencies, excesses, or imbalances of tear components. Ocular diseases or syndromes that merit investigating for possible relationships to qualitative tear abnormalities include superficial punctate keratitis, idiopathic vascularizing keratopathies, corneal epithelial erosions, certain breed-predisposed corneal dystrophies (e.g., as seen in Shetland Sheepdogs), feline corneal sequestration (nigrum) syndrome, and degenerative lipodystrophies. It is quite possible that a number of these entities as well as other presently unexplained chronic corneal diseases may be manifestations of qualitative disorders of the preocular tear film. For this reason, investigations of the biochemical components of the tear fluid in both normal and diseased animal eyes is encouraged. Furthermore, because of the frequent occurrences and considerable similarities between tear film diseases of humans and small companion animals and the mutual interest of physician and veterinary ophthalmologists in tear biochemistry, comparative studies should be pursued in search of appropriate animal models for study of tear film diseases.
SUMMARY Abnormalities of the lipid and mucin components of the preocular tear film may result from diseases of the eyelid margins and conjunctiva. Chronic keratoconjunctivitis with epithelial edema and superficial corneal neovascularization, with or without ulceration, characterizes qualitative tear diseases. Tear components other than lipid and mucin that carry probable clinical significance include tear proteins, all-trans retinal, cholesterol, glucose, and electrolytes. Although less common than quantitative or aqueous deficiencies, qualitative abnormalities are recognized as primary or secondary causes of ocular surface disease in companion animals.
REFERENCES 1. 2. 3. 4.
Adams AD: The morphology of human ocular mucus. Arch Ophthalmol 97:730, 1979 Andrews JS: Human tear film lipids. Exp Eye Res 10:223, 1970 Barrie KP: Eyelid pyogranulomas in four dogs. J Am Anim Hosp Assoc 15:433, 1979 Benedetto DA, Shah DO, Kaufman HE: The instilled fluid dynamics and surface chemistry of polymers in the preocular tear film. Invest Ophthalmol Vis Sci 14:887, 1975
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5. Bistner SI: Corneal pathology. In Peiffer RL: Comparative Ophthalmic Pathology. Springfield, IL, Chas C Thomas, 1983, p 103 6. Bradford MM : A rapid and sensitive method for the quantitation of microgram quantities of protein utilizating the principle of protein-dye binding. Anal Biochem 72:248, 1976 7. Brightman AH, Washsstock RS : Lysozyme concentrations in the tears of cattle, goats, and sheep. Presented to the American College of Veterinary Ophthalmologists, San Francisco, CA, 1986, p 422 8. Bron AJ, Tripathi RC: Cystic disorders of the corneal epithelium. I. Clinical aspects. Br J Ophthalmol 57:361, 1973 9. Bron AJ, Mehgher LS: Congenital deficiency of meibomian glands. Br J Ophthalmol 71:312, 1987 10. Carney LG, Hill RM: Human tear pH: Diurnal variations. Arch Ophthalmol 94:821, 1976 11. Carrington SD, Bedford PGC, Guillon JP, et al: Polarized light biomicroscopic observations on the precorneal tear film . l. The normal tear film of the dog. J Small Anim Pract 28:605, 1987 12. Carrington SD, Bedford PGC, Guillon JP, et al: Polarized light biomicroscopic observations on the precorneal tear film . 2. Keratoconjunctivitis sicca in the dog. J Small Anim Pract 28:671, 1987 13. Carrington SD, Bedford PGC, Guillon JP, et al: Polarized light biomicroscopic observations on the precorneal tear film . 3. The normal tear film of the cat. J Small Anim Pract 28:821, 1987 14. Chambers ED, Severin GA: Staphylococcal bacterin for treatment of chronic staphylococcal blepharitis in the dog. J Am Vet Med Assoc 185:422, 1984 15. Chao CW, Butala S: Isolation and preliminary characterization of tear prealbumin and human ocular mucus. Exp Eye Res 5:895, 1986 16. Coles WH, Jaros PA: Dynamics of ocular surface pH. Br J Ophthalmol 68:549, 1984 17. Dilly PN: On the nature and the role of the subsurface vesicles in the outer epithelial cells of the conjunctiva. Br J Ophthalmol 69:477, 1985 18. Dilly PN: Contribution of the epithelium to the stability of the tear film. Trans Ophthalmol Soc UK 104:381, 1985 19. Fischer FH , Wiederholt M: Human precorneal tear film pH measured by microelectrodes. Graefe's Arch Clin Exp Ophthalmol 218:168, 1982 20. Fullard RJ: Normal protein profiles and diurnal variations in human tear fluid. Invest Ophthalmol Vis Sci (suppl)28:157; 1987 21. Fullard RJ: Identification of proteins in small volumes with and without size exclusion HPLC fractionation. Curr Eye Res 7:163, 1988 22. Gilbard JP, Farris RL, Santamaria J: Osmolarity of tear microvolumes in keratoconjunctivitis sicca. Arch Ophthalmol 96:677, 1978 23. Hassel JR, Newsone DA: Vitamin A-induced alterations in corneal and conjunctival epithelial glycoprotein biosynthesis. Ann NY Acad Sci, 1981, p 358 24. Helper LC, Magrane WG, Koehm J, et al: Surgical induction of keratoconjunctivitis sicca in the dog. JAm Vet Med Assoc 165:172, 1974 25. Hill RM: Tear film analysis. Am J Optom Physiol Opt 58:609, 1981 26. Holly FJ: Tear film physiology. Am J Optom Physiol Opt 57:252, 1980 27. Holly FJ: Tear film physiology. Int Ophthalmol Clin 27:2, 1987 28. Janssen PT, van Bijsterveld PO: Origin and biosynthesis of human tear fluid proteins. Invest Ophthalmol Vis Sci 24:623, 1983 29. Johnson BW: Dermatoses of the canine eyelids. Contin Educ Pract Vet 11:385, 1989 30. Kaswan RL, Martin CL, Chapman WL: Keratoconjunctivitis sicca: Hi~topathologic study of nictitating membrane and lacrimal glands from 28 dogs. Am J Vet Res 45:112, 1984 31. Lamberts DW: Keratoconjunctivitis sicca. In Smolin G, Throft RA: The Cornea. Boston, Little Brown, 1983, p 293 32. Lemp MA, Hamil JR: Factors affecting tear film breakup in normal eyes. Arch Ophthalmol 89:103, 1973 33. McCulley JP, Scialis GF: Meibomian keratoconjunctivitis. Am J Ophthalmol 84:788, 1977 34. Mackie lA, Seal DV: Diagnostic implications of tear protein profiles. Br J Ophthalmol 68:321, 1984 35. Milder B: The lacrimal apparatus. In Moses RA (ed): Adle/s Physiology of the Eye, ed 8. St. Louis, CV Mosby, 1987, p 18
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36. Michel G: Beitrag zur Anatomie der Tranenorgane von Hund und Katze. Dtsch Wochenschr 62:347, 1955 37. Moore CP, Collier LL: Mucin-deficient external eye disease in dogs. Presented to the American College of Veterinary Ophthalmologists, San Francisco, CA, 1985, p 152 38. Moore CP, Wilsman NJ, Nordhein EV, et a!: Density and distribution of canine conjunctival goblet cells. Invest Ophthalmol Vis Sci 28:1925, 1987 39. Moore CP, Collier LL: Ocular surface disease associated with loss of conjunctival goblet cells in dogs. J Am Anim Hosp Assoc (in press) 40. Moore JC, Tiffany JM: Human ocular mucus. Origins and preliminary characteristics. Exp Eye Res 33:203, 1981 41. Nelson JD, Wright JC: Conjunctival goblet cell densities in ocular surface disease. Arch Ophthalmol 102:1049, 1984 42. Nelson JD, Farris RL: Sodium hyaluronate and polyvinyl alcohol artificial tear preparations. Arch Ophthalmol 106:484, 1988 43. Nichols BA, Chiappino ML, Dawson CR: Demonstration of the mucous layer of the tear film by electron microscopy. Vis Sci 26:464, 1985 44. Nichols B, Dawson CR, Togni B: Surface features of the conjunctiva and cornea. Invest Ophthalmol Vis Sci 24:570, 1983 45. Peiffer RL: Feline ophthalmology. In Gelatt KN (ed): Textbook of Veterinary Ophthalmology. Philadelphia, Lea & Febiger, 1981, p 522 46. Pollock RVH: Eyelids. In Evans HE, Christensen GC (eds): Anatomy of the Dog, ed 2. Philadelphia, WB Saunders, 1979, p 1099 47. Rismondo V, Edelhauser HF, Ubels JL: Ionic composition of tears and lacrimal gland fluid. Invest Ophthalmol Vis Sci (suppl) 28:157, 1987 48. Roberts SR, Erickson OF: Dog tear secretion and tear proteins. J Small Anim Pract 3:1, 1962 49. Robin JB, Jester JV, Nobe J, et a!: In vivo transillumination biomicroscopy and photography of meibomian gland dysfunction. Ophthalmology (Rochester) 92:1423, 1985 50. Seal DV, McGill Jl, Jacobs P, eta!: Microbial and immunological investigations of chronic nonulcerative blepharitis and meibomianitis. Br J Ophthalmol 69:604, 1985 51. Selinger OS, Selinger RC, Reed WP: Resistance to infection of the external eye: The role of tears. Surv Ophthalmol 24:33, 1979 52. Sherman Ml: Evidence that retinoids control epithelial differentiation. In Retinoids and Cell Differentiation. Boca Raton , FL, CRC Press, 1986, p 30 53. Sommer A, Emran N: Topical retinoic acid in the treatment of corneal xerophthalmia. Am J Ophthalmol 86:615, 1978 54. Speek AJ : Microdetermination of vitamin A in human plasma using high-performance liquid chromatography with fluorescence detection. J Chromatogr 382:284, 1986 55. Tseng SCG, Hirst LW, Maumenee AD, eta!: Possible mechanisms for the loss of goblet cells in mucin-deficient disorders. Ophthalmology 91:545, 1984 56. Thorig L, van Agtmaal EJ, Glasius E, eta!: Comparison of tears and lacrimal gland fluid in the rabbit and guinea pig. Curr Eye Res 4:13, 1985 57. Tiffany J, Dart J: Normal and abnormal functions of meibomian secretions. Royal Society of Medicine International Congress and Symposium Series, No. 40, London, Royal Society of Medicine, 1981 58. Van Haeringen NJ: Clinical biochemistry of tears. Surv Ophthalmol 26:84, 1981 59. Vanley GT, Irving LH, Gregg TH: lntrepretation of tear film breakup. Arch Ophthalmol 95:445, 1977 60. Vaughan D, Asbury T: Marginal blepharitis. In General Ophthalmology, ed 8. Los Altos, CA, Lange Medical Publications, 1977, p 46 61. Wright P, Mackie lA: Mucus in the healthy and diseased eye. Trans Ophthalmol Soc UK 97:1, 1977
Address reprint requests to Cecil P. Moore, DVM, MS Department of Veterinary Medicine and Surgery College of Veterinary Medicine University of Missouri-Columbia Columbia, MO 65211
0195-5616/90 $0.00 + .20
Qualitative Tear Film Disease Cecil P. Moore, DVM, MS*
Deficiency of aqueous tear, or keratoconjunctivitis sicca, is a widely recognized disease or group of eye diseases of small animals, particularly dogs. It is characterized by desiccation of the ocular surface with accompanying inflammation, pain, progressive corneal disease, and reduced vision. Problematic cases of canine keratoconjunctivitis occur when aqueous tear volume is adequate and when other recognized causes of surface disease-that is, infection, frictional irritation, and ineffectual blinkingare excluded as primary causes. In such cases, qualitative tear abnormalities may be primary or contributory causes of the surface disease.
TEAR FILM PHYSIOLOGY Source and Components of the Preocular Tear Film Preocular tear is a complex trilaminar fluid film consisting of lipid, aqueous, and mucin layers. 35 The outer lipid layer is relatively thin at 0.1 J.Lm, the intermediate aqueous layer is the thickest at approximately 7 J.Lm, and the innermost mucous layer has variable thickness ranging from 1 J.Lm over the cornea to 2 J.Lm or greater thickness over the conjunctiva. 35• 43 The superficial lipid layer, secreted by the tarsal (meibomian) glands, provides a thin oily covering over the aqueous tear, retards evaporation, and promotes stable, even spread of tear fluid over the cornea. 35 The lipid tear layer is composed predominantly of cholesterol and waxy lipids with some polar lipids. 2 The molecular weight of meibomian lipids (meibum) is higher and its polarity is lower than those of sebum. 2 Meibomian secretions in humans contain lower levels of triglycerides and free fatty acids and more cholesterol than sebaceous secretions. 2 Meibum is· fluid at eyelid temperature. 57 The meibomian glands are holocrine glands arranged linearly within *Diplomate, American College of Veterinary Ophthalmologists; Associate Professor, Department of Veterinary Medicine and Surgery, College of Veterinary Medicine, and Head, Ophthalmology Section, Veterinary Teaching Hospital, University of Missouri-Columbia, Columbia, Missouri Veterinary Clinics of North America: Small Animal Practice-Vol. 20, No. 3, May 1990
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the dense connective tissues of the eyelid margin (tarsal plate). These modified sebaceous glands are more highly developed in the upper eyelid. 46 Typically, 20 to 40 glands are located in each eyelid. 46 Within the tarsal plate, the meibomian glandular acini form beige grape-like clusters that open into central ductules. These ductules, which lie at right angles to the eyelid margin, deliver meibum to the surface of the eyelid through small orifices on the lid margins (Fig. 1). 35• 46 Each meibomian gland orifice is surrounded by a cuff of epithelial cells. The openings are normally even with the lid margin and form a trough or line, sometimes referred to as the "gray line," just anterior to the mucocutaneous junction (see Fig. 1). Gentle pressure applied to the eyelid margin will normally allow a clear fluid lipid to be expressed from the glandular openings. Although compression of the eyelids during normal blinking may contribute to release of meibum, the precise physiologic mechanism controlling secretion of meibomian lipid is not well understood. The aqueous tear component is the intermediate tear layer, which, in small animals, is produced by the orbital and third eyelid lacrimal glands. 46 Aqueous tear accounts for most of the total tear fluid volume and consists of water, inorganic salts, glucose, urea, surface active polymers, glycoproteins, and some serum proteins. 58 These components have not been quantified for canine or feline tears. Aqueous tear contains proteins secreted by the lacrimal gland-that is, lactoferrin, tear-specific prealbumin, and secretory immunoglobulin A. 15· 28 · 35 Varying amounts of lysozyme are also produced by the lacrimal glands of mammals. 7· 35• 48 The total protein content
Figure l. Normal canine eye. Note row of meibomian gland openings (gray line), glistening corneal surface, tear meniscus along the ve ntral eyelid margin, and mucous thread at the medial canthus. These features indicate the presence of normal tear components.
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of tear is slightly less than l %, which is considerably less than the percentage of protein present in plasma. 35 Aqueous tear serves the metabolic needs of the avascular cornea by supplying glucose, electrolytes, oxygen, and water to the superficial corneal layers. 35 Aqueous tear lubricates and cleanses the ocular surface. It flushes away metabolites, such as C0 2 and lactic acid, in addition to particulate debris and bacteria that would otherwise accumulate on the ocular surface. Antibodies (secretory lgA) 51 and lysozyme (in some species?' 48 present in aqueous fluid protect against microbial invasion. Lysozyme is present in very low levels in canine tear. 48 Third eyelid (TE) and orbital (or main) lacrimal glands are tubuloacinar and histologically similar in dogs . 30 Ductules from these glands deliver aqueous tear secretions into the conjunctival fornices .46 Three to five ductules from the orbital lacrimal glands open into the dorsal lateral conjunctival fornix . 36 The TE gland delivers aqueous tears into the ventral fornices near the base of the TE. The exact number and location of the ductular openings of the TE gland have not been determined. The relative contribution of each of the lacrimal glands to total reflex tear secretion was investigated in dogs by surgically removing one or both glands and then measuring tears with the Schirmer tear test. 24 It was concluded that the contribution of each gland in reflex tear production varies from dog to dog-that is, although the orbital lacrimal gland is the primary source of reflex aqueous tear in some dogs, the TE gland may be the main source in others. When either gland was removed separately, a compensatory increase in production by the remaining gland was apparent. The role of each gland in the production of basal secretions (versus reflex tears) was not determined. Removal of both glands resulted in a total absence of secretions, indicating that accessory conjunctival glands play no appreciable role in aqueous secretions. The deepest tear layer consists of mucin, a hydrated glycoprotein produced by conjunctival goblet cells. Mucin forms an interface between the aqueous tear and the hydrophobic corneal epithelium. 44 In dogs, goblet cells are found in highest density in the conjunctival fornices (Fig. 2). 38 Mature goblet cells are filled with mucin glycoprotein in small droplets that are packaged in the supranuclear Golgi complex. These droplets coalesce into larger droplets as the secretions move toward the cell's apex. Because droplets of mucin are delivered to the epithelial surface through the apices of globlet cells, they are classified as apocrine secretory cells. Goblet cells eject mucin droplets onto the conjunctival surface as the result of normal cell maturation and ocular motion. Follow,ing deposition of secretions onto the conjunctival surface, goblet cells lose their connections to the epithelial basement membrane and are desquamated. The exact life span of conjunctival goblet cells is not known . As glycoproteins are released onto the epithelial surface, secretions of adjacent goblet cells coalesce into mucous threads that consist of a network of interconnecting strands of intracellular and surface mucin . 1 Microfilaments present in the surface microvilli of conjunctival epithelium are hypothesized to facilitate microvillar contraction and thereby aid in spread of the surface mucin. 18• 44 Superficial epithelial cells synthesize a mucopro-
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Figure 2. Histologic section of normal canine conjunctiva (PAS/H&E stain; X 100). Goblet cell mucoprotein stains intensely (PAS positive) and reveals a high density of secretory cells in the ventral conjunctival fornix .
tein packaged as subsurface vesicles that fuse with the cell membrane, become part of the microvilli, and bond to the overlying mucous layer. 17 The role of the subsurface vesicle mucus, therefore, is to anchor goblet cell mucin to the surface me mbranes of the epithelial cells. 18 A relatively small amount of soluble mucin is also a component of aqueous tear. The soluble mucin is believed to be a secretory product of mucous cells of lacrimal glands and accounts for the glycoprotein portion of the aqueous layer. This dissolved mucin decreases the surface tension of tear fluid and enhances spread , stability, and coherence of the aqueous layer to the cornea. 26 Goblet cell-derived preocular mucin fills in irregularities of the corneal surface, resulting in an optically smooth ocular surface. 43• 44 Bacteria and foreign particles are trapped by mucoproteins. Mucin harbors immunoglobulins (lgA) and lysozyme and aids in lubrication and hydration of the conjunctiva and cornea. 61 Dynamics of Tear Film Formation In addition to the presence of normal secretory components, formation of the tri-stratified composite tear film is dependent on the integrity of the eyelid margins, normal ocular movements, and an intact blink mechanism. As the eyelids close, the superficial lipid accumulates on the lid margins where it is compressed into a relatively thick layer between the upper and lower eyelid margins. Basal levels of aqueous tear are secreted into the conjunctival cul-de-ces and bathe the globe. Most of the tear volume-that is, basal secretion-remains under the eyelids. As the lids open, an aqueous
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tear surface is formed over the globe, upon which the compressed lipids rapidly spread. The aqueous layer therefore should normally be coveted by a lipid monolayer. 27 In addition, as movement of the eyelids occurs with concurrent movements of the globe, contact between palpebral conjunctiva and corneal and bulbar conjunctival surfaces results in spread of goblet cell mucin directly over the glycocalyces of surface cells. The continuous distribution of this mucous glycoprotein over the ocular surface is essential for providing a compatible interface between aqueous tear and the hydrophobic corneal and conjunctival epithelial cell membranes. 1• 4 • 27 • 40
PATHOPHYSIOLOGY OF TEAR FILM DISEASE Because of the complex interactions among the lipid, aqueous, and mucin tear components, abnormalities in the quantity or quality of any of these three primary components may alter the fluid dynamics and thus compromise the functions of the tear fluid. Hypertonicity and dehydration of conjunctival and corneal epithelia are the most common events associated with deficiency of one or more of the tear components. 31 Hypoxia of the corneal epithelium and subepithelial corneal stroma may then occur. Tear-deficient eyes have an increased susceptibility to ocular surface infections caused by colonization of either opportunistic or pathogenic microorganisms. Lack of efficient moisturization may result in frictional irritation to surface cells by the eyelids or TE. Potentially toxic tissue metabolites-that is, lactic acid, desquamated cells, denatured mucus, and other organic debris-may accumulate on the ocular surface. Because an overwhelmingly' high percentage of the total tear volume is aqueous tear, quantitative tear deficiency is considered synonymous with insufficient production of aqueous fluid . Abnormalities or deficiencies of tear components other than aqueous fluid are considered to be qualitative disorders. Diseases causing decreased secretion of aqueous tears are diseases of the lacrimal glands or the nerve supply to these glands. Dry-eye states associated with lacrimal gland hyposecretion are collectively referred to as keratoconjunctivitis sicca (see separate article). Distributional abnormalities resulting from lagophthalmos, buphthalmos, eyelid paresis, corneal anesthesia, TE deformities, or frictional irritants may result in an abnormal tear covering or rupture of the tear film with surface drying. These disorders therefore must be distinguished from primary quantitative or qualitative tear deficiencies.
QUALITATIVE ABNORMALITIES Lipid Abnormalities Disturbances of the tarsal or meibomian glands.may result in abnormalities in the superficial lipid layer. Inflammation of the mucocutaneous junction, often referred to as marginal blepharitis, frequently involves the
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meibomian glands. Marginal blepharitis, blepharoconjunctivitis, and meibomianitis are most commonly caused by suppurative bacteria-for example, Staphylococcus sp.-that result in swelling of the eyelid margin, accumulation of exudates, and abnormal lipid secretions. 14· 50 In humans, yeasts are also common infectious causes of marginal blepharitis. 60 In the author's experience, inflammatory diseases of the eyelids of dogs and cats may also be caused by gram-positive aerobic bacteria and yeasts. Diseases of the meibomian glands have been associated with chronic epithelial keratopathy (cystic epithelial keratopathy) and epithelial keratitis in humans. 8 The corneal pathology observed was presumably a manifestation of a deficient lipid layer and an inability to conserve tear by preventing aqueous evaporation. Meibomian gland infections also produce abnormal lipid degradation products that may be toxic to ·surface cells, further contributing to the pathogenesis of the disease. 33 Abnormalities in preocular lipid occur when the tear is contaminated with highly polar lipids from abnormal skin secretions or infected meibomian glands. These abnormal polar lipids are disruptive to the normal nonpolar lipids and may result in premature dispersion of the aqueous layer. 27 Dogs and cats with acute meibomianitis typically have swollen eyelid margins (Fig. 3) with slight "pointing" of the meibomian gland openings. Affected openings may be plugged with dried and discolored meibum. Chronic meibomianitis may result in rupture of glandular acini and release of lipid secretions into the periacinar tissues. The formation of multiple chalazia and lipid granulomas3 may frictionally irritate the eye and complicate any surfacing abnormality that is present (Figs. 4 and 5).
Figure 3. Acute meibomianitis and conjunctivitis in a dog (right upper eyelid). The swollen, rounded eyelid margin and "pointing" of the gland openings (arrows) characterize diffuse meibomian gland inflammation.
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Figure 4. Chalazion of right upper eyelid. The focal enlargement and beige discoloration of meibomian gland, as seen through the tarsal conjunctiva, indicate chronic granulomatous inflammation, with probable rupture of the meibomian gland. .
In cases of chronic meibomianitis, superficial keratitis may be noted. The ke ratopathy may be characterized by somewhat subtle clinical findings, including faint diffuse epithelial edema (see Fig. 5), small multifocal punctate areas of roughe ned epithelium (which may or may not retain fluorescein stain), and a fine superficial perilimbal vascular infiltrate. The degree to which the corneal disease is directly attributable to deficient lipid and unstable tear film is unclear.
Figure 5. Multiple chalazia of right upper eyelid. Note several adjacent meibomian glands with inspissated contents (black arrows). Mild superficial corneal edema may be a manifestation of chronic meibomianitis (white arrows).
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It is probable that surface disease results from a combination of insults, including poor surfacing of the tear and frictional or toxic effects from inflamed meibomian glands. It should be noted that some patients have serious meibomian disease with relatively little or no apparent corneal changes. Most of these patients have an abundant production of aqueous tear, suggesting that reflex tearing may compensate for the lack of tear conservation. It is also possible that toxic inflammatory products and abnormal lipids are simply diluted or flushed away by the aqueous tears. Deficiency of meibomian secretion resulting from maldevelopment of meibomian glands, although rare, has been reported in humans. 9 Eyelid agenesis occurs as a congenital condition of small companion animals where affected areas of the eyelid are devoid of normal eyelid tissue, including meibomian glands. Although relatively uncommon in small animals, eyelid agenesis appears to occur more commonly in cats than dogs. 45 Epiphora, conjunctivitis, and superficial keratitis are the predominant clinical signs. Surface pathology presumably results from a combination of insults, including an absence of meibomian secretions, exposure, trichiasis, and, in some cases, spastic entropion. Diagnosis. Diagnosis of lipid tear abnormalities depends on abnormal findings from a detailed examination. To be most informative, a thorough examination of the eyelid margins should be performed with a focused light and a magnifying source. Although a slit lamp biomicroscope is recommended for this purpose, binocular magnifying loupes and a separate focal light source, such as a Finoff transilluminator, are adequate. During the course of the ocular examination, particular attention is focused on the appearance of the eyelid margins and meibomian glands (see Fig. 1). With the eyelid everted, the examiner will normally note (with magnification) the serial arrangement of multiple meibomian glands just beneath the tarsal conjunctiva. Although each gland appears parallel to the adjacent glands and perpendicular to the eyelid margin, when considering the multitude of glands, the alignment is essentially radial with respect to the axis of the eye. Visualization of the glands may be enhanced by transillumination of the eyelid. An infrared technique of photographing meibomian gland profiles (termed meibomoscopy) recently has been developed. 49 Swollen, rounded eyelid margins indicate acute or subacute marginal blepharitis (see Fig. 2). Hyperemia of the mucocutaneous junction with dry, crusty porphyrin-stained exudates on the lid margins are also indicative of marginal blepharitis. During the detailed examination, the examiner should note the occurrence of any elevated, focal , beige subconjunctival masses typical of chalazia (see Fig. 3 and Fig. 4). When p~esent, chalazia indicate chronic meibomianitis of affected glands with periglandular granuloma formation . Following the administration of topical anesthetic solution, gentle manipulation of the eyelid margin with blunt-tipped forceps (with shallow serrations) will allow inspection of secretions expressed from the meibomian glands. Normal meibomian lipid appears as a clear _viscous oil similar in appearance to clear vegetable oil. Abnormal meibomian secretions are typically thick and opaque or may appear inspissated with a cheesy
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consistency. Expression of coiled semisolid strands of abnormal lipid is not uncommon with chronic meibomian disease. The technique of polarized light biomicroscopy recently was used to characterize surface lipid morphology from normal dogs and cats and to study surface abnormalities of dogs with confirmed or suspected tear film disease. 11- 13 Patterns of lipid distribution were described and the overall thicknesses of surface lipid were determined in normal dogs (between 0.013 and 0.581 j.Lm) and cats (0.089 j.Lm). This technique has been proposed as a practical means of examining the functional integrity of the tear film. In addition to enhancing our understanding of tear film diseases, it has been suggested that polarized biomicroscopy may provide a more effective means of monitoring patients undergoing treatment for tear film abnormalities. 12 Treatment. Treatment of lipid tear abnormalities depends on the specific meibomian disorder present. Bacterial meibomianitis should be treated with both topical and systemic antibiotics. Topical broad spectrum antibiotic ointment-that is, gentamicin or triple antibiotic combinationshould be applied three times daily for a minimum of 3 weeks. Ideally, antibiotic selection is based on antibacterial susceptibility testing. Because opportunistic or pathogenic bacteria may be involved in cases of acute meibomianitis, aerobic bacterial cultures are necessary for definitive diagnosis. It is most rewarding to culture secretions expressed directly from the affected meibomian glands. Systemic and topical antibiotics are also indicated in cases of granulomatous blepharitis secondary to ruptured or abscessed meibomian glands. 3 Oral cloxacillin or trimethaprim-sulfadiazine may be prescribed initially for 2-3 weeks. If the latter is used, the owner should be alerted to the possibility of drug-induced lacrimal hyposecretion. As with acute meibomianitis, topical bacteriocidal agents such as gentamicin or triple antibiotic ointment should be applied topically three times daily. Concurrent administration of systemic corticosteroids may be necessary to resolve the extensive, diffuse eyelid inflammation. 29 Immunostimulant therapy has also been advocated for cases of chronic bacterial blepharitis. 14 Surgical curettage of chalazia and removal of sequestered secretions may be necessary to permit resolution of focal granulomas. Bacterial meibomianitis is regrettably often recurrent and requires intermittent intensive treatments or continuous maintenance therapy. In cases of chronic meibomianitis, periodic manual expression of meibomian material from the glands with blunt-tipped or mildly serrated thumb forceps may be helpful in evacuating inspissated secretions. Treatment of chronic or recurrent infections should be based on antibiotic selection following culture and susceptibility testing. ' Exudates on the eyelid margins may be softened and removed by the use of warm moist compresses to the eyelids for several minutes two or three times daily. This is particularly helpful during the initial treatment period. Application of warm moist compresses results in local vasodilation and improved blood flow to affected areas and, therefore, presumably augments healing. . In treating diffuse meibomian diseases, topical ointments provide the benefit of serving as a lipid substitute. Ophthalmic ointments (or emollients) are usually petrolatum-based preparations but may also contain vegetable
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oil, mineral oil, liquid lanolin, or some combination of these ingredients. In addition to serving as lipid substitutes, emollients lubricate the ocular surface, which is especially beneficial in cases where irregular conjunctiva, rough eyelid margins, or conjunctival granulomas are associated with chalazia. Congenital absence of meibomian glands associated with eyelid agenesis may require both medical and surgical therapy. Grafting of periocular skin, using either a pedicle or sliding skin flap, into the defeCtive area will generally restore eyelid function. 45 In addition to surgical reconstruction, topical application of lubricant ointments will provide a desired lipid-like effect to enhance wetting of the ocular surface. Mucin Deficiency Insufficient production of mucous glycoprotein by conjunctival goblet cells results in deficiency of preocular mucin in humans and dogs . 37• 39• 41 • 61 Human diseases resulting in preocular mucin deficiency include vitamin A deficiency, alkali burns, cicatricial pemphigoid, Stevens-Johnson syndrome, trachoma, and radiation injury. 31 Practolol and echothiophate have been reported as drug-induced causes of mucin insufficiency. 31 These abnormalities result in loss of tear film stability with resultant corneal desiccation. In the dog, the pathogenesis of spontaneously occurring mucin-deficient dry eye disease may vary among cases. 37• 39 Diffuse infiltration of chronic inflammatory cells into the conjunctival mucosa and submucosa may markedly reduce or eliminate goblet cells. Although causes for these infiltrates are often not determined, infectious and immune-mediated causes have been hypothesized. 39 Conjunctival hypoplasia was suspected in one of the reported cases. 39 Squamous metaplasia of the conjunctiva, which may be produced experimentally by vitamin A deficiency, results in abnormal keratinization of the secretory epithelium and loss of conjunctival goblet cells. 52• 53 Severe cicatrization following diffuse ulcerative conjunctival disease is another possible cause of abnormal epithelium and goblet cell loss. Furthermore, it has been hypothesized that the relative avascularity of the conjunctiva that may occur with scarring might result in local deficiency of an essential nutrient, such as vitamin A, to the conjunctival mucosa. 55 Clinical features of canine mucin deficiency are chronic keratoconjunctivitis, corneal ulceration, absence of ocular discharge, and adequate aqueous tear secretions (Fig. 6). 39 The notable absence of appreciable ocular discharge is presumably related to lack of mucin production. Histopathologic studies of conjunctival specimens from affected eyes reveal s,parse numbers or a total absence of goblet cells (Fig. 7). Diagnosis. The clinical diagnosis of canine ocular mucin deficiency is supported by performing tear film breakup time (BUT) tests and confirmed by biopsying conjunctiva and quantifying epithelial goblet cells. 39 Tear film BUT tests are performed by instilling one or two drops of fluorescein stain into the eye and then manually holding the eyelids open. The time is recorded from the last blink to the appearance of the. first dry spot, which appears as a dark area in the yellow-green fluorescent film. A cobalt blue
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Figure 6. Chronic keratitis associated with mucin deficie ncy. Absence of appreciable ocular discharge, chronic inflammation of the cornea and conjunctiva, and nonresponse to symptomatic topical therapy characterize mucin deficiencv. Although not present in this e"xample, nonresponsive or recurrent corneal ulceration may also occur.
filter should be used when viewing the cornea. Normal BUT in dogs is approximately 20 seconds (19 ± 5 sees). 38 In affected animals, tear breakup occurs in usually less than 5 seconds. 39 Although BUT is regarded as a clinically useful test, the reliability of this test has been questioned. Erroneously rapid BUTs may be recorded when corneal surface irregularities are present. Corneal exposure, corneal anesthesia, or surface frictional irritants may result in evaporation or abnormal distribution of tear fluid and rapid tear BUT. 59 Preservatives in ophthalmic irrigating solutions and fluorescein preparations may artificially lower the BUT. 32 These possible sources of variability have limited the usefulness of tear BUT as a diagnostic and research tool. Determination of goblet cell frequency from a conjunctival biopsy is an indirect measure of mucin production. Collecting a sample of conjunctiva for histopathology involves instillation of topical anesthetic solution, grasp-
Figure 7. Conjunctival biopsy from the eye shown in Figure 6. Note absence of goblet cells and infiltration of mononuclear inflammatory cells. Biopsy was taken from ventral conjunctival fornix just anterior to the third eyelid, whe re goblet cells are normally present at high densities (see Fig. 2).
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ing and tenting (with Bishop-Harmon forceps) conjunctiva from the ventral fornix just anterior to the base of the TE, and removing a 3 mm X 4 mm specimen with conjunctival scissors. Because a heterogenous density and distribution of conjunctival goblet cells has been demonstrated in dogs, conjunctival sites with the .highest and most predictable density of goblet cells are most appropriate for sampling (see Fig. 2). 38 The optimal area for biopsy in the dog is the lower fornix conjunctiva between the TE and the lower eyelid. 38 Conjunctival biopsy specimens are gently stretched over a flat surface, such as a section of tongue depressor, and fixed with iO% buffered formalin . Sections of conjunctival epithelium are stained with periodic acid Schiff and hematoxylin reagents. Goblet cells and nongoblet epithelial cells are counted and an index of goblet cell density (the ratio of goblet cells to all epithelial cells) is determined. The highest index has been demonstrated to occur in lower conjunctival fornix (anterior to the TE) where the value is 0. 29-0.3. 38 In severely affected animals the index at this site may be as low as 0.05 or lessY· 39 Treatment. Treatment for mucin-deficient keratoconjunctivitis consists of topical mucin replacements (mucinomimetics), symptomatic treatment of corneal ulcers, if present, and topical anti-inflammatory therapy in selected cases. 37 • 39 The use of topical mucinomimetics applied at 4-6 hour intervals is the mainstay of therapy. Commercially available artificial tears with a patented polymer base (e.g., Adsorbobase, Alcon Laboratories, Ft. Worth, TX) have mucinomimetic properties. Viscoelastic substances containing a hyaluronate component42 or 2% methylcellulose (Hydroxypropyl Methylcellulose 2% in BSS, Department of Pharmaceutics, University of Tennessee, Memphis, TN) rnay also be effective wetting agents. These lubricants have extended contact time with the ocular surface and, therefore, provide enhanced wetting of the ocular surface. Given that the presence of conjunctival inflammatory cells appears to be inhibitory to the existence of normal goblet cell populations, 37 • 39 topical corticosteroids may be indicated when marked mucosal and submucosal mononuclear inflammatory cell infiltrates are noted histologically. When ulcerative keratitis is present, corticosteroids should not be administered until ulcerations have healed, as evidenced by negative fluorescein stain retention. Any concurrent infections must be treated simultaneously with appropriate antimicrobials. The use of topical retinoic acid in the treatment of mucin-deficient ocular surface disease. is controversial. In experimentally induced vitamin A deficiency that is characterized by squamous metaplasia and absence of conjunctival goblet cells, topical 0.1% retinoic acid was demonstrated to be beneficial in reversing the conjunctival metaplasia and rejuvenation of a secretory mucosa. 53 The benefits of topical retinoic acid in treating mucindeficient disease not specifically caused by vitamin A deficiency is debatable, however. Because naturally occurring vitamin A deficiency is extremely uncommon in small animals and the availability of retinoic acid products suitable for topical application has been limited to experimental protocols, topical vitamin A cannot be recommended at pre.sent for routine use in treating mucin-deficient surface disease.
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Lacrimomimetics are usually not indicated when treating mucindeficient keratoconjunctivitis because, in most cases, aqueous tear production is adequate. 39 In instances where both aqueous and mucin deficiencies are present, combination therapy aimed at treating both deficiencies is appropriate. Effects of lacrimostimulant drugs, such as pilocarpine or cyclosporine, on conjunctival mucous secretions are unknown, but their potential usefulness in treating ocular mucin deficiency merits investigation. Other Potential Qualitative Disorders In problematic cases of canine keratoconjunctivitis where recognized causes of surface disease have been excluded, other possible qualitative abnormalities of the tear fluid must be considered. It is probable that specific tear components other than lipid and mucin, when present in abnormal quantities, may cause overt ocular disease or contribute substantially to the pathogenesis of presently enigmatic eye diseases of small animals. Additional important components of tear fluid essential to its multiple functions are tear proteins, a number of miscellaneous organic constituents, and electrolytes. These components influence osmolality and tear pH, which are also discussed briefly here. Specific levels of tear proteins vary considerably among species. 35 • 48 • 56 Tear proteins in most mammals consist of three main componentsglobulins, albumin (tear-specific prealbumin), and lysozyme. 35 • 48 • 56 Globulins and lysozyme are protective (antimicrobial), while tear prealbumin is believed to be a retinol-binding protein that allows delivery of vitamin A to the cornea. 15 The role of several proteins present in smaller quantitiesthat is, lactoferrin, transferrin, cendoplasmin, and soluble glycoproteinsis unclear. Separation, identification, and· quantification of tear proteins are possible with sodium dodecylsulfate-polyacrylamide gel electrophoresis and high performance liquid chromatography (HPLC) systems. 6 • 20• 21 • 34 In the future, detection of abnormal distributions and/or quantities of tear proteins may prove to be quite useful in the accurate diagnosis and prognosis of ocular surface diseases. · Additional organic components of tears of possible clinical significance are vitamin A, cholesterol, and glucose. The role of vitamin A in the regulation and differentiation of epithelial tissues is well documented. 23 Deficiency of vitamin A in humans and animals leads to reduction or loss of normal secretions with subsequent keratinization of the conjunctiva and cornea (xerophthalmia). 23• 52• 53 Desquamation of cells with ulceration and progressive loss of tissue followed by rupture of the eye may also occur. Repletion of vitamin A to deficient animals restores conjunctival and corneal health and cells return to their normal nonkeratinized states. 53 Using HPLC methods, tear all-trans retinal quantitation is possible from microvolumes of tear fluid. 54 Normal levels of all-trans retinal have been determined for tear fluids of companion patients. Elevated tear cholesterol levels may occur with systemic diseases of local metabolic disturbances and could partially account for deposition of cholesterol in the cornea, resulting in substantial corneal opacities. In human tears, cholesterol is normally present in considerably lower levels
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than those found in serum. The role of tear cholesterol in the development of degenerative keratopathies (lipodystrophies) with stromal crystalline deposits is unknown. Similarly, it is not known whether elevated tear cholesterol is involved in crystalline keratopathy observed in dogs treated long-term with topical corticosteroids. Sampling tear glucose has been pursued mainly as a means of detecting or monitoring a patient's diabetic status. Of equal or greater interest may be quantifYing tear glucose as a measure of surface normalcy. Chemical, osmotic, or mechanical insults may shift tear glucose levels dramaticallythat is, the shift is usually to lower values in reflex tearing and to higher values in states of surface dehydration. 25 A spectrophotometer may be used for tear glucose measurements by transferring tear from a collecting capillary tube into a measuring pipette. 25 Regrettably, spectrophotometer units capable of managing microvolumes are relatively expensive. Several ions found in tears are essential to the osmotic regulation of the extracellular and intracellular spaces. The primary cations of tear are sodium and potassium, and the predominant anions are chloride and bicarbonate. 35 Although sodium in tears is about equal to plasma, potassium is considerably higher than corresponding plasma concentrations. 58 This indicates an active secretion of potassium into the tear. Calcium and magnesium are other cations present in tear, although they occur at relatively low concentrations. 47 These cations play an important role in regulating membrane permeability and are presumably associated with cofactor and enzyme functions of ocular surface cells. In humans, tear calcium is normally lower than the free fraction in plasma. 58 Increases in tear calcium may occur with diminution of tear volume or with changes in the ocular surface environment. The author has encountered cases in which elevated tear pH was associated with calcium deposits on the cornea. Corneal calcification has been noted in primary canine corneal dystrophies and in keratopathies secondary to chronic inflammation. 5 Excess calcium may precipitate preocular mucus and reduce its effectiveness as a wetting agent (Robinson J: Personal communication, 1989). By influencing the osmotic pressure of tears, the particles within tears (especially the ions) affect the turgescence (i.e., state of hydration) of the cornea. 35 Because of evaporation, the open-eye tear osmotic pressure is equivalent to 0. 97% NaCl solution. 25 This is slightly higher than the 0. 9% reference level for many ophthalmic solutions. When tear osmolality falls below 0. 9%, the cornea may become edematous. Methods are available for measuring the osmolality of nanoliter volumes of tear; the equipment is expensive, however. . Tear pH is an important factor in determining patient comfort, the tolerance of various topical eye medications, and susceptibility of eyes to certain infections. 25 Clinical methods for measuring tear pH have generally not been reliable. Tear pH shifts may normally occur to such an extent in a given patient as to make interpretation of findings very difficult. 16 In one group of normal human subjects, the pH of unstimulated tears was 7.45, with most variability among subjects and in a given subject occurring at different times of the day. 10 A more acidic pH was noted after prolonged lid closure-that is, after sleeping.
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Accurate measurement of tear pH involves use of micromethods with temperature regulation and isolation of the sample from the air. 19· 25 Tears can be measured and monitored to within 0.02 of a pH unit. The cost and availability of suitable equipment and available technical support have limited the use of tear pH measurements in clinical settings.
IMPLICATIONS FOR FUTURE RESEARCH The need for basic research into components of tear in healthy and diseased eyes is apparent. It is unfortunate that more is not known about the biochemical composition of canine and feline tear fluid . Until additional basic information becomes available, it will not be possible to precisely define cause and effect relationships between ocular surface diseases and deficiencies, excesses, or imbalances of tear components. Ocular diseases or syndromes that merit investigating for possible relationships to qualitative tear abnormalities include superficial punctate keratitis, idiopathic vascularizing keratopathies, corneal epithelial erosions, certain breed-predisposed corneal dystrophies (e.g., as seen in Shetland Sheepdogs), feline corneal sequestration (nigrum) syndrome, and degenerative lipodystrophies. It is quite possible that a number of these entities as well as other presently unexplained chronic corneal diseases may be manifestations of qualitative disorders of the preocular tear film. For this reason, investigations of the biochemical components of the tear fluid in both normal and diseased animal eyes is encouraged. Furthermore, because of the frequent occurrences and considerable similarities between tear film diseases of humans and small companion animals and the mutual interest of physician and veterinary ophthalmologists in tear biochemistry, comparative studies should be pursued in search of appropriate animal models for study of tear film diseases.
SUMMARY Abnormalities of the lipid and mucin components of the preocular tear film may result from diseases of the eyelid margins and conjunctiva. Chronic keratoconjunctivitis with epithelial edema and superficial corneal neovascularization, with or without ulceration, characterizes qualitative tear diseases. Tear components other than lipid and mucin that carry probable clinical significance include tear proteins, all-trans retinal, cholesterol, glucose, and electrolytes. Although less common than quantitative or aqueous deficiencies, qualitative abnormalities are recognized as primary or secondary causes of ocular surface disease in companion animals.
REFERENCES 1. 2. 3. 4.
Adams AD: The morphology of human ocular mucus. Arch Ophthalmol 97:730, 1979 Andrews JS: Human tear film lipids. Exp Eye Res 10:223, 1970 Barrie KP: Eyelid pyogranulomas in four dogs. J Am Anim Hosp Assoc 15:433, 1979 Benedetto DA, Shah DO, Kaufman HE: The instilled fluid dynamics and surface chemistry of polymers in the preocular tear film. Invest Ophthalmol Vis Sci 14:887, 1975
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5. Bistner SI: Corneal pathology. In Peiffer RL: Comparative Ophthalmic Pathology. Springfield, IL, Chas C Thomas, 1983, p 103 6. Bradford MM : A rapid and sensitive method for the quantitation of microgram quantities of protein utilizating the principle of protein-dye binding. Anal Biochem 72:248, 1976 7. Brightman AH, Washsstock RS : Lysozyme concentrations in the tears of cattle, goats, and sheep. Presented to the American College of Veterinary Ophthalmologists, San Francisco, CA, 1986, p 422 8. Bron AJ, Tripathi RC: Cystic disorders of the corneal epithelium. I. Clinical aspects. Br J Ophthalmol 57:361, 1973 9. Bron AJ, Mehgher LS: Congenital deficiency of meibomian glands. Br J Ophthalmol 71:312, 1987 10. Carney LG, Hill RM: Human tear pH: Diurnal variations. Arch Ophthalmol 94:821, 1976 11. Carrington SD, Bedford PGC, Guillon JP, et al: Polarized light biomicroscopic observations on the precorneal tear film . l. The normal tear film of the dog. J Small Anim Pract 28:605, 1987 12. Carrington SD, Bedford PGC, Guillon JP, et al: Polarized light biomicroscopic observations on the precorneal tear film . 2. Keratoconjunctivitis sicca in the dog. J Small Anim Pract 28:671, 1987 13. Carrington SD, Bedford PGC, Guillon JP, et al: Polarized light biomicroscopic observations on the precorneal tear film . 3. The normal tear film of the cat. J Small Anim Pract 28:821, 1987 14. Chambers ED, Severin GA: Staphylococcal bacterin for treatment of chronic staphylococcal blepharitis in the dog. J Am Vet Med Assoc 185:422, 1984 15. Chao CW, Butala S: Isolation and preliminary characterization of tear prealbumin and human ocular mucus. Exp Eye Res 5:895, 1986 16. Coles WH, Jaros PA: Dynamics of ocular surface pH. Br J Ophthalmol 68:549, 1984 17. Dilly PN: On the nature and the role of the subsurface vesicles in the outer epithelial cells of the conjunctiva. Br J Ophthalmol 69:477, 1985 18. Dilly PN: Contribution of the epithelium to the stability of the tear film. Trans Ophthalmol Soc UK 104:381, 1985 19. Fischer FH , Wiederholt M: Human precorneal tear film pH measured by microelectrodes. Graefe's Arch Clin Exp Ophthalmol 218:168, 1982 20. Fullard RJ: Normal protein profiles and diurnal variations in human tear fluid. Invest Ophthalmol Vis Sci (suppl)28:157; 1987 21. Fullard RJ: Identification of proteins in small volumes with and without size exclusion HPLC fractionation. Curr Eye Res 7:163, 1988 22. Gilbard JP, Farris RL, Santamaria J: Osmolarity of tear microvolumes in keratoconjunctivitis sicca. Arch Ophthalmol 96:677, 1978 23. Hassel JR, Newsone DA: Vitamin A-induced alterations in corneal and conjunctival epithelial glycoprotein biosynthesis. Ann NY Acad Sci, 1981, p 358 24. Helper LC, Magrane WG, Koehm J, et al: Surgical induction of keratoconjunctivitis sicca in the dog. JAm Vet Med Assoc 165:172, 1974 25. Hill RM: Tear film analysis. Am J Optom Physiol Opt 58:609, 1981 26. Holly FJ: Tear film physiology. Am J Optom Physiol Opt 57:252, 1980 27. Holly FJ: Tear film physiology. Int Ophthalmol Clin 27:2, 1987 28. Janssen PT, van Bijsterveld PO: Origin and biosynthesis of human tear fluid proteins. Invest Ophthalmol Vis Sci 24:623, 1983 29. Johnson BW: Dermatoses of the canine eyelids. Contin Educ Pract Vet 11:385, 1989 30. Kaswan RL, Martin CL, Chapman WL: Keratoconjunctivitis sicca: Hi~topathologic study of nictitating membrane and lacrimal glands from 28 dogs. Am J Vet Res 45:112, 1984 31. Lamberts DW: Keratoconjunctivitis sicca. In Smolin G, Throft RA: The Cornea. Boston, Little Brown, 1983, p 293 32. Lemp MA, Hamil JR: Factors affecting tear film breakup in normal eyes. Arch Ophthalmol 89:103, 1973 33. McCulley JP, Scialis GF: Meibomian keratoconjunctivitis. Am J Ophthalmol 84:788, 1977 34. Mackie lA, Seal DV: Diagnostic implications of tear protein profiles. Br J Ophthalmol 68:321, 1984 35. Milder B: The lacrimal apparatus. In Moses RA (ed): Adle/s Physiology of the Eye, ed 8. St. Louis, CV Mosby, 1987, p 18
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36. Michel G: Beitrag zur Anatomie der Tranenorgane von Hund und Katze. Dtsch Wochenschr 62:347, 1955 37. Moore CP, Collier LL: Mucin-deficient external eye disease in dogs. Presented to the American College of Veterinary Ophthalmologists, San Francisco, CA, 1985, p 152 38. Moore CP, Wilsman NJ, Nordhein EV, et a!: Density and distribution of canine conjunctival goblet cells. Invest Ophthalmol Vis Sci 28:1925, 1987 39. Moore CP, Collier LL: Ocular surface disease associated with loss of conjunctival goblet cells in dogs. J Am Anim Hosp Assoc (in press) 40. Moore JC, Tiffany JM: Human ocular mucus. Origins and preliminary characteristics. Exp Eye Res 33:203, 1981 41. Nelson JD, Wright JC: Conjunctival goblet cell densities in ocular surface disease. Arch Ophthalmol 102:1049, 1984 42. Nelson JD, Farris RL: Sodium hyaluronate and polyvinyl alcohol artificial tear preparations. Arch Ophthalmol 106:484, 1988 43. Nichols BA, Chiappino ML, Dawson CR: Demonstration of the mucous layer of the tear film by electron microscopy. Vis Sci 26:464, 1985 44. Nichols B, Dawson CR, Togni B: Surface features of the conjunctiva and cornea. Invest Ophthalmol Vis Sci 24:570, 1983 45. Peiffer RL: Feline ophthalmology. In Gelatt KN (ed): Textbook of Veterinary Ophthalmology. Philadelphia, Lea & Febiger, 1981, p 522 46. Pollock RVH: Eyelids. In Evans HE, Christensen GC (eds): Anatomy of the Dog, ed 2. Philadelphia, WB Saunders, 1979, p 1099 47. Rismondo V, Edelhauser HF, Ubels JL: Ionic composition of tears and lacrimal gland fluid. Invest Ophthalmol Vis Sci (suppl) 28:157, 1987 48. Roberts SR, Erickson OF: Dog tear secretion and tear proteins. J Small Anim Pract 3:1, 1962 49. Robin JB, Jester JV, Nobe J, et a!: In vivo transillumination biomicroscopy and photography of meibomian gland dysfunction. Ophthalmology (Rochester) 92:1423, 1985 50. Seal DV, McGill Jl, Jacobs P, eta!: Microbial and immunological investigations of chronic nonulcerative blepharitis and meibomianitis. Br J Ophthalmol 69:604, 1985 51. Selinger OS, Selinger RC, Reed WP: Resistance to infection of the external eye: The role of tears. Surv Ophthalmol 24:33, 1979 52. Sherman Ml: Evidence that retinoids control epithelial differentiation. In Retinoids and Cell Differentiation. Boca Raton , FL, CRC Press, 1986, p 30 53. Sommer A, Emran N: Topical retinoic acid in the treatment of corneal xerophthalmia. Am J Ophthalmol 86:615, 1978 54. Speek AJ : Microdetermination of vitamin A in human plasma using high-performance liquid chromatography with fluorescence detection. J Chromatogr 382:284, 1986 55. Tseng SCG, Hirst LW, Maumenee AD, eta!: Possible mechanisms for the loss of goblet cells in mucin-deficient disorders. Ophthalmology 91:545, 1984 56. Thorig L, van Agtmaal EJ, Glasius E, eta!: Comparison of tears and lacrimal gland fluid in the rabbit and guinea pig. Curr Eye Res 4:13, 1985 57. Tiffany J, Dart J: Normal and abnormal functions of meibomian secretions. Royal Society of Medicine International Congress and Symposium Series, No. 40, London, Royal Society of Medicine, 1981 58. Van Haeringen NJ: Clinical biochemistry of tears. Surv Ophthalmol 26:84, 1981 59. Vanley GT, Irving LH, Gregg TH: lntrepretation of tear film breakup. Arch Ophthalmol 95:445, 1977 60. Vaughan D, Asbury T: Marginal blepharitis. In General Ophthalmology, ed 8. Los Altos, CA, Lange Medical Publications, 1977, p 46 61. Wright P, Mackie lA: Mucus in the healthy and diseased eye. Trans Ophthalmol Soc UK 97:1, 1977
Address reprint requests to Cecil P. Moore, DVM, MS Department of Veterinary Medicine and Surgery College of Veterinary Medicine University of Missouri-Columbia Columbia, MO 65211