- Email: [email protected]
Congenital Ocular Disease in the Foal
Symposium on Large Animal Ophthalmology
Congenital Ocular Disease in the Foal
G.A. Munroe, B.V.Sc. (Hons), M.R.C.V.S.,* and K.C. Barnett, M.A., Ph.D. , B.Sc., F.R.C.V.S . , D.V. Ophtholmol.t
Congenital ocular abnormalities in the foal, although not as common in the dog and cat, nevertheless form an interesting and important group of equine eye conditions. Within the last 10 years, several surveys of the eye of the foal have been published in an attempt to establish its normal appearance and possible variations and to identify and record the incidence of various abnormalities. 1.5. 13. 15
THE NORMAL EYE OF THE FOAL The eyes of the foal may be examined for a purchaser or insurance company, as part of a general stud preventative medicine regimen or for a specific eye problem. On either of these occasions, it is essential for the veterinarian to possess a knowledge of the normal foal eye and its variations , as well as of possible disease entities . The early recognition and diagnosis of congenital eye disease in the foal is important to prevent unnecessary suffe ring by the foal and, in some conditions, to expedite treatment. The owner is most likely to be concerned with prognosis for both normal performance and breeding; therefore , the possible progression or inheritance of a given problem is of vital importance. The following article has been written in an attempt to bring the latest information on congenital eye disease of th e foal to the veterinary practitioner so that some of these problems can be bette r resolved.
THE EYE OF THE NEONATE The ophthalmoscopic appearance of the normal eye of the newborn foal has been little studied until rece ntly, when several surveys in the Thor' Private Practitioner, The Tortington Centre , Arundel, West Sussex, England tU ni! of Comparative Ophthalmology, The Animal Healt h Trust , Lanwades Park, Suffolk, England
Ve terinary Clinics of North America: Large Animal Practice-Vol. 6, No.3, November 1984
519
520
G.A.
MUNROE AND
K.C.
BARNETI
oughbred,I.5.13.18 and in thoroughbreds , standardbreds, and saddlebreds l5 have been undertaken. Reflexes The eye of the foal is open at birth. Response to a menacing gesture is present in all foals over the age of 5 days.15 The direct and consensual pupillary light reflexes are absent at birth and are sluggish and incomplete for at least 48 hours. During this time, the pupil is semidilated and Gircular. The normal adult responses of the pupil are generally present in 5-day-old foals . 15 Eyelids Vibrissae are present below the eye in most foals and above the eye to a lesser extent. Cilia are usually long on the upper lid. The edges of the eyelids rest in close proximity to the cornea. There should be no ocular discharge , alopecia, or depigmentation of the lids . The Nictitating Membrane The leading margin of this structure is usually pigmented9 . 15 and lies immediately above the distal extension of the nictitating cartilage. Lack of margin pigmentation may be associated with absence of conjunctival and limbal pigment. The caruncle is located at the medial canthus at the base of the membrane. Cornea The normal cornea of the foal is clear, oval, and broader nasally. Corneal thickness is variable, being thinnest at the center. The scleral shelf or overhang is more prominent dorsally and ventrally, which makes visualization of the iridocorneal angle more difficult., Conjunctiva This structure is the same as in the adult; irregular pigmentation is usually concentrated at the limbus. Aqueous Humor Normally, the aqueous humor is optically clear. Iris In almost all foals, the iris is dark brown and has little association with coat color except in heterochromia irides. The brown color may be homogenous or mottled with tan or grey toward the periphery. Corpora nigra (granula iridica) are present in all foals dorsally, but their occurrence ventrally is more sporadic and less striking. 13.15.18 The pupils of the newborn foal that is less than 24-hours old should be equal, large, and circular (in a position near maximal dilation. )1.5.9. 18The normal adult resting position and oval shape of the pupil are generally present in 3-day-old foals . u5. ls The pupil is wider nasally. The Lens Two structures are visible in the lens of the foal that are less commonly seen in the adult horse. In the majority of foals , the lens suture pattern is
CONGENITAL O CU LAR DISEASE IN THE FOAL
521
Figure 1. Normal fundus in a 2-dayold normal Thoroughbred foal. (See also color plate lA .)
apparent with slit-lamp biomicroscopy. Marked variations can occur among foals and between the two eyes of the foal. Generally, the anterior structure has a Y configuration, and the posterior suture varies from an inverted Y to a sawhorse or stellate with feathering. 14.15 In the majority of Thoroughbred foals, the anterior suture is not visualized. 14.15 A structure present in the vitreous body and attached to the posterior lens capsule is the hyaloid system. The hyaloid artery is present in all foals under the age of 24 hours and can be recognized as a fine, dark, curving line transversing the vitreous from the center of the optic disc to the posterior aspect of the lens. Occasionally, it may contain blood or appear black, although even as early as 2 hours after birth no blood or pulsations may be observed within the artery. 5 Remnants of the artery are also easily observed on the posterior lens capsule during the first 24 hours but disappear and are less prominent in older foals. The Fundus The ocular fundus may be divided ophthalmoscopically into the retinal blood vessels, optic nerve head or disc, tapetal fundus , and nontapetal fundus (Fig. 1). Retinal Vasculature. Thirty to 60 small retinal vessels radiate from the optic nerve in all directions, with fewer emerging in the mid ventral region. 9 . 15,23 Vessels traverse the retina 1 to 2 disc diameters in approximately straight lines although they can be tortuous, particularly on the surface of the optic disc (see Fig. 1). Arterioles and venules cannot be differentiated, Optic Disc. The optic nerve head is situated in the non tapetal fundus and is usually oval, although it may possibly be round, nasally blunted, and football-shaped . 15 Optic disc color is consistent between the two eyes and changes with age , The color within the first 24 hours can vary between pale pink and deep red (see Fig, 1), As the foal ages, the disc becomes lighter (pale pink to salmon pink) with a darker border. l,5 A white rim may be present along the midventral aspect of the disc. 15 The surface of the disc may have fine irregularities that, in later life, take on a more fibrous appearance. 23 The Tapetal Fundus. The tapetal color variation is considerable, but
522
G.A.
MUNROE AND
K.C.
BARNETI
the predominant color is green. Variations include primary yellow, nasal yellow with temporal green, green with a yellow periphery, yellow-green, and yellow with green islands. Foals with cream and light-colored hair coats may have more yellow in the tapetal fundus; whereas chestnut or bay horses can be yellow-green, and black or grey animals can have a slightly more bluish coloration. In saddlebreds, the tapetal coloration varies from one area to another with a relatively well-defined junction between areas . 15 Pigment mottling can occur dorsally and nasally. Penetration of the tapetum by small choroidal vessels appears as brown dots (the stars of Winslow) distributed throughout the tapetal fundus . Choroidal vessels , represented by red or blue streaks extending dorsally to the disc, can be seen particularly well when there is peripapillary hypopigmentation , a thin tapetum, and lack of choroidal or retinal pigment. Partial to total albinism may occur in both the tapetal and non tapetal fundus . When tapetal fundus is involved, light yellow is often observed with red stars of Winslow. In extreme cases, no tapetum is present and the choroidal vessels can be seen against a pale cream background (sclera). The junction between the tapetal and nontapetal areas is usually prominent in foals and may be "ragged" in appearance. The tapetal fundus can occasionally extend ventrally towards the dorsal aspect of the optic nerve head and, in this area, may have a different coloration from the majority of the tapetum. The Nontapetal Fundus. In the majority of foals, the non tapetal fundus is a dark brown or chocolate color. Light grey streaks that curve nasally and temporally away from the central disc margin have been reported recentlyl5 and may reflect the course of retinal vessels . Albinism and subalbinism can involve the nontapetal fundus , which can lead to a diffuse brown-red appearance in mild cases. With greater degrees of albinism the underlying choroidal vessels become visible and, in extreme cases, may be observed lying in front of the sclera. The area of tapetum nigrum immediately above the optic disc, which always lies within the non tapetal fundus and below the tapetum lucidum, is commonly less densely pigmented; parts of choroidal vessels often may be apparent or the area may appear more red than the rest of the nontapetal fundus. The disc may also have irregular, light grey, translucent zones irradiating from one or both sides. These represent either increased retinal thickness 9 or an increased amount of myelin . I . 9 "Islands" or "spots" may be seen in the non tapetal fundus just ventral to its junction with the tapetal fundus. 13 These zones , which represent areas of reduced pigmentation, appear to be nonprogressive, white , variable in size, and histologically show no evidence of inflammatory changes.9. 13.23 It has been suggested that these foci could be the result of focal inflammatory lesions. 13.22 If tapetal structure is present in these areas, they are known as tapetal islands. Koch has described them as atypical colobomas or focal areas of pigment dystrophy. 13 THE ABNORMAL EYE OF THE FOAL The Globe and Orbit Conditions of the globe and orbit can be divided into those that produce either exophthalmos (prominence of the globe) or enophthalmos (eye is
CONGENITAL O CU LAR DISEASE IN THE FOAL
523
Figure 2. Two microphthalmic eyes from a Thoroughbred foal showing diffe ring degrees of microphthalmos.
recessed). In congenital eye disease in the foal , only the latter are mentioned in the literature . Microphthalmos Microphthalmos may be unilateral or bilateral; it has been described in many species and is encountered not infrequently in foals . 21 It can be an isolated finding or, more frequently, can be associated with other congenital abnormalities-particularly cataracts . The size of the globe varies considerably from a near normal structure with normal function to a microscopic structure with only pigmented remnants (Fig. 2). 3.6 True enophthalmos is extremely rare . 23 Microphthalmos can be divided into three categories: 2 1. Pure microphthalmos (nanophthalmos), in which the eye is small but otherwise normal. 2. Colobomatous microphthalmos , in which the defect is associated with failure of involution of the optic vesicle or closure of the embryonic cleft. 3. Complicated microphthalmos, in which anomalous development occurs unrelated to fissure closure. The condition is usually recognized in newborn foals as a smaller than normal palpebral fissure that often has a prominent nictitating membrane. The cornea and globe are sometimes not visible or may be abnormally pigmented, and the animal may be blind or partially sighted in the affected eye(s). Treatment is not possible; in severe, bilateral cases, euthanasia is the only course of action. In unilateral cases, however, the animal may manage quite well and adapt as it ages. There are many examples of Thoroughbreds winning races with unilateral microphthalmia. If the affected eye is very small, drainage from the eye may be inadequate, which will lead to ocular discharge and the necessity for enucleation. Many cases are sporadic and idiopathic, but in other cases, the cause may be toxic, mechanical , infectious, or even nutritional. Although all breeds are affected, some breeds appear to have a higher incidence (the Thor-
524
G.A.
MUNROE AND
K.C .
BARNEIT
Figure 3. Entropion oflower eyelid in a SkewbaJd foal. (Courtesy of Dr. D. Wilson. )
oughbred, for example). However, the role of inheritance has not been proven in the horse 3 as it has in man and the dog. Strabismus Strabismus is rarely reported in the horse. In a survey conducted on horses and mules in 1940, it was only detected in males at a frequency of 0.5 per cent. Nearly all the mules exhibited nonsymmetric convergent strabismus (esotropia). Congenital strabismus occurs in the horse and is predominantly of the convergent type. 9 Two Appaloosa yearlings with bilateral hypertropia (vertical strabismus) of apparent congenital origin have been reported. 8 Affected horses deviated the head as if to compensate for the abnormal visual axis, stumbled, and had nervous temperaments. In two Appaloosas with hypertropia, surgical correction was performed by recession of the dorsal rectus muscle and resection of the ventral rectus muscle. Surgery resulted in rotation of the ocular axis to a more normal plane; vision and temperament were improved.
THE EYELIDS The eyelids of the horse are important for protection of the eye, tear film distribution , and cosmetic appearance of the head. Lesions of the eyelids, including congenital disease, are common in the horse. The eyelids of the foal may be affected at birth as part of the microphthalmia complex. Other conditions affecting the eyelids of the foal are entropion, ectropion, and dermoids. Entropion Entropion is an inversion of the eyelid margin and eyelashes (Fig. 3). The problem may involve one or both eyes, and the lower lid is much more
CO NGENITAL O C ULAH DISEASE IN TilE FOAL
525
commonly affected. The associated irritation may cause lacrimation, conjunctivitis, ble pharospasm, and corneal disease including keratitis, ulceration , and perforation. 2o The condition may be seen in weak, premature, or systemically ill foals as a secondary problem due to enophthalmos. This sinking of the globe into the orbit forms part of the clinical picture of dehydration and anorexia. Entropion also occurs in normal healthy foals as a primary and congenital lesion occurring in the first few days of life. Whether the entropion is primary or secondary, it is necessary at an early stage to determine if the condition can be corrected medically or surgically. In some foals , the condition can be managed for a few weeks with topical antibiotic ointments, allowing time for the rapid growth processes to gradually correct the eyelid defect. Occasionally, it is possible to correct the entropion in the first 12- to 24-hour period by gently everting the eyelid several tim es a day together with a regimen of protective antibiotic ophthalmic ointment three times a day. In lids that do not respond to manipulation, correction may be attained by infiltrating procaine penicillin under th e skin of the affected lid. 25 With the foal restrained in lateral recumbency and/or sedation, approximately 0.5 to 1 ml of procaine penicillin is injected under the skin of the lower lid. The resultant bleb is milked toward the entire length of the eyelid margin causing the conjunctiva to swell and mechanical eversion of the lid. The swelling decreases over a 24to 36-hour period. Repeated procedures are usually unnecessary. A local tissue reaction may occur later which will further evert the inturned lid. A simple technique of eversion involves placement of several temporary horizontal mattress re te ntion sutures through the skin of the affected lid. 23 For nonresponsive congenital and acquired entropion, surgical intervention may b e required. Surgery is best performed under general anesthesia and with the cornea protected with ophthalmic ointment and the skin clipped and surgically scrubbed. The area and extent of excision should be ascertained carefully prior to anesthesia. Overcorrection must be avoided . Many methods are available for correction, but the Hotz-Celsus procedure provides a satisfactory result. 9 .20 .23 This technique involves the removal of a section of skin with the initial incision parallel to and within 3 to 5 mm of the affected lid margin. Some surgeons also prefer to remove a strip of orbicularis oculi muscle. Hemorrhage may be controlled by hemostats or electrocautery (not ligation), and skin (and muscle , if resected) may be opposed with simple interrupted nonabsorbable sutures . Ophthalmic antibiotic ointment is applied postope ratively, and assessme nt of the surgery is made some 6 to 8 weeks after surgery. Ectropion Ectropion, an e ve rsion or turning out of the eyelid margin , abnormally exposes the palpebral and bulbar conjunctivae and the cornea and interferes with normal drainage from the eye. Only the lower eyelid is involved. Both congenital and acquired types occur and are rare in horses. The condition mayor may not lead to corneal or conjunctival disease . Treatment may be conservative (that is , cleaning away of discharge and the use of ophthalmic antibiotic ointm e nts) or, if necessary, surgery. No specific procedure has
526
G.A.
MUNROE AND
K .C.
BARNED
Figure 4. Small, pigmented, nonhairy dermoid and corneal opacity in a Thoroughbred foal.
been reported for the foal, but it is possible to adapt one or more of the canine surgical methods. Dermoids Congenital dermoids are focal masses that resemble skin and can affect the eyelid margin, palpebral and bulbar conjunctivae, nictitating membrane, and, more commonly, the cornea (Fig. 4). They are described in more detail in the section on the cornea.
THE NASOLACRIMAL SYSTEM The nasolacrimal excretory system drains tears from the eye into the nose. It consists of two lacrimal punta, one on each eyelid margin, that connect together at a poorly formed nasolacrimal sac. The nasolacrimal duct passes from the sac downwards through the maxillary bone to emerge at the mucocutaneous junction on the nasal floor near the nostril. Congenital blockage of this system does occur at various points but less commonly than do the acquired forms (swelling of duct mucosa and/or inflammatory plugs). 10.16 In the foal, atresia or agenesis usually occurs at, or a few centimeters from, the nasal opening of the duct. The condition can be unilateral or bilateral, and the earliest sign of persistent epiphora or mucoid discharge may be apparent at any time from birth to 1 year of age. Later presenting signs may include conjunctivitis and blepharitis. The use of a fluorescein dye will help recognize patency of the nasolacrimal system if the dye appears at the nasal orifice. Examination on the floor of the nostril may reveal no orifice. Passage of a fine catheter or monofilament nylon from the upper punctum may help delineate the site of agenesis. Dacryocystorhinography can also be undertaken with several radiographic views of anesthetized foals in which 4 to 16 ml of a suitable positive contrast media has been injected via the upper lacrimal puncta. With distal atresia, surgery under general anesthesia can be performed either through the nostril or, if necessary, through a paramedian incision in the dorsal wall of the nostril. 9 . 10.16 A catheter is passed down the duct from
CONGENITAL OCULAR DISEASE IN THE FOAL
527
the upper punctum and an incision is made over the distal end or distended Ruid swelling. Apposition of the duct mucosa to the nasal mucosa is not usually possible, so an indwelling catheter should be positioned in the nasolacrimal apparatus using one of the various techniques available 19 and left for up to 3 or 4 weeks. Daily Rushing with antibiotic-corticosteroid preparation is ideal to prevent fibrosis and stenosis during healing. This may not be necessary, however. Complete agenesis of the nasolacrimal system may be corrected surgically by performing a conjunctivorhinostomy. 23
THE NICTITATING MEMBRANE This structure is part of the protective mechanism of the eye, and its gland helps to produce tears. Congenital abnormalities are rare. The membrane can be hypoplastic, usually as part of the microphthalmia complex of disorders. Congenital adhesions can form between the membrane and the bulbar conjunctiva, causing limitations of movement. These adhesions are thought to occur prenatally owing to failure of separation between the two structures or possible prenatal inRammation. 9 If these adhesions are discrete, it may be possible to separate them surgically.
THE CONJUNCTIVA Congenital anomalies of this part of the eye are rare in the horse and consist almost entirely of dermoids. Treatment is by excision before secondary eye disease occurs. Apposition of the wound edges to prevent excessive granulation tissue formation and adhesions is important.
THE CORNEA Congenital abnormalities are relatively rare 21 and must be distinguished from lesions acquired in early life, which are more common. 15 Microcornea Microcornea is usually present in the microphthalmia complex but has been noted as occurring in apparently normal-sized globes in foals and yearlings with visual impairment. 9 Corneal Melanosis The pigmentation is usually central in the cornea and involves the epithelium and superficial anterior stroma. It is present at birth or just afterwards and is usually nonprogressive. Neovascularization of the cornea may be present. Treatment by superficial keratectomy is often successful. The author (G.A.M.) has recently seen a case of melanosis in association with an atypical corneal dermoid and uveal strand in a newborn foal (see Fig. 3).
528
G.A.
MUNROE AND
K.C.
BARNETI
Persistent Pupillary Membranes These structures will be discussed more fully under the iris section. Persistent pupillary membranes (PPM) consist of mesodermal tissue that arises from the anterior iridal surface and on occasion attaches to the endothelial surface of the cornea, resulting in focal opacities in the cornea. Treatment is not indicated, and vision is usually unaffected. Dermoids Congenital dermoids, mentioned in earlier sections, most commonly affect the ventral and lateral aspects of the cornea and can be unilateral or bilateral. Extension onto adjacent areas from the original site (limbus) is common. Dermoids consist of pigmented, focal masses that frequently contain hair, sebaceous glands, and other dermal structures (see Fig. 4). Impairment of vision can occur, and irritation is common. Treatment consists of superficial keratectomy and local excision from the adjacent structures. The equine cornea has a tendency to scar; therefore, postoperative topical and/or subconjunctival corticosteroids are indicated after epithelialization of the corneal wound. After removing the dermoid from the adjacent structures, any defect in the conjunctiva should be sutured to prevent symblepharon formation . Iridocorneal Angle Anomalies Angle anomalies occasionally affect the lateral and medial aspects of the corneal limbus . 9 Slight opacities in this area, which appear as a grey line, represent the insertions of the pectinate ligaments into the cornea at the termination of Descemet's membrane. In a recently published survey, these lines were apparent around the entire limbal circumference in 17 foals and were less obvious when conjunctival pigment was absent. 1.5 These anomalies are non progressive and apparently do not predispose the horse to glaucoma. These might more appropriately be considered as normal variants rather than as anomalies. Congenital Corneal Opacities The final group of congenital corneal anomalies has been mentioned in recent publications. 9.28 These corneal opacities are defined as linear and band opacities and are due to a thin, dense Descemet's membrane that results in edema immediately anterior to the defective membrane. The anomaly is not associated with inflammatory changes and is not progressive.
THE IRIS AND CILIARY BODY In contrast to acquired diseases affecting this part of the eye, particularly inflammations, congenital anomalies are rare. 23 Although the majority are of little clinical importance, they nevertheless form an interesting and easily recognizable group of defects .
CONGENITAL O C ULAR DISEASE IN THE FOAL
529
Anomalies of Pigmentation These anomalies are observed most frequently in lighter-colored horses, especially white, cream , spotted, or palamino types. There is evidence that pigme ntation variations may be inherited in white horses. Heterochromia irides occur occasionally and may be present in one or both eyes. This is a relatively common observation in Welsh ponies. Part of the iris ("wall eye" or albinismus partialis) or all of the iris ("china eye") may be involved. Albinism , or "glass eye," refers to the condition in which the iris has no pigment and appears pink. Particular ophthalmic conditions associated with anomalies of iridal pigmentation, except for iris cysts, have not been reported. Anomalies of iris pigmentation frequently occur in combination with retinal and choroidal pigment anomalies and should be considered normal variants , rather than abnormalities, although they are frequently confused as such. Aniridia Aniridia (the partial or complete absence of the iris) is a rare, inherited condition and is often associated with other ocular defects . II Bilateral aniridia with associated secondary cataracts and inherited as an autosomal dominant trait has been reported in Belgium draft horses . 4 Clinical signs include photophobia (abnormally large circular pupils). The pupils were unresponsive to light, thereby allowing visualization of the lens margin . Varying degrees of lens opacity were noted to develop at different times after 2 months of age. Vision mayor may not be affectedY·11 Coloboma of the Iris This common structural defect occurs when part of the iris is lacking. In some species, it may be hereditary. Iris colobomata may produce variations in pupil size and shape. They may appear as single or multiple apertures in the width of the iris . The condition can be bilateral or unilateral and may be associated with other ocular defects . Uveal Cysts In the horse , uveal cysts can be congenital or associated with inflammation . Cysts of the iridal pigmented epithelium do occur more frequently in animals with heterochromic irides; there is a predilection for the supe rior basal portion of the iris. Cysts can also occur along the pupillary margins, including the corpora nigra, may often be smaller, and may detach to be free-floating. 23 They usually arise from the heavily pigmented posterior iris epithelium (Fig. 5). Both types can be unilateral or bilateral and are evidenced clinically as focal enlargements of the iris that may distort the pupillary aperture, especially after mydriasis. The cyst is typically thin-walled and transillumination is usually possible, thereby aiding differentiation from the more solid pigmented tumor (melanoma). Although rarely necessary, treatm ent consists of removal of a small portion of the anterior wall. Spontaneous rupture may occur with inte nsive mydriatic therapy.
530
G.A.
MUNROE AND
K.C.
BARNETI
Figure 5. Uveal cysts in a Thoroughbred foal. (Courtesy of Dr. D. Wilson. )
Persistent Pupillary Membrane Although infrequently mentioned in standard textbooks, a recent survey of 144 foals noted the presence of persistent pupillary membrane (PPM) tags arising from the iris collarette unilaterally or bilaterally in 47 foals aged 5 days to 17 weeks. IS Embryologically, the membrane is derived from the iris mesenchyme, constituting the anterior vascular sheath for the lens and forming a delicate membrane stretching across the pupil. 27 The membrane normally atrophies in early life, but where this is incomplete, remnants persist as strands attached to the lesser circle of the iris. Besides attaching to the iris, they may span the pupil, insert into the anterior lens capsule, which produces focal cataracts, orJorm congenital anterior synechiae which produce focal corneal opacities. PPMs are normally of no significance and cause no inconvenience or vision defect to the affected animal.
THE LENS The normal lens is biconvex (but with a different radius of curvature front and back), clear, colorless, and suspended behind the iris . It is bounded anteriorly by the aqueous humor and posteriorly by the vitreous. The lens reacts to insult by death of cells and lens fibers, as well as by disruption in maintenance of its water content. Failure of this system allows localized accumulations of water with resulting vacuolation and subsequent loss of transparency. This opacity is termed a "cataract." Cataracts Cataracts may be classified according to the anatomic position of the opacity within the lens (capsular, cortical, or nuclear), by degree of maturity, by age of the animal, and by possible cause. They can be unilateral or bilateral, primary or secondary to another eye disease, stationary or progressive , and are usually permanent. In a report of congenital ocular defects,
CONGENITAL O CU LAR DISEASE IN THE FOAL
531
cataracts comprised 35.29 per cent of all defects in horses. 21 They have been described as relative ly frequent in foals-that is , 12 cases out of 34 horses with congenital ocular defects . 23 However, some recent surveys have shown incidence to be much less: 3 out of 82 13 and 1 in 144 foals examined. 15 Congenital cataracts, by definition , are present at birth and still must be considered the most common congenital ocular defect in the foal. Usually both eyes are affected and the cataract may exist with other anomalies such as microphthalmia,29 persistent pupillary membranes 23 and aniridia. 4.11 The precise etiology of most congenital cataracts in the horse are unknown . Possible causes include heredity,4.26 trauma (prenatal and foaling), poor nutrition , in utero infections, radiation, and metabolic and toxic effects. 29 In recent years, a higher proportion of congenital cataracts have occurred in the Arabian horse, but proof of inheritance is lacking and the cataracts have not been consistent types (as is the usual occurrence with hereditary cataracts in the dog) , varying as they did from partial to total. Congenital cataracts may be subdivided into four types : mature complete cortical, nuclear, suture line , and those associated with persistent hyaloid vasculature. The latter two are of minor importance, for they are small and have minimal effect on vision. They are usually nonprogressive ; therefore, treatment is unnecessary. Cataract secondary to a persistent hyaloid artery is usually a small concave opacity in the posterior lens capsule, but occasionally a small portion of the lens cortex may also be involved. The congenital suture line cataract is a Y-shaped opacity that is apparent in the anterior and/or posterior lens suture. Three out of 82 foals examined by Koch l3 had this type of opacity, and two were siblings. The cause of such an opacity may be increased lens ground substance or reduced lens fiber length. Under magnification, it had a vacuolar appearance .9 Mature total congenital cataracts usually produce blindness in foals and are noticed soon after birth . However, the cataracts may not be discovered until weaning owing to the close relationship of foal and mother. As a result of its dense nature, the cataract prevents examination of the fundus. The entire lens is affected; thus , mydriasis does not improve the foal's vision . Foals with this type of cataract may be candidates for surgery. Congenital nuclear cataracts affect only the center of the lens. They may appear as focal opacities or as concentric, irregular rings, with the periphery of the lens remaining transparent. Because visualization of the unaffected le ns periphery increases as the pupil dilates , the visual impairment accompanying this condition may be less evident in dim light or with mydriasis . Progression of congenital nuclear cataract is unlikely; furthermore , some do regress with age. Lens Dislocation Dislocation or luxation of the lens occurs occasionally in the foal and can be associated with other congenital anomalies. Displacement may be anterior, posterior, or a subluxation (loss of some zonular attachments but lens is still within the patellar fossa) . Cataract formation and concomitant subluxation of the lens have been observed in the left eye of a foal affected with multiple congenital defects .6 More recently , a case of bilateral cataract formation and subluxation in a 6-month-old part Arab foal was reported by
532
G.A.
MUNROE AND
K.C.
BARNETT
Matthews and Hanscombe. 17 Removal of the luxated or subluxated lens is possible by intracapsular methods . Coloboma of the Lens Congenital absence of a segment of the lens periphery and its associated zonules has been seen in a Thoroughbred foal (K.C .B.). VITREOUS BODY The normal vitreous body consists of a clear gel that occupies the space between the lens and retina. The only congenital lesion recorded that affects this structure is persistence of the hyaloid vasculature. In a recent survey of 144 foals of mixed breeding, 20 were found to have persistent hyaloid structures. 15 The hyaloid artery is normally found in the fetus and for a period shortly after birth. Regression of the hyaloid vascular system usually takes place in the first 2 to 3 weeks of life. Examination of the young foal's eye will show the hyaloid system as faint branches just behind the posterior lens capsule, running together at the pole of the lens into a larger structure that passes through the vitreous to the optic disc. Various parts of this entire structure may persist. Distinguishing between hyaloid remnants and posterior capsular cataracts can be difficult. THE FUNDUS Variations in the normal equine ocular fundus are numerous and have been discussed earlier in this article . A knowledge of these normal variants is essential before any fundus can be evaluated. The incidence of congenital fundus lesions is low. Equine Night Blindness Equine night blindness is a congenital, apparently nonprogressive disease that has been reported increasingly in the Appaloosa breed in the United States. It occurs to a lesser extent in other breeds . 12.30 The condition has been seen in siblings and is believed to be recessively inherited. The ocular fundi of these animals are generally normal both ophthalmoscopically and histologically. 23 However, vision deficit may cover a broad range of illumination including, in some horses , significant day blindness. Some animals may have smaller than normal globe size, others display dorsal strabismus and nystagmus. The clinical history may relate to multiple episodes of injury occurring in familiar surroundings at night. The diagnosis can be tentatively suggested from the history and ocular examination but can only be confirmed by electroretinography. The defect may be a neuroretinal transmission problem . No treatment is available. Optic Nerve Hypoplasia Hypoplasia can occur unilaterally or bilaterally in foals with otherwise normal eyes in combination with other ocular anomalies, including microphthalmia, cataracts, and re tinal detachment. (i .H. 12. 2.3 Affected foals present with impaired vision or blindness and have slow to nonexistent pupillary reflexes.
CONGENITAL OCULAR DISEASE IN THE FOAL
533
Figure 6. Multiple re tinal hemorrhages in a Thoroughbred foal with maladjustment syndrom e. (See also color plate IB )
The optic disc is smaller and paler than normal, abnormal in shape, with retinal vessels decreased in number and size. Animals may collide with obstacles in a maze test. Optic nerve hypoplasia can be difficult to differentiate clinically from acquired optic atrophy, which can sometimes occur early in life. However, histologically the difference is obvious: hypoplasia is characterized by few ganglion cells, an abnormally thin retinal fiber layer, and a small optic disc. No treatment is available. Retinal Hemorrhage and Papilledema Multiple hemorrhages in the retina and sclera, and papilledema, together with variable pupil size, have been described in association with the convulsive foal or neonatal maladjustment syndrome (Fig. 6).1 .5.24 A recent survey of 180 Thoroughbred foals (normal and abnormal) examined within 3 days of birth showed retinal hemorrhage to be present in 8 per cent and papilledema in 5.5 per cent. 5 It has been suggested that retinal hemorrhage is of little clinical significance in newborn foals, whereas papilledema and changes in optic disc coloration seem to be more closely related to cerebral edema, the underlying factor in the maladjustment syndrome. It is our experience that a few small retinal hemorrhages in Thoroughbred foals up to a few days of age are of no significance and can be found in apparently normal animals with no evidence of the maladjustment syndrome (Fig. 7). Miscellaneous Congenital Lesions of Fundus Congenital chorioretinitis that leads to pigmentary changes throughout the fundus has been reported. 23 The inactive lesions are seen in foals born to dams that have been exposed to respiratory disease in late gestation. Congenital peripheral retinal cysts are rare. They are a component of some multiple intraocular anomalies that may result from defects in the ora serrata. 23 Congenital retinal dysplasia and detachment occurs rarely in foals . The detached retina that results in blindness can be seen in the vitreous as numerous folds in front of the fundus. It may occur with other congenital ocular anomalies and has been seen in the Hackney pony in two successive foals born to the same parents (K.C.B.) (Fig. 8).
534
G.A.
Figure 7.
MUNROE AND
K.C.
BARNETT
A. Resorbing retinal hemorrhages in a normal I-day-old Thoroughbred foal.
B, Streak hemorrhage across disc in a normal 5-day-old Thoroughbred foal with resorbing retinal hemorrhage. (See also color plates Ie and ID).
CONGENITAL OCULAR DISEASE IN TIlE FOAL
535
Figure 8. A and B, Retinal detachment folds in both eyes of a 3-week-old Hackney pony foal. (See also color plates IE and IF).
536
G.A.
MUNROE AND
K.C .
BARNEIT
REFERENCES 1. Barnett , K.C.: The eye of the newborn foa!' J. Reprod. Ferti!. Supp!., 23:701-702, 1975. 2. Duke-Elde r, S.: Normal and abnormal development. In Duke-Elder System of Ophthalmology. Vol III. Part 2. St. Louis , Missouri , C. V. Mosby, Co., 1963. 3. Dziezyc, J. , Kern, T.J. , and Wolf, E.D.: Microphthalmia in a foal. III Equine Ophthalmology, [Supp!.] London , England, British Veterinary Association , 1983. 4. Eriksson, K.: Hereditary aniridia with secondary cataract in horse. Nord. Vet. Med. , 7:773-793, 1955. 5. Frauenfelder, H. C.: A survey of eyes in newborn Thoroughbred foals with special reference to papilloedema and retinal hemorrhage . Personal communication, 1983. 6. Garner, A. , and Griffiths, P.: Bilateral ocular defects in a foal. Br. J. Ophthalmo!., 53:513- 517, 1969. 7. Gelatt, K. N. , Leipold, H. W. , and Coffman , J. R: Bilateral optic nerve hypoplasia in a colt. J. Am. Vet. Med. Assoc. , 155:627- 631 , 1969. 8. Gelatt, K.N. , and McClure, J.R.: Congenital strabismus and its correction in two Appaloosa horses. J. Equine Med. Surg. , 3(5):240-244, 1979. 9. Gelatt, K.N.: The eye. In Equine Medicine and Surgery. Edition 3. Santa Barbara, California, American Veterinary Publications, Inc. , 1982. 10. Hjorth, P.: Aplasia of the nasolacrimal duct in the horse . Nord. Vet. Med., 23:216, 1971. 11. Joyce, J. R , Aniridia in a quarter horse. In Equine Veterinary Journal supplement of Equine Ophthalmology. London, England, British Veterinary Association, 1983. 12. Kern, T.J.: Ocular fundus and central nervous system causes of blindness. In Robinson , N.E. (ed. ): Current Therapy in Equine Medicine. Edition 1. Philadelphia, W. B. Saunders Co. , 1983. 13. Koch , S.A., Reynolds Cowes, R. , Schmidt, G.R, et al.: Ocular disease in the newborn horse : A preliminary report. J. Equine Surg. , 2(4):167-170, 1978. 14. Kunze, D.J.: Cataracts. In Robinson, N.E. (ed. ): Current Therapy in Equine Medicine. Edition 1. Philadelphia, W. B. Saunders Co. , 1983. 15. Latimer, C.A., Wyman , N., and Hamilton, J.: Ophthalmic survey of the neonatal horse. In Equine Veterinary Journal supplement of Equine Ophthalmology. London , England, British Veterinary Association , 1983. 16. Lundvall, R. L. , and Carter, J. D.: Atresia of the nasolacrimal meatus in the horse. J. Am. Vet. Med. Assoc., 1971. 17. Matthews, A.G. , and Hanscombe, M.C.: Bilateral cataract formation and subluxation of the lens in a foal-a case report. In Equine Veterinary Journal supplement of Equine Ophthalmology. London , England, British Veterinary Association , 1983. 18. Munroe, G.A.: Unpublished data as part of prospective fellowship thesis for the Royal College of Veterinary Surgeons, 1983. 19. Murphy, J. M.: Administration of ocular therapy. In Robinson , N.E. (ed. ): Current Therapy in Equine Medicine. Edition 1. Philadelphia, W.B. Saunders Co. , 1982. 20. Peiffer, R L. , et al.: Correction of congenital entropion in a foa!' Vet. Med. Small Anim. Clin. , 72:1219, 1977. 21. Priester, W.A.: Congenital ocular defects in cattle, horses , cat and dogs. J. Am . Vet. Med. Assoc., 160:1504-1511 , 1972. 22. Rubin , L. F. : Atlas of Vete rinary Ophthalmoscopy. Philadelphia, Lea & Febige r, 1974. 23. Riis, R.C.: Equine ophthalmology. In Gelatt, K.N. (ed. ): Veterinary Ophthalmology. Philadelphia, Lea & Febige r, 1981. 24. Rossdale, P. D.: Clinical studies on four newborn thoroughbred foals suffering convulsions with special reference to blood gas chemistry and pulmonary ventilation. Res. Vet. Sci., 10:279, 1969. 25. Serk, G .W. : Ocular discharge in young horses. In Robinson , N.E. (ed. ): Curre nt Therapy in Equine Medicin e . Edition 1. Philadelphia, W. B. Saunders Co. , 1983. 26. Slatter, D . H . : Fundame ntals of Veterinary Ophthalmology. Philadelphia, W. B. Saunders Co. , 1981. 27. Startup, F.G.C. : Diseases of the canine eye. London , England, Balliere, Tindall , 1964. 28. Walde , I.: Band opacities. In Equine Veterinary Journal supplement of Equine Ophthalmology. London , England, British Veterinary Association , 1983. 29. Whitley, RD. , Moore, C . P. , and Slone, D.E.: Cataract surgery in the horse. In Equine Veterinary Journal supplement of Equine Ophthalmology. London , England, British Veterinary Association , 1983.
CONGENITAL OCULAR DISEASE IN THE FOAL
537
20. Witzel, D.A. , Joyce , J.R. , and Smith, J. L.: Electroretinography of congenital night blindness in an Appaloosa filly. J. Equine Vet. Med. Surg. , 1:226-269, 1977. The Tortington Centre Arundel West Sussex, England BN18 OBG
PLATE I
A
B
c
o
E
F
A, Normal fundus in a 2-day-old normal Thoroughbred foal (see p. 521). B , Multiple retinal hemorrhages in a Thoroughbred foal with maladjustment syndrome (see p. 533). C, Resorbing retinal hemorrhages in a normal I-day-old Thoroughbred foal (see p. 534). D, Streak hemorrhages across disc in a normal 5-day-old Thoroughbred foal with resorbing retinal hemorrhage (see p. 534). E and F , Retinal detachment folds in both eyes of a 3-week-old Hackney pony foal (see p. 535).
538
PLATE II
A
B
c
D
E
F
A, Normal equine fundus (see p. 542). B, Variation of normal equine optic nerve (see p. 542). C, Subalbinotic equine fundus (see p. 543). D, Absence of tapetal area, exposing vortex veins of normal choroidal circulation (see p. 543). E, Peripapillary areas of focal retinal and choroidal atrophy in non tapetal portion of retina (see p. 544). F, Fundus of an older horse (over 20 years old) with proliferative optic neuropathy (see p. 549).
539
Congenital Ocular Disease in the Foal
G.A. Munroe, B.V.Sc. (Hons), M.R.C.V.S.,* and K.C. Barnett, M.A., Ph.D. , B.Sc., F.R.C.V.S . , D.V. Ophtholmol.t
Congenital ocular abnormalities in the foal, although not as common in the dog and cat, nevertheless form an interesting and important group of equine eye conditions. Within the last 10 years, several surveys of the eye of the foal have been published in an attempt to establish its normal appearance and possible variations and to identify and record the incidence of various abnormalities. 1.5. 13. 15
THE NORMAL EYE OF THE FOAL The eyes of the foal may be examined for a purchaser or insurance company, as part of a general stud preventative medicine regimen or for a specific eye problem. On either of these occasions, it is essential for the veterinarian to possess a knowledge of the normal foal eye and its variations , as well as of possible disease entities . The early recognition and diagnosis of congenital eye disease in the foal is important to prevent unnecessary suffe ring by the foal and, in some conditions, to expedite treatment. The owner is most likely to be concerned with prognosis for both normal performance and breeding; therefore , the possible progression or inheritance of a given problem is of vital importance. The following article has been written in an attempt to bring the latest information on congenital eye disease of th e foal to the veterinary practitioner so that some of these problems can be bette r resolved.
THE EYE OF THE NEONATE The ophthalmoscopic appearance of the normal eye of the newborn foal has been little studied until rece ntly, when several surveys in the Thor' Private Practitioner, The Tortington Centre , Arundel, West Sussex, England tU ni! of Comparative Ophthalmology, The Animal Healt h Trust , Lanwades Park, Suffolk, England
Ve terinary Clinics of North America: Large Animal Practice-Vol. 6, No.3, November 1984
519
520
G.A.
MUNROE AND
K.C.
BARNETI
oughbred,I.5.13.18 and in thoroughbreds , standardbreds, and saddlebreds l5 have been undertaken. Reflexes The eye of the foal is open at birth. Response to a menacing gesture is present in all foals over the age of 5 days.15 The direct and consensual pupillary light reflexes are absent at birth and are sluggish and incomplete for at least 48 hours. During this time, the pupil is semidilated and Gircular. The normal adult responses of the pupil are generally present in 5-day-old foals . 15 Eyelids Vibrissae are present below the eye in most foals and above the eye to a lesser extent. Cilia are usually long on the upper lid. The edges of the eyelids rest in close proximity to the cornea. There should be no ocular discharge , alopecia, or depigmentation of the lids . The Nictitating Membrane The leading margin of this structure is usually pigmented9 . 15 and lies immediately above the distal extension of the nictitating cartilage. Lack of margin pigmentation may be associated with absence of conjunctival and limbal pigment. The caruncle is located at the medial canthus at the base of the membrane. Cornea The normal cornea of the foal is clear, oval, and broader nasally. Corneal thickness is variable, being thinnest at the center. The scleral shelf or overhang is more prominent dorsally and ventrally, which makes visualization of the iridocorneal angle more difficult., Conjunctiva This structure is the same as in the adult; irregular pigmentation is usually concentrated at the limbus. Aqueous Humor Normally, the aqueous humor is optically clear. Iris In almost all foals, the iris is dark brown and has little association with coat color except in heterochromia irides. The brown color may be homogenous or mottled with tan or grey toward the periphery. Corpora nigra (granula iridica) are present in all foals dorsally, but their occurrence ventrally is more sporadic and less striking. 13.15.18 The pupils of the newborn foal that is less than 24-hours old should be equal, large, and circular (in a position near maximal dilation. )1.5.9. 18The normal adult resting position and oval shape of the pupil are generally present in 3-day-old foals . u5. ls The pupil is wider nasally. The Lens Two structures are visible in the lens of the foal that are less commonly seen in the adult horse. In the majority of foals , the lens suture pattern is
CONGENITAL O CU LAR DISEASE IN THE FOAL
521
Figure 1. Normal fundus in a 2-dayold normal Thoroughbred foal. (See also color plate lA .)
apparent with slit-lamp biomicroscopy. Marked variations can occur among foals and between the two eyes of the foal. Generally, the anterior structure has a Y configuration, and the posterior suture varies from an inverted Y to a sawhorse or stellate with feathering. 14.15 In the majority of Thoroughbred foals, the anterior suture is not visualized. 14.15 A structure present in the vitreous body and attached to the posterior lens capsule is the hyaloid system. The hyaloid artery is present in all foals under the age of 24 hours and can be recognized as a fine, dark, curving line transversing the vitreous from the center of the optic disc to the posterior aspect of the lens. Occasionally, it may contain blood or appear black, although even as early as 2 hours after birth no blood or pulsations may be observed within the artery. 5 Remnants of the artery are also easily observed on the posterior lens capsule during the first 24 hours but disappear and are less prominent in older foals. The Fundus The ocular fundus may be divided ophthalmoscopically into the retinal blood vessels, optic nerve head or disc, tapetal fundus , and nontapetal fundus (Fig. 1). Retinal Vasculature. Thirty to 60 small retinal vessels radiate from the optic nerve in all directions, with fewer emerging in the mid ventral region. 9 . 15,23 Vessels traverse the retina 1 to 2 disc diameters in approximately straight lines although they can be tortuous, particularly on the surface of the optic disc (see Fig. 1). Arterioles and venules cannot be differentiated, Optic Disc. The optic nerve head is situated in the non tapetal fundus and is usually oval, although it may possibly be round, nasally blunted, and football-shaped . 15 Optic disc color is consistent between the two eyes and changes with age , The color within the first 24 hours can vary between pale pink and deep red (see Fig, 1), As the foal ages, the disc becomes lighter (pale pink to salmon pink) with a darker border. l,5 A white rim may be present along the midventral aspect of the disc. 15 The surface of the disc may have fine irregularities that, in later life, take on a more fibrous appearance. 23 The Tapetal Fundus. The tapetal color variation is considerable, but
522
G.A.
MUNROE AND
K.C.
BARNETI
the predominant color is green. Variations include primary yellow, nasal yellow with temporal green, green with a yellow periphery, yellow-green, and yellow with green islands. Foals with cream and light-colored hair coats may have more yellow in the tapetal fundus; whereas chestnut or bay horses can be yellow-green, and black or grey animals can have a slightly more bluish coloration. In saddlebreds, the tapetal coloration varies from one area to another with a relatively well-defined junction between areas . 15 Pigment mottling can occur dorsally and nasally. Penetration of the tapetum by small choroidal vessels appears as brown dots (the stars of Winslow) distributed throughout the tapetal fundus . Choroidal vessels , represented by red or blue streaks extending dorsally to the disc, can be seen particularly well when there is peripapillary hypopigmentation , a thin tapetum, and lack of choroidal or retinal pigment. Partial to total albinism may occur in both the tapetal and non tapetal fundus . When tapetal fundus is involved, light yellow is often observed with red stars of Winslow. In extreme cases, no tapetum is present and the choroidal vessels can be seen against a pale cream background (sclera). The junction between the tapetal and nontapetal areas is usually prominent in foals and may be "ragged" in appearance. The tapetal fundus can occasionally extend ventrally towards the dorsal aspect of the optic nerve head and, in this area, may have a different coloration from the majority of the tapetum. The Nontapetal Fundus. In the majority of foals, the non tapetal fundus is a dark brown or chocolate color. Light grey streaks that curve nasally and temporally away from the central disc margin have been reported recentlyl5 and may reflect the course of retinal vessels . Albinism and subalbinism can involve the nontapetal fundus , which can lead to a diffuse brown-red appearance in mild cases. With greater degrees of albinism the underlying choroidal vessels become visible and, in extreme cases, may be observed lying in front of the sclera. The area of tapetum nigrum immediately above the optic disc, which always lies within the non tapetal fundus and below the tapetum lucidum, is commonly less densely pigmented; parts of choroidal vessels often may be apparent or the area may appear more red than the rest of the nontapetal fundus. The disc may also have irregular, light grey, translucent zones irradiating from one or both sides. These represent either increased retinal thickness 9 or an increased amount of myelin . I . 9 "Islands" or "spots" may be seen in the non tapetal fundus just ventral to its junction with the tapetal fundus. 13 These zones , which represent areas of reduced pigmentation, appear to be nonprogressive, white , variable in size, and histologically show no evidence of inflammatory changes.9. 13.23 It has been suggested that these foci could be the result of focal inflammatory lesions. 13.22 If tapetal structure is present in these areas, they are known as tapetal islands. Koch has described them as atypical colobomas or focal areas of pigment dystrophy. 13 THE ABNORMAL EYE OF THE FOAL The Globe and Orbit Conditions of the globe and orbit can be divided into those that produce either exophthalmos (prominence of the globe) or enophthalmos (eye is
CONGENITAL O CU LAR DISEASE IN THE FOAL
523
Figure 2. Two microphthalmic eyes from a Thoroughbred foal showing diffe ring degrees of microphthalmos.
recessed). In congenital eye disease in the foal , only the latter are mentioned in the literature . Microphthalmos Microphthalmos may be unilateral or bilateral; it has been described in many species and is encountered not infrequently in foals . 21 It can be an isolated finding or, more frequently, can be associated with other congenital abnormalities-particularly cataracts . The size of the globe varies considerably from a near normal structure with normal function to a microscopic structure with only pigmented remnants (Fig. 2). 3.6 True enophthalmos is extremely rare . 23 Microphthalmos can be divided into three categories: 2 1. Pure microphthalmos (nanophthalmos), in which the eye is small but otherwise normal. 2. Colobomatous microphthalmos , in which the defect is associated with failure of involution of the optic vesicle or closure of the embryonic cleft. 3. Complicated microphthalmos, in which anomalous development occurs unrelated to fissure closure. The condition is usually recognized in newborn foals as a smaller than normal palpebral fissure that often has a prominent nictitating membrane. The cornea and globe are sometimes not visible or may be abnormally pigmented, and the animal may be blind or partially sighted in the affected eye(s). Treatment is not possible; in severe, bilateral cases, euthanasia is the only course of action. In unilateral cases, however, the animal may manage quite well and adapt as it ages. There are many examples of Thoroughbreds winning races with unilateral microphthalmia. If the affected eye is very small, drainage from the eye may be inadequate, which will lead to ocular discharge and the necessity for enucleation. Many cases are sporadic and idiopathic, but in other cases, the cause may be toxic, mechanical , infectious, or even nutritional. Although all breeds are affected, some breeds appear to have a higher incidence (the Thor-
524
G.A.
MUNROE AND
K.C .
BARNEIT
Figure 3. Entropion oflower eyelid in a SkewbaJd foal. (Courtesy of Dr. D. Wilson. )
oughbred, for example). However, the role of inheritance has not been proven in the horse 3 as it has in man and the dog. Strabismus Strabismus is rarely reported in the horse. In a survey conducted on horses and mules in 1940, it was only detected in males at a frequency of 0.5 per cent. Nearly all the mules exhibited nonsymmetric convergent strabismus (esotropia). Congenital strabismus occurs in the horse and is predominantly of the convergent type. 9 Two Appaloosa yearlings with bilateral hypertropia (vertical strabismus) of apparent congenital origin have been reported. 8 Affected horses deviated the head as if to compensate for the abnormal visual axis, stumbled, and had nervous temperaments. In two Appaloosas with hypertropia, surgical correction was performed by recession of the dorsal rectus muscle and resection of the ventral rectus muscle. Surgery resulted in rotation of the ocular axis to a more normal plane; vision and temperament were improved.
THE EYELIDS The eyelids of the horse are important for protection of the eye, tear film distribution , and cosmetic appearance of the head. Lesions of the eyelids, including congenital disease, are common in the horse. The eyelids of the foal may be affected at birth as part of the microphthalmia complex. Other conditions affecting the eyelids of the foal are entropion, ectropion, and dermoids. Entropion Entropion is an inversion of the eyelid margin and eyelashes (Fig. 3). The problem may involve one or both eyes, and the lower lid is much more
CO NGENITAL O C ULAH DISEASE IN TilE FOAL
525
commonly affected. The associated irritation may cause lacrimation, conjunctivitis, ble pharospasm, and corneal disease including keratitis, ulceration , and perforation. 2o The condition may be seen in weak, premature, or systemically ill foals as a secondary problem due to enophthalmos. This sinking of the globe into the orbit forms part of the clinical picture of dehydration and anorexia. Entropion also occurs in normal healthy foals as a primary and congenital lesion occurring in the first few days of life. Whether the entropion is primary or secondary, it is necessary at an early stage to determine if the condition can be corrected medically or surgically. In some foals , the condition can be managed for a few weeks with topical antibiotic ointments, allowing time for the rapid growth processes to gradually correct the eyelid defect. Occasionally, it is possible to correct the entropion in the first 12- to 24-hour period by gently everting the eyelid several tim es a day together with a regimen of protective antibiotic ophthalmic ointment three times a day. In lids that do not respond to manipulation, correction may be attained by infiltrating procaine penicillin under th e skin of the affected lid. 25 With the foal restrained in lateral recumbency and/or sedation, approximately 0.5 to 1 ml of procaine penicillin is injected under the skin of the lower lid. The resultant bleb is milked toward the entire length of the eyelid margin causing the conjunctiva to swell and mechanical eversion of the lid. The swelling decreases over a 24to 36-hour period. Repeated procedures are usually unnecessary. A local tissue reaction may occur later which will further evert the inturned lid. A simple technique of eversion involves placement of several temporary horizontal mattress re te ntion sutures through the skin of the affected lid. 23 For nonresponsive congenital and acquired entropion, surgical intervention may b e required. Surgery is best performed under general anesthesia and with the cornea protected with ophthalmic ointment and the skin clipped and surgically scrubbed. The area and extent of excision should be ascertained carefully prior to anesthesia. Overcorrection must be avoided . Many methods are available for correction, but the Hotz-Celsus procedure provides a satisfactory result. 9 .20 .23 This technique involves the removal of a section of skin with the initial incision parallel to and within 3 to 5 mm of the affected lid margin. Some surgeons also prefer to remove a strip of orbicularis oculi muscle. Hemorrhage may be controlled by hemostats or electrocautery (not ligation), and skin (and muscle , if resected) may be opposed with simple interrupted nonabsorbable sutures . Ophthalmic antibiotic ointment is applied postope ratively, and assessme nt of the surgery is made some 6 to 8 weeks after surgery. Ectropion Ectropion, an e ve rsion or turning out of the eyelid margin , abnormally exposes the palpebral and bulbar conjunctivae and the cornea and interferes with normal drainage from the eye. Only the lower eyelid is involved. Both congenital and acquired types occur and are rare in horses. The condition mayor may not lead to corneal or conjunctival disease . Treatment may be conservative (that is , cleaning away of discharge and the use of ophthalmic antibiotic ointm e nts) or, if necessary, surgery. No specific procedure has
526
G.A.
MUNROE AND
K .C.
BARNED
Figure 4. Small, pigmented, nonhairy dermoid and corneal opacity in a Thoroughbred foal.
been reported for the foal, but it is possible to adapt one or more of the canine surgical methods. Dermoids Congenital dermoids are focal masses that resemble skin and can affect the eyelid margin, palpebral and bulbar conjunctivae, nictitating membrane, and, more commonly, the cornea (Fig. 4). They are described in more detail in the section on the cornea.
THE NASOLACRIMAL SYSTEM The nasolacrimal excretory system drains tears from the eye into the nose. It consists of two lacrimal punta, one on each eyelid margin, that connect together at a poorly formed nasolacrimal sac. The nasolacrimal duct passes from the sac downwards through the maxillary bone to emerge at the mucocutaneous junction on the nasal floor near the nostril. Congenital blockage of this system does occur at various points but less commonly than do the acquired forms (swelling of duct mucosa and/or inflammatory plugs). 10.16 In the foal, atresia or agenesis usually occurs at, or a few centimeters from, the nasal opening of the duct. The condition can be unilateral or bilateral, and the earliest sign of persistent epiphora or mucoid discharge may be apparent at any time from birth to 1 year of age. Later presenting signs may include conjunctivitis and blepharitis. The use of a fluorescein dye will help recognize patency of the nasolacrimal system if the dye appears at the nasal orifice. Examination on the floor of the nostril may reveal no orifice. Passage of a fine catheter or monofilament nylon from the upper punctum may help delineate the site of agenesis. Dacryocystorhinography can also be undertaken with several radiographic views of anesthetized foals in which 4 to 16 ml of a suitable positive contrast media has been injected via the upper lacrimal puncta. With distal atresia, surgery under general anesthesia can be performed either through the nostril or, if necessary, through a paramedian incision in the dorsal wall of the nostril. 9 . 10.16 A catheter is passed down the duct from
CONGENITAL OCULAR DISEASE IN THE FOAL
527
the upper punctum and an incision is made over the distal end or distended Ruid swelling. Apposition of the duct mucosa to the nasal mucosa is not usually possible, so an indwelling catheter should be positioned in the nasolacrimal apparatus using one of the various techniques available 19 and left for up to 3 or 4 weeks. Daily Rushing with antibiotic-corticosteroid preparation is ideal to prevent fibrosis and stenosis during healing. This may not be necessary, however. Complete agenesis of the nasolacrimal system may be corrected surgically by performing a conjunctivorhinostomy. 23
THE NICTITATING MEMBRANE This structure is part of the protective mechanism of the eye, and its gland helps to produce tears. Congenital abnormalities are rare. The membrane can be hypoplastic, usually as part of the microphthalmia complex of disorders. Congenital adhesions can form between the membrane and the bulbar conjunctiva, causing limitations of movement. These adhesions are thought to occur prenatally owing to failure of separation between the two structures or possible prenatal inRammation. 9 If these adhesions are discrete, it may be possible to separate them surgically.
THE CONJUNCTIVA Congenital anomalies of this part of the eye are rare in the horse and consist almost entirely of dermoids. Treatment is by excision before secondary eye disease occurs. Apposition of the wound edges to prevent excessive granulation tissue formation and adhesions is important.
THE CORNEA Congenital abnormalities are relatively rare 21 and must be distinguished from lesions acquired in early life, which are more common. 15 Microcornea Microcornea is usually present in the microphthalmia complex but has been noted as occurring in apparently normal-sized globes in foals and yearlings with visual impairment. 9 Corneal Melanosis The pigmentation is usually central in the cornea and involves the epithelium and superficial anterior stroma. It is present at birth or just afterwards and is usually nonprogressive. Neovascularization of the cornea may be present. Treatment by superficial keratectomy is often successful. The author (G.A.M.) has recently seen a case of melanosis in association with an atypical corneal dermoid and uveal strand in a newborn foal (see Fig. 3).
528
G.A.
MUNROE AND
K.C.
BARNETI
Persistent Pupillary Membranes These structures will be discussed more fully under the iris section. Persistent pupillary membranes (PPM) consist of mesodermal tissue that arises from the anterior iridal surface and on occasion attaches to the endothelial surface of the cornea, resulting in focal opacities in the cornea. Treatment is not indicated, and vision is usually unaffected. Dermoids Congenital dermoids, mentioned in earlier sections, most commonly affect the ventral and lateral aspects of the cornea and can be unilateral or bilateral. Extension onto adjacent areas from the original site (limbus) is common. Dermoids consist of pigmented, focal masses that frequently contain hair, sebaceous glands, and other dermal structures (see Fig. 4). Impairment of vision can occur, and irritation is common. Treatment consists of superficial keratectomy and local excision from the adjacent structures. The equine cornea has a tendency to scar; therefore, postoperative topical and/or subconjunctival corticosteroids are indicated after epithelialization of the corneal wound. After removing the dermoid from the adjacent structures, any defect in the conjunctiva should be sutured to prevent symblepharon formation . Iridocorneal Angle Anomalies Angle anomalies occasionally affect the lateral and medial aspects of the corneal limbus . 9 Slight opacities in this area, which appear as a grey line, represent the insertions of the pectinate ligaments into the cornea at the termination of Descemet's membrane. In a recently published survey, these lines were apparent around the entire limbal circumference in 17 foals and were less obvious when conjunctival pigment was absent. 1.5 These anomalies are non progressive and apparently do not predispose the horse to glaucoma. These might more appropriately be considered as normal variants rather than as anomalies. Congenital Corneal Opacities The final group of congenital corneal anomalies has been mentioned in recent publications. 9.28 These corneal opacities are defined as linear and band opacities and are due to a thin, dense Descemet's membrane that results in edema immediately anterior to the defective membrane. The anomaly is not associated with inflammatory changes and is not progressive.
THE IRIS AND CILIARY BODY In contrast to acquired diseases affecting this part of the eye, particularly inflammations, congenital anomalies are rare. 23 Although the majority are of little clinical importance, they nevertheless form an interesting and easily recognizable group of defects .
CONGENITAL O C ULAR DISEASE IN THE FOAL
529
Anomalies of Pigmentation These anomalies are observed most frequently in lighter-colored horses, especially white, cream , spotted, or palamino types. There is evidence that pigme ntation variations may be inherited in white horses. Heterochromia irides occur occasionally and may be present in one or both eyes. This is a relatively common observation in Welsh ponies. Part of the iris ("wall eye" or albinismus partialis) or all of the iris ("china eye") may be involved. Albinism , or "glass eye," refers to the condition in which the iris has no pigment and appears pink. Particular ophthalmic conditions associated with anomalies of iridal pigmentation, except for iris cysts, have not been reported. Anomalies of iris pigmentation frequently occur in combination with retinal and choroidal pigment anomalies and should be considered normal variants , rather than abnormalities, although they are frequently confused as such. Aniridia Aniridia (the partial or complete absence of the iris) is a rare, inherited condition and is often associated with other ocular defects . II Bilateral aniridia with associated secondary cataracts and inherited as an autosomal dominant trait has been reported in Belgium draft horses . 4 Clinical signs include photophobia (abnormally large circular pupils). The pupils were unresponsive to light, thereby allowing visualization of the lens margin . Varying degrees of lens opacity were noted to develop at different times after 2 months of age. Vision mayor may not be affectedY·11 Coloboma of the Iris This common structural defect occurs when part of the iris is lacking. In some species, it may be hereditary. Iris colobomata may produce variations in pupil size and shape. They may appear as single or multiple apertures in the width of the iris . The condition can be bilateral or unilateral and may be associated with other ocular defects . Uveal Cysts In the horse , uveal cysts can be congenital or associated with inflammation . Cysts of the iridal pigmented epithelium do occur more frequently in animals with heterochromic irides; there is a predilection for the supe rior basal portion of the iris. Cysts can also occur along the pupillary margins, including the corpora nigra, may often be smaller, and may detach to be free-floating. 23 They usually arise from the heavily pigmented posterior iris epithelium (Fig. 5). Both types can be unilateral or bilateral and are evidenced clinically as focal enlargements of the iris that may distort the pupillary aperture, especially after mydriasis. The cyst is typically thin-walled and transillumination is usually possible, thereby aiding differentiation from the more solid pigmented tumor (melanoma). Although rarely necessary, treatm ent consists of removal of a small portion of the anterior wall. Spontaneous rupture may occur with inte nsive mydriatic therapy.
530
G.A.
MUNROE AND
K.C.
BARNETI
Figure 5. Uveal cysts in a Thoroughbred foal. (Courtesy of Dr. D. Wilson. )
Persistent Pupillary Membrane Although infrequently mentioned in standard textbooks, a recent survey of 144 foals noted the presence of persistent pupillary membrane (PPM) tags arising from the iris collarette unilaterally or bilaterally in 47 foals aged 5 days to 17 weeks. IS Embryologically, the membrane is derived from the iris mesenchyme, constituting the anterior vascular sheath for the lens and forming a delicate membrane stretching across the pupil. 27 The membrane normally atrophies in early life, but where this is incomplete, remnants persist as strands attached to the lesser circle of the iris. Besides attaching to the iris, they may span the pupil, insert into the anterior lens capsule, which produces focal cataracts, orJorm congenital anterior synechiae which produce focal corneal opacities. PPMs are normally of no significance and cause no inconvenience or vision defect to the affected animal.
THE LENS The normal lens is biconvex (but with a different radius of curvature front and back), clear, colorless, and suspended behind the iris . It is bounded anteriorly by the aqueous humor and posteriorly by the vitreous. The lens reacts to insult by death of cells and lens fibers, as well as by disruption in maintenance of its water content. Failure of this system allows localized accumulations of water with resulting vacuolation and subsequent loss of transparency. This opacity is termed a "cataract." Cataracts Cataracts may be classified according to the anatomic position of the opacity within the lens (capsular, cortical, or nuclear), by degree of maturity, by age of the animal, and by possible cause. They can be unilateral or bilateral, primary or secondary to another eye disease, stationary or progressive , and are usually permanent. In a report of congenital ocular defects,
CONGENITAL O CU LAR DISEASE IN THE FOAL
531
cataracts comprised 35.29 per cent of all defects in horses. 21 They have been described as relative ly frequent in foals-that is , 12 cases out of 34 horses with congenital ocular defects . 23 However, some recent surveys have shown incidence to be much less: 3 out of 82 13 and 1 in 144 foals examined. 15 Congenital cataracts, by definition , are present at birth and still must be considered the most common congenital ocular defect in the foal. Usually both eyes are affected and the cataract may exist with other anomalies such as microphthalmia,29 persistent pupillary membranes 23 and aniridia. 4.11 The precise etiology of most congenital cataracts in the horse are unknown . Possible causes include heredity,4.26 trauma (prenatal and foaling), poor nutrition , in utero infections, radiation, and metabolic and toxic effects. 29 In recent years, a higher proportion of congenital cataracts have occurred in the Arabian horse, but proof of inheritance is lacking and the cataracts have not been consistent types (as is the usual occurrence with hereditary cataracts in the dog) , varying as they did from partial to total. Congenital cataracts may be subdivided into four types : mature complete cortical, nuclear, suture line , and those associated with persistent hyaloid vasculature. The latter two are of minor importance, for they are small and have minimal effect on vision. They are usually nonprogressive ; therefore, treatment is unnecessary. Cataract secondary to a persistent hyaloid artery is usually a small concave opacity in the posterior lens capsule, but occasionally a small portion of the lens cortex may also be involved. The congenital suture line cataract is a Y-shaped opacity that is apparent in the anterior and/or posterior lens suture. Three out of 82 foals examined by Koch l3 had this type of opacity, and two were siblings. The cause of such an opacity may be increased lens ground substance or reduced lens fiber length. Under magnification, it had a vacuolar appearance .9 Mature total congenital cataracts usually produce blindness in foals and are noticed soon after birth . However, the cataracts may not be discovered until weaning owing to the close relationship of foal and mother. As a result of its dense nature, the cataract prevents examination of the fundus. The entire lens is affected; thus , mydriasis does not improve the foal's vision . Foals with this type of cataract may be candidates for surgery. Congenital nuclear cataracts affect only the center of the lens. They may appear as focal opacities or as concentric, irregular rings, with the periphery of the lens remaining transparent. Because visualization of the unaffected le ns periphery increases as the pupil dilates , the visual impairment accompanying this condition may be less evident in dim light or with mydriasis . Progression of congenital nuclear cataract is unlikely; furthermore , some do regress with age. Lens Dislocation Dislocation or luxation of the lens occurs occasionally in the foal and can be associated with other congenital anomalies. Displacement may be anterior, posterior, or a subluxation (loss of some zonular attachments but lens is still within the patellar fossa) . Cataract formation and concomitant subluxation of the lens have been observed in the left eye of a foal affected with multiple congenital defects .6 More recently , a case of bilateral cataract formation and subluxation in a 6-month-old part Arab foal was reported by
532
G.A.
MUNROE AND
K.C.
BARNETT
Matthews and Hanscombe. 17 Removal of the luxated or subluxated lens is possible by intracapsular methods . Coloboma of the Lens Congenital absence of a segment of the lens periphery and its associated zonules has been seen in a Thoroughbred foal (K.C .B.). VITREOUS BODY The normal vitreous body consists of a clear gel that occupies the space between the lens and retina. The only congenital lesion recorded that affects this structure is persistence of the hyaloid vasculature. In a recent survey of 144 foals of mixed breeding, 20 were found to have persistent hyaloid structures. 15 The hyaloid artery is normally found in the fetus and for a period shortly after birth. Regression of the hyaloid vascular system usually takes place in the first 2 to 3 weeks of life. Examination of the young foal's eye will show the hyaloid system as faint branches just behind the posterior lens capsule, running together at the pole of the lens into a larger structure that passes through the vitreous to the optic disc. Various parts of this entire structure may persist. Distinguishing between hyaloid remnants and posterior capsular cataracts can be difficult. THE FUNDUS Variations in the normal equine ocular fundus are numerous and have been discussed earlier in this article . A knowledge of these normal variants is essential before any fundus can be evaluated. The incidence of congenital fundus lesions is low. Equine Night Blindness Equine night blindness is a congenital, apparently nonprogressive disease that has been reported increasingly in the Appaloosa breed in the United States. It occurs to a lesser extent in other breeds . 12.30 The condition has been seen in siblings and is believed to be recessively inherited. The ocular fundi of these animals are generally normal both ophthalmoscopically and histologically. 23 However, vision deficit may cover a broad range of illumination including, in some horses , significant day blindness. Some animals may have smaller than normal globe size, others display dorsal strabismus and nystagmus. The clinical history may relate to multiple episodes of injury occurring in familiar surroundings at night. The diagnosis can be tentatively suggested from the history and ocular examination but can only be confirmed by electroretinography. The defect may be a neuroretinal transmission problem . No treatment is available. Optic Nerve Hypoplasia Hypoplasia can occur unilaterally or bilaterally in foals with otherwise normal eyes in combination with other ocular anomalies, including microphthalmia, cataracts, and re tinal detachment. (i .H. 12. 2.3 Affected foals present with impaired vision or blindness and have slow to nonexistent pupillary reflexes.
CONGENITAL OCULAR DISEASE IN THE FOAL
533
Figure 6. Multiple re tinal hemorrhages in a Thoroughbred foal with maladjustment syndrom e. (See also color plate IB )
The optic disc is smaller and paler than normal, abnormal in shape, with retinal vessels decreased in number and size. Animals may collide with obstacles in a maze test. Optic nerve hypoplasia can be difficult to differentiate clinically from acquired optic atrophy, which can sometimes occur early in life. However, histologically the difference is obvious: hypoplasia is characterized by few ganglion cells, an abnormally thin retinal fiber layer, and a small optic disc. No treatment is available. Retinal Hemorrhage and Papilledema Multiple hemorrhages in the retina and sclera, and papilledema, together with variable pupil size, have been described in association with the convulsive foal or neonatal maladjustment syndrome (Fig. 6).1 .5.24 A recent survey of 180 Thoroughbred foals (normal and abnormal) examined within 3 days of birth showed retinal hemorrhage to be present in 8 per cent and papilledema in 5.5 per cent. 5 It has been suggested that retinal hemorrhage is of little clinical significance in newborn foals, whereas papilledema and changes in optic disc coloration seem to be more closely related to cerebral edema, the underlying factor in the maladjustment syndrome. It is our experience that a few small retinal hemorrhages in Thoroughbred foals up to a few days of age are of no significance and can be found in apparently normal animals with no evidence of the maladjustment syndrome (Fig. 7). Miscellaneous Congenital Lesions of Fundus Congenital chorioretinitis that leads to pigmentary changes throughout the fundus has been reported. 23 The inactive lesions are seen in foals born to dams that have been exposed to respiratory disease in late gestation. Congenital peripheral retinal cysts are rare. They are a component of some multiple intraocular anomalies that may result from defects in the ora serrata. 23 Congenital retinal dysplasia and detachment occurs rarely in foals . The detached retina that results in blindness can be seen in the vitreous as numerous folds in front of the fundus. It may occur with other congenital ocular anomalies and has been seen in the Hackney pony in two successive foals born to the same parents (K.C.B.) (Fig. 8).
534
G.A.
Figure 7.
MUNROE AND
K.C.
BARNETT
A. Resorbing retinal hemorrhages in a normal I-day-old Thoroughbred foal.
B, Streak hemorrhage across disc in a normal 5-day-old Thoroughbred foal with resorbing retinal hemorrhage. (See also color plates Ie and ID).
CONGENITAL OCULAR DISEASE IN TIlE FOAL
535
Figure 8. A and B, Retinal detachment folds in both eyes of a 3-week-old Hackney pony foal. (See also color plates IE and IF).
536
G.A.
MUNROE AND
K.C .
BARNEIT
REFERENCES 1. Barnett , K.C.: The eye of the newborn foa!' J. Reprod. Ferti!. Supp!., 23:701-702, 1975. 2. Duke-Elde r, S.: Normal and abnormal development. In Duke-Elder System of Ophthalmology. Vol III. Part 2. St. Louis , Missouri , C. V. Mosby, Co., 1963. 3. Dziezyc, J. , Kern, T.J. , and Wolf, E.D.: Microphthalmia in a foal. III Equine Ophthalmology, [Supp!.] London , England, British Veterinary Association , 1983. 4. Eriksson, K.: Hereditary aniridia with secondary cataract in horse. Nord. Vet. Med. , 7:773-793, 1955. 5. Frauenfelder, H. C.: A survey of eyes in newborn Thoroughbred foals with special reference to papilloedema and retinal hemorrhage . Personal communication, 1983. 6. Garner, A. , and Griffiths, P.: Bilateral ocular defects in a foal. Br. J. Ophthalmo!., 53:513- 517, 1969. 7. Gelatt, K. N. , Leipold, H. W. , and Coffman , J. R: Bilateral optic nerve hypoplasia in a colt. J. Am. Vet. Med. Assoc. , 155:627- 631 , 1969. 8. Gelatt, K.N. , and McClure, J.R.: Congenital strabismus and its correction in two Appaloosa horses. J. Equine Med. Surg. , 3(5):240-244, 1979. 9. Gelatt, K.N.: The eye. In Equine Medicine and Surgery. Edition 3. Santa Barbara, California, American Veterinary Publications, Inc. , 1982. 10. Hjorth, P.: Aplasia of the nasolacrimal duct in the horse . Nord. Vet. Med., 23:216, 1971. 11. Joyce, J. R , Aniridia in a quarter horse. In Equine Veterinary Journal supplement of Equine Ophthalmology. London, England, British Veterinary Association, 1983. 12. Kern, T.J.: Ocular fundus and central nervous system causes of blindness. In Robinson , N.E. (ed. ): Current Therapy in Equine Medicine. Edition 1. Philadelphia, W. B. Saunders Co. , 1983. 13. Koch , S.A., Reynolds Cowes, R. , Schmidt, G.R, et al.: Ocular disease in the newborn horse : A preliminary report. J. Equine Surg. , 2(4):167-170, 1978. 14. Kunze, D.J.: Cataracts. In Robinson, N.E. (ed. ): Current Therapy in Equine Medicine. Edition 1. Philadelphia, W. B. Saunders Co. , 1983. 15. Latimer, C.A., Wyman , N., and Hamilton, J.: Ophthalmic survey of the neonatal horse. In Equine Veterinary Journal supplement of Equine Ophthalmology. London , England, British Veterinary Association , 1983. 16. Lundvall, R. L. , and Carter, J. D.: Atresia of the nasolacrimal meatus in the horse. J. Am. Vet. Med. Assoc., 1971. 17. Matthews, A.G. , and Hanscombe, M.C.: Bilateral cataract formation and subluxation of the lens in a foal-a case report. In Equine Veterinary Journal supplement of Equine Ophthalmology. London , England, British Veterinary Association , 1983. 18. Munroe, G.A.: Unpublished data as part of prospective fellowship thesis for the Royal College of Veterinary Surgeons, 1983. 19. Murphy, J. M.: Administration of ocular therapy. In Robinson , N.E. (ed. ): Current Therapy in Equine Medicine. Edition 1. Philadelphia, W.B. Saunders Co. , 1982. 20. Peiffer, R L. , et al.: Correction of congenital entropion in a foa!' Vet. Med. Small Anim. Clin. , 72:1219, 1977. 21. Priester, W.A.: Congenital ocular defects in cattle, horses , cat and dogs. J. Am . Vet. Med. Assoc., 160:1504-1511 , 1972. 22. Rubin , L. F. : Atlas of Vete rinary Ophthalmoscopy. Philadelphia, Lea & Febige r, 1974. 23. Riis, R.C.: Equine ophthalmology. In Gelatt, K.N. (ed. ): Veterinary Ophthalmology. Philadelphia, Lea & Febige r, 1981. 24. Rossdale, P. D.: Clinical studies on four newborn thoroughbred foals suffering convulsions with special reference to blood gas chemistry and pulmonary ventilation. Res. Vet. Sci., 10:279, 1969. 25. Serk, G .W. : Ocular discharge in young horses. In Robinson , N.E. (ed. ): Curre nt Therapy in Equine Medicin e . Edition 1. Philadelphia, W. B. Saunders Co. , 1983. 26. Slatter, D . H . : Fundame ntals of Veterinary Ophthalmology. Philadelphia, W. B. Saunders Co. , 1981. 27. Startup, F.G.C. : Diseases of the canine eye. London , England, Balliere, Tindall , 1964. 28. Walde , I.: Band opacities. In Equine Veterinary Journal supplement of Equine Ophthalmology. London , England, British Veterinary Association , 1983. 29. Whitley, RD. , Moore, C . P. , and Slone, D.E.: Cataract surgery in the horse. In Equine Veterinary Journal supplement of Equine Ophthalmology. London , England, British Veterinary Association , 1983.
CONGENITAL OCULAR DISEASE IN THE FOAL
537
20. Witzel, D.A. , Joyce , J.R. , and Smith, J. L.: Electroretinography of congenital night blindness in an Appaloosa filly. J. Equine Vet. Med. Surg. , 1:226-269, 1977. The Tortington Centre Arundel West Sussex, England BN18 OBG
PLATE I
A
B
c
o
E
F
A, Normal fundus in a 2-day-old normal Thoroughbred foal (see p. 521). B , Multiple retinal hemorrhages in a Thoroughbred foal with maladjustment syndrome (see p. 533). C, Resorbing retinal hemorrhages in a normal I-day-old Thoroughbred foal (see p. 534). D, Streak hemorrhages across disc in a normal 5-day-old Thoroughbred foal with resorbing retinal hemorrhage (see p. 534). E and F , Retinal detachment folds in both eyes of a 3-week-old Hackney pony foal (see p. 535).
538
PLATE II
A
B
c
D
E
F
A, Normal equine fundus (see p. 542). B, Variation of normal equine optic nerve (see p. 542). C, Subalbinotic equine fundus (see p. 543). D, Absence of tapetal area, exposing vortex veins of normal choroidal circulation (see p. 543). E, Peripapillary areas of focal retinal and choroidal atrophy in non tapetal portion of retina (see p. 544). F, Fundus of an older horse (over 20 years old) with proliferative optic neuropathy (see p. 549).
539