Ocular Surface Influences on Corneal Topography

Clinical Science GARY N. FOULKS, MD, SECTION EDITOR

Ocular Surface Influences on Corneal Topography LORETTA SZCZOTKA-FLYNN, OD, MS

ABSTRACT The shape of a cornea, as measured by corneal topography, can be influenced by a variety of factors. Direct and obvious influences on corneal shape include corneal pathology, corneal surgery, and contact lenses. With the modern and widespread use of computerized videokeratoscopy, subtle topographic changes from other external and systemic influences can now be detailed. The purpose of this review is to examine ocular surface influences and indirect surgical, pathological, and pharmacological factors that affect the corneal shape and acquired topographic maps. The clinical consequences of altered corneal topography will be highlighted throughout. KEY WORDS axial algorithm, corneal topography, dry eye, elevation maps, tangential algorithm, tears,

I. INTRODUCTION he shape of a cornea, as measured by corneal topography, can be influenced by a variety of ocular surface interactions, systemic associations, and ocular pathology or surgery. The corneal shape can be purposefully manipulated, as is commonly seen after keratorefractive surgery, penetrating keratoplasty, or orthokeratology. Other direct and obvious influences on the corneal shape include corneal pathology and unintended corneal molding with contact lenses. However, for

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Accepted for publication March 2004. From the Department of Ophthalmology, University Hospitals of Cleveland and Case Western Reserve University, Cleveland, Ohio Supported by Research to Prevent Blindness, and the Ohio Lion’s Eye Research Foundation. The author has no proprietary interest in any product or concept discussed in this article. Single copy reprint requests to: Loretta Szczotka-Flynn, OD, MS (address below). Corresponding author: Loretta Szczotka-Flynn, OD, MS, Associate Professor, Department of Ophthalmology, Case Western Reserve University, 11100 Euclid Avenue, Bolwell Bldg., Suite 3200, Cleveland, OH 44106. Tel: 216844-3609. Fax: 216-844-7117. Email: [email protected]. Abbreviations are printed in boldface where they first appear with their definitions. ©2004 Ethis Communications, Inc. The Ocular Surface ISSN: 15420124. Szczotka L. Ocular surface influences on corneal topography. 2004;2(3):188-200.

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over 30 years, it has been suggested that the corneal surface shape can also be affected by less obvious factors, such as age, medications, prolonged near work, natural mechanical forces, eye rubbing, environmental temperature, and heart beat.1 With the modern and widespread use of corneal topography or, more formally, computerized videokeratoscopy (CVK), these external and systemic influences on the corneal shape can be better characterized. The purpose of this review is to examine ocular surface influences and indirect surgical, pathological, and pharmacological factors that affect the corneal shape and acquired CVK maps. The clinical consequences of altered corneal topography will be highlighted throughout. II. COMPUTERIZED VIDEOKERATOSCOPY Before considering the true ocular surface and related topographic changes demonstrated by CVK, one should be aware of the principles and characteristics of the technology that influence the appearance of a given CVK map. A. Data Acquisition

In clinical practice, the two most common measuring systems in CVK technology are reflective devices and slit scanning devices. Other techniques, such as rasterstereography and interferometry, are used less clinically than in the research laboratory. Rasterstereography uses a projected grid of light to illuminate a surface, and the shape of the surface is determined by its distortion of the projected grid.2 The basic map produced is an anterior corneal elevation map. The grid is projected, not reflected, off the cornea; hence, a smooth epithelial surface is not required for image acquisition, and the image will be not be adversely affected by a highly irregular cornea.2 The most popular rasterstereographic-based system is the PAR Corneal Topography System (PAR CTS, PAR Vision Systems, New Hartford, NY), which is no longer manufactured for clinical office use, but is an excellent tool for intra-operative corneal topographic assessment.3,4 Purely reflective devices include the placido-based systems, which use the corneal tear film as a convex mirror to reflect a series of illuminated annular rings. These are by far the most popular instruments in clinical practice. The position, shape, size, and spacing of the rings in the reflected image are determined by the corneal shape (Figure 1). Since

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OCULAR SURFACE INFLUENCES ON CORNEAL TOPOGRAPHY / Szczotka-Flynn OUTLINE I. Introduction II. Computerized videokeratoscopy A. Data acquisition B. Color-coded data displays C. Quantifying corneal topography III. Ocular surface influences on corneal topography A. Eyelid pressure B. Tears and dry eye C. External influences D. Systemic influences E. Pathological influences F. Surgical influences IV. Summary

placido ring technology relies on the reflective properties of the corneal surface, any abnormality at the anterior corneal surface (actually the air-tear layer interface) will reduce the quality of the image obtained. Because the convex surface of the pre-corneal tear film serves as one of the most important refractive interfaces in the ocular media, the importance of a smooth and regular optical interface between the air and the tears is critical. Any irregularity in corneal topography due to a deficient or altered tear layer will have a direct effect on visual function. Common examples of anterior corneal abnormalities include epithelial defects, scarring, and highly irregular corneal surfaces, which make it difficult to digitize the reflected rings for quantitative analysis and accurate CVK maps.2 Placido devices measure rate of change of the corneal slope by calculating the positions and distances of the reflected rings from the center of the image to the periphery in a radial fashion.5 They can acquire information only from the anterior corneal surface, and they require many assumptions in their calculations. For example, placido disk devices do not provide “measured” dioptric values for the power of the anterior cornea. Instead, they give a calculated value based on the actual measured radius of curvature of the anterior cornea and a combined index of

refraction of 1.3375, which represents the combined airtear interface, tear-cornea interface, and cornea-aqueous interface.6 This results in an averaged total corneal curvature, incorporating the anterior and posterior corneal curvatures. Reflective devices also suffer from other ambiguities. Because of the inherent symmetry of the illuminated rings, placido-based corneal topography cannot disambiguate a central hill from a central depression on the cornea. Both appear as a local increase in curvature on the CVK map.5 Nevertheless, they remain the most commonly used instruments and provide relatively accurate and TABLE 1. repeatable data. COMMONLY USED Slit scanning systems TERMS IN CVK MAP DISPLAYS (Reproduced enable the acquisition of elwith permission from evation topography from Elsevier Science) the anterior and posterior corneal surfaces, as well as Color Scales: from the anterior surface of the crystalline lens. The Absolute scale Standard scale Orbscan II system (Bausch Normalized scale & Lomb, Rochester, NY) is Color map currently the only commerAutosize scale cially available example of Adjustable scale this technology. The optiCustomized scale cal acquisition head scans the eye using scanning opCurvature Maps: tical slits that are projected at 45-degree angles to the Axial distance Sagittal map corneal surface. Twenty Color map slits are projected sequenDefault map tially on the eye from the Tangential curvature map left and 20 slits from the Instantaneous map 7 right for a total of 40 slits. Local map In Orbscan II, the slit scanTrue map ning technique is combined with reflective CVK Other Displays: technology by the addition Elevation map of a placido disk attachHeight map ment, which acquires the 3D map front surface curvature Refractive map data. A computer algoPower map rithm computes the anteSnell’s law rior and posterior corneal surface elevations by comparison to a pre-calibrated known spatial position. The corneal thickness is then determined by the difference in elevation from the anterior and posterior surfaces. B. Color-Coded Data Displays

Figure 1. Placido image of a keratoconus cornea displaying altered mire reflections from the irregular corneal surface.

Regardless of the technology used to acquire the data, the thousands of points generated from a CVK instrument are usually transformed into a familiar color-coded dioptric or elevation display. These color-coded displays were developed to allow rapid interpretation of data by the clinician. Although the various maps allow a rapid view of corneal shape, they mainly provide a subjective analysis, which can be in-

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changes. Alternatively, axial curvature is derived directly from CVK, making it much more robust to noise (i.e., less likely to show errors in derivation of the curvature), and it is a global and spherically biased descriptor of shape.8,9 Although both of these representations of curvature are commonly used in CVK analysis, axial and tangential maps differ significantly in curvature readings (especially away from the vertex normal) due to the algorithms applied (Figure 2). Tangential curvature is based on a standard mathematical formula for a local radius at a given point along a curve. Although tangential radius of curvature is a derivative of axial data, it is proportional to the local curvaFigure 2. Axial and tangential (true) maps of the same cornea with with-the-rule astigmatism. ture, as it is axis-independent.8,10 Compared to its axial counterpart, in fluenced by the scale or algorithm used to derive the map. diseased or surgically altered corneas, a tangential map Therefore, although using the same raw data, CVK maps and more accurately depicts abrupt and localized shape patterns can be significantly influenced by mathematical machanges. It provides “true,” not averaged, data, which benipulations that produce more or less detail, and global or comes important in detecting any subtle irregularities, critilocal shape variations. The clinician may be led to believe cal aspects of topography, and in detecting or monitoring that the corneal shape has changed on serial map comparidisease progression.11-15 The axial “curvature” is not a true curvature. It is the sons, when, actually, only a different interpretation of the data distance along the normal from the corneal surface to the is presented. Therefore, one should be aware of some curvaoptic axis.8,16,17 The axial algorithm effectively averages ture and scale options common to most CVK systems so that all curvatures from the center to some peripheral location, true corneal changes, to be discussed in subsequent sections using spherical references. Because of this averaging naof this review, can be recognized. Topographic indices have ture of the algorithm, local curvature changes may be been developed by various manufacturers in an attempt to masked and are often underestimated. quantify the vast amount of data generated into a single, meanElevation maps display data differently than curvaingful number. These indices may be useful in assessing subtle corneal change and will be discussed below. There is no consistency among topography system manufacturers in the methods or terminology used to describe the data that is ultimately displayed. However, several common maps and terms exist; terminology used in the industry for these data displays are listed in Table 1. The underlined names will be used throughout this review, as they are the most common terms used in the literature today. Two methods of analyzing corneal curvature from CVK have been described, namely, axial and tangential topographic data displays. CVK maps derived from the tangential algorithm provide instantaneous radius of curFigure 3. Elevation map (upper left) versus anterior curvature map (lower left) of a normal vature data that are more precise in with-the-rule cornea. measuring local corneal shape 190

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ture maps (Figure 3.) Here, the elevation of the cornea is typically measured relative to a best-fit reference sphere selected by the computer. In more sophisticated programs, a more complex surface may serve as the reference, such as a sphero-cylinder. Elevation maps will significantly differ based on the selection of the reference surface and the point of reference. For a normal prolate cornea, the most common display features a central red peak on an elevation map, signifying elevation above the best fit reference sphere (Figure 4). Prolateness of the normal cornea causes it to rise above the best-fit reference sphere, creating a central hill. Immediately surrounding the “central hill,” the cornea dips Figure 4. Elevation map (upper left image) of normal prolate cornea. below the reference sphere, resulting in an “annular sea.” In the far periphture and elevation maps.4,20,21 Even in normal eyes with no ery, the cornea once again rises above the reference sur5 history of contact lens wear, almost 40% display asymmetric face, resulting in “peripheral highlands” (Figure 5). Color scaling can alter the appearance of any map, or irregular curvatures on the topographic pattern.20,21 Alwhether a curvature, elevation, or even pachymetry map is ternatively, many clinicians and researchers rely on manufacviewed. Briefly, an absolute scale always assigns the same color turer-produced preexisting regularity and symmetry indices to a given data value and forces the data to fit within a predeto qualify and quantify topographic shape. Most of these intermined dioptric range. Absolute scales are instrument-spedices are machine-specific, as each proprietary index was cific, and the scales are consistent each time the absolute map developed and implemented by a specific instrument manuis employed; thus, it allows a direct and rapid comparison facturer. Many of these indices have not been externally veribetween the color maps taken on the same instrument. A fied, particularly because the algorithms used are not always normalized scale automatically adjusts and subdivides the publicly available. map into multiple equal intervals, based on the range of valThe two most common indices referred to in the litues found for that cornea. A normalized display usually alerature are the Surface Regularity Index (SRI) and the Surlows more detail, because the color intervals can be much face Asymmetry Index (SAI) produced from Tomey Techsmaller than the corresponding absolute maps; however, the nology TMS indices. These indices are available on the normalized scale can produce a misleading map, because it TMS-1 original unit, as well as on later models, including can take a normal cornea and exaggerate its shape to look the TMS-2 and TMS-3. The SRI algorithm first evaluates abnormal (Figures 6A and B).18 the frequency distribution of powers along each of 256 Lastly, topographic maps can be altered depending on hemi-meridians of the analyzed corneal surface. The first the alignment of the CVK system with the various axes of the ocular system. There can be vast differences in the corneal shape when the CVK instrument is aligned with standard alignment versus alignment along the corneal sighting axis or the corneal apex.19 C. Quantifying Corneal Topography

A variety of methods exist to assess the regularity of the corneal surface. Both qualitative and quantitative approaches have been used. Qualitative classification schemes have been proposed and used to classify both curva-

Figure 5. Schematic representation of a prolate corneal surface compared to a reference sphere (reproduced by permission from Elsevier Science)

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Figure 6a. Absolute scale, on an axial map, of a normal eye.

ten mire rings are used to calculate the SRI, which usually covers a 3mm-diameter of the central corneal area. Any difference in power gradient between mire pairs is assigned a positive value and added to the running sum. This process is continued for the innermost 10 mires for all 256 hemi-meridians.22 For a perfectly smooth surface, SRI would approach zero; for normal corneas, the SRI is typically below 0.5.22,23 The SAI is the centrally weighted average of the summation of differences in corneal power between corresponding points on individual mires 180 degrees apart from 90 equally spaced meridians.22,24 Similar to the SRI, the SAI value is <0.5 in normal corneas, which reflects a smooth corneal surface with a high degree of symmetry. These indices, as well as other methodology, will be used to describe corneal topographic changes that may be influenced by the tear layer and other external factors, as described below. III. OCULAR SURFACE INFLUENCES ON CORNEAL TOPOGRAPHY A. Eyelid Pressure

In studies in which the topography of normal eyes was classified as irregular or asymmetric, explanations included poor fixation, misalignment between the corneal apex and visual axis, or tear film abnormalities.20 In this section, eyelid pressure, blink, and tear film influences on the resulting corneal topography will be discussed. Normal eyelid pressure may have a direct influence on corneal shape and astigmatism.25-27 Alternatively, lid pressures may simply induce tear film interactions that produce changes observed on placido-based CVK images. It has been hypothesized that lids have a direct effect on the corneal shape.26,27 Gray and Yap measured induced astigmatism in subjects with deliberately narrowed lid positions and found a statistically significant increase of withthe-rule astigmatism when the lid aperture was narrowed.26 Similarly, Buehren et al attributed corneal topographic changes to forces applied by the lid margins at the upper and lower edges of 8 mm tangential CVK maps (consistent with the natural position of the patient’s lid margins).27 192

Figure 6b. Normalized scale of the same eye in 6a; highlights and exaggerates subtle curvature variations.

Curvature changes were often more than 1 diopter (D) near the area of lid contact with the cornea. They felt that the displacement of corneal epithelium by the eyelid forces caused corneal curvature shifts, similar to forces used in orthokeratology, where contact lenses displace the epithelial cells via a mechanical mechanism.28 Other evidence that eyelid pressure has a direct effect on corneal shape includes the changes observed in corneal refractive status secondary to chalazia.29-31 Although not usually considered a risk factor for refractive disorders other than astigmatism, chalazia of the upper eyelid have been recently shown to produce a decrease in vision associated with reversible central corneal flattening and acquired hyperopia.29,31 In one case,29 a post-LASIK patient had decreased vision from a chalazion of the upper eyelid. A hyperopic shift of 1.25 D was induced, evidenced by central corneal flattening corresponding to the location of the chalazion. The chalazion-induced hyperopic change on topography disappeared once the chalazion resolved completely. Additionally, lid surgery, such as lid-lowering procedures used in the treatment of lagophthalmos3234 or ptosis surgery,32,35 can induce topographic changes and astigmatism. Brown et al32 showed that after ptosis repair, the average spherical topographic curvature shift was approximately 0.60 D, and nearly 30% of these patients showed transient astigmatic changes greater than 1 D. Others have also reported temporary increases in astigmatism after ptosis surgery, predominantly with-the-rule astigmatism, which dissipates approximately 6 weeks post-operatively.35 The clinical consequences of lid distortion on corneal shape have been well-described by Lieberman and Grierson.36 The premise of refractive procedures, they state, is that the corneal shape is the same or unaffected by the presence of lids on the cornea. Yet, they demonstrated that the corneal topography differs with and without lid retraction. During refractive surgery performed with a speculum in place, the lids are not in contact with the cornea, and, therefore, the surgical corneal shape may differ from that measured during a pre-operative examination performed without lid retraction. Therefore, lids should be

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retracted when topographic data are acquired prior to a refractive surgical procedure; otherwise, unexpected postoperative ablation results may be found when the lids return to their normal position at the conclusion of surgery. Lastly, lids may influence the corneal topography simply by displacing the globe during serial image acquisition. During topographic acquisition, the anteroposterior position of the globe can be directly affected by the position of the eyelids. The globe has been shown to retract with eyelid closure and move forward with eye widening.37 These effects are independent of lid forces and are caused by cocontraction and relaxation of the extraocular muscles. Alternatively, lid influences on corneal shape may be due not to mechanical impingement of the eyelids against the cornea, but to tear film disruption at the edge of the eyelid apposed to the tear layer. This hypothesis has been explored in a series of papers regarding the occurrence of monocular diplopia and corneal aberrations during reading. Monocular diplopia has been linked to corneal distortion after near work in various studies dating back over 35 years.38-45 Most believe that the problem stems from tear film interactions with the corneal surface during sustained, close work. On topographic maps, a wave-like distortion has been described with a clear association to the position of the eyelids during the reading task, which influences the corneal wavefront.46 Buehren et al believe that such changes observed in the corneal topography appear to be directly related to the forces exerted by the eyelids.46 However, others hypothesize that similar findings are secondary to focal tear film disturbances associated with the resting reading lid position, as during this period of concentration, the blink rate decreases and the lid stays at one location for a prolonged period of time.42 The topographic changes induced during the onset of diplopia include significantly increased SRI and SAI indices (0.25 and 0.5, respectively) and focal power shifts as high as 2.5 D within the entrance pupil.42

Figure 7a. Absolute scale, on an axial map, of a normal eye taken 5 seconds after a blink.

Figure 7b. Same eye in Figure 7a, 15 seconds after a blink. Note curvature changes secondary to tear film disruption.

B. Tears and Dry Eye

Blinking and the resultant tear layer distribution can significantly influence reflective CVK topographic images.47,48 During a pause in blinking, significant changes in corneal irregularity can be measured by topography. Nemeth et al used a high-speed CVK method to record the regularity of the tear film in the 15-second post-blink interval.47 It takes the tear film approximately 3-10 seconds after each blink to reach its most regular state, which they termed the tear film buildup time. The tear film regularity was quantified in the post blink interval. Specifically, the SRI and SAI decreased (improved) to their minimum levels at 7.1 and 5.4 seconds after a blink, respectively. This implies that after the eyelids are open, it takes the tear film some time to build up and reach its highest regularity and optical quality. At the end of the 15-second period, there is a significant increase in SRI and SAI, an indicator of imminent tear film breakup. At this point, the SRI can almost double, indicating enough irregularity to

Figure 7c. Same eye in Figure 7a, after 15 seconds of eye rubbing. Note corneal flattening and decreased astigmatism after rubbing.

account for a loss of at least one letter on a Snellen chart.48 Thus, for the most consistent and accurate CVK measurement, image acquisitions should be taken 5-7 seconds after the blink (Figure 7 A and B). In dry eye patients, corneal thinning as well as increased astigmatism, corneal irregularity, and asymmetry compared with normals, has been documented.7,49,50 Most clinicians

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believe that dry eye affects visual function in a limited manner, because the best-corrected visual acuity of dry eye patients appears to be clinically normal by ordinary visual acuity testing techniques.23 However, patients with dry eye frequently complain of decreased vision during activities of increased concentration, such as reading and computer work. Since a stable tear layer over the corneal surface is essential for regular surface optics and clear visual imagery, CVK may be used to measure the irregular surface changes attributable to tear film disturbances. In fact, the SRI measurement has frequently been used for this purpose, because it records a local fluctuation of central corneal power and is a correlate to potential visual acuity.23,50 For example, during sustained eye opening (10 seconds), SRI can almost triple in dry eye patients, which significantly impacts the ability to see clearly during periods of concentration.23 In some cases, keratoconus-like topographic changes have been seen in patients with keratoconjunctivitis sicca and dry eye patients with ocular rosacea.51,52 Orbscan corneal topography has been used to show that corneal thickness is significantly reduced in dry eyes. Liu and Plugfelder7 found that the central and mid-peripheral corneal thickness is significantly decreased in dry eyes. The reason for the apparent decreased corneal thickness has not been fully established and may represent a combination of factors. It is possible that a chronic state of desiccation and immune activation in dry eye contributes to thinning in the corneal stroma, induced by factors such as inflammatory cytokines released by the diseased corneal epithelium. Alternatively, the decreased corneal thickness may be partially due to decreased tear film thickness. The Orbscan system assesses corneal thickness by calculating the distance between the anterior air-tear film interface and the posterior corneal surface. Since the tear film structure is that of a hydrated mucous gel, any thinning of this hydrated gel will cause an apparent thinning of the corneal surface. Therefore, a decreased hydrated mucin gel layer will contribute in part to the observed decrease in corneal thickness. Clinically, the difference is approximately 0.04 mm, with normals measuring approximately 0.57 mm in the central cornea, and dry eye patients measuring approximately 0.53 mm. These findings are clinically relevant if slit scanning systems are used to assess corneal thickness in glaucoma suspects or candidates for keratorefractive surgery. In extreme cases, keratoconus-like topographic changes have been seen in advanced keratitis sicca (Figure 8). Two cases have been reported in which chronic ocular desiccation and aqueous tear deficiency produced inferior corneal steepening and high astigmatism resembling keratoconus.51,52 In one case, lubricating and hydrating the regional corneal desiccation reversed the topographic changes to normal.52 The treatment of dry eye classically involves the use of artificial tear preparations. However, artificial tear instillation before image acquisition produces mixed results on the accuracy of CVK maps. Some studies report an im194

Figure 8. Pseudo-keratoconus in a patient with severe punctuate epithelial keratopathy.

provement in surface regularity after instillation of tears, whereas others report worsening. The effect of artificial tears of differing viscosity on slit scanning CVK acquisitions have been tested. Nonviscous aqueous artificial tears can cause an imbalanced and uneven precorneal tear film. Viscous artificial tears (such as sodium hyaluronate solutions) provide a uniform distribution of tears.53 Additionally, the effects of artificial tears on CVK-measured corneal regularity vary by patient. Specifically, artificial tears affect normal, diseased, and postoperative corneas differently. In normal patients, corneal topography shows greater irregularity after instillation of artificial tears, such that imaging should be performed prior to artificial tear application.54,55 However, irregular corneal surfaces, such as those that have undergone penetrating keratoplasty, have improved surface regularity after instillation of tears.56 Dry eye patients with punctate epithelial keratopathy have significant improvements in surface regularity and symmetry (SRI and SAI) after instillation of tears, but in dry eye patients without punctate epithelial keratopathy, there is

Figure 9. Corneal topography after retinal detachment surgery (OS) compared to un-operated OD. Note steeper cornea and increased corneal irrregularity on the tangential map OS.

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actually a worsening of these indices.55 Therefore, artificial tears should be used prior to topographic measurement only in the most irregular postsurgical corneas or severe dry eye. C. External Influences

External influences, such as ocular rubbing, pharmacologic agents, and hypoxia, have been associated with significant changes in corneal shape. Chronic ocular rubbing has long been implicated in the pathogenesis of keratoconus, and even short-term ocular rubbing shows a transient effect on the cornea. Short-term eye rubbing can transiently increase the SRI and SAI three-fold, and astigmatism by 0.5 diopters (Figure 7C).57 The etiology of this shape change is traditionally thought to be from mechanical corneal deformity, but increased mucous production from meibomian gland secretion that significantly disrupts the tear film has also been implicated. Pharmacological dilation and constriction of the pupil prior to CVK measurement can alter the findings and increase the variability of measurement.58 During cycloplegia in children, corneal power increases in hyperopic eyes and decreases in myopic eyes.59 In other words, the act of accommodation affects corneal power obtained by CVK. Lastly, corneal hypoxia induces corneal curvature changes in some corneas. In post-radial keratotomy eyes exposed to hypoxia, a significant change in anterior and posterior corneal curvatures associated with a significant hyperopic shift has been reported.6 Normal control eyes show no corneal curvature changes associated with hypoxia alone. D. Systemic Influences

Both gender and age have been shown to influence corneal topography. Sex hormones are thought to play a role in the gender-related changes in corneal structure. Although some investigators were not able to detect variations in corneal topography related to the female menstrual cycle,60 more recent studies detected differences in corneal topography related to these hormonal variations.61,62 For example, corneal curvature has been found to be significantly flatter in females during certain time points during the menstrual cycle. Additionally, diurnal variations in corneal curvature of approximately 0.83 diopters have been reported to be associated with changes in the menstrual cycle.61 Handa concludes that these diurnal variations are significant enough to affect planning of refractive surgery and contact lens fitting, and should be taken into consideration. CVK-measured corneal astigmatism changes with age and varies by gender.62 Older men have flatter corneas and higher potential for against-the-rule astigmatism than older women. Decreases in the levels of sex hormones with age may play a role in these gender-related changes. However, in both genders, corneal irregularity, as measured via the SRI and Irregular Astigmatism Index, increases with age. These changes in the corneal surface may be related

to the degradation of the ocular optics, as corneal aberrations have been shown to significantly increase with age.63,64 Specifically, spherical aberration, coma, and other higher order aberrations increase in the aging eye,63 and there is a loss of balance between corneal and internal surface aberrations.65 However, the increases in corneal surface aberrations are not enough to explain the total increases found in ocular aberrations that ultimately reduce retinal image quality with age. Instead, internal surface aberrations increase and compound with higher corneal aberrations, degrading the overall optics of the aging eye. E. Pathological Influences

It is well documented that corneal pathology and corneal surgery can directly influence corneal topography. However, other anterior and posterior segment ocular pathology, not directly related to the cornea, can unexpectedly alter the corneal surface. Specifically, pterygium, cataracts, cataract surgery, extraocular muscle surgery, and retina and vitreous surgery have all been shown to have either temporary or permanent influences on corneal shape. Pterygium, a fibrovascular connective tissue overgrowth of the bulbar conjunctiva onto the peripheral cornea, can produce marked changes in refractive state and corneal curvature long before entering the optical zone of the eye. The most common topographic abnormality is asymmetric, with with-the-rule astigmatism caused by flattening of the cornea in the direction of the pterygium.66,67 However, the correlation between topographic astigmatism and astigmatism measured by manifest refraction is poor, with topographic changes correlating better with subjective visual symptoms.68-71 The cause of the astigmatism appears to be an alteration of the tear film on the placido image, rather than the traction on the cornea by the lesion.72 As the head of the pterygium approaches the apex of the cornea, a tear meniscus develops between the corneal apex and the elevated pterygium, causing an apparent flattening of the normal corneal curvature in that area.67 The induced astigmatism is usually irregular and isolated on the side of the pterygium. Additionally, there is a significant correlation between the pterygium size and the corneal power, with larger pterygia depressing a much wider area of the cornea, leading to decreased corneal spherical power, and increased astigmatism, asymmetry, and irregularity.66,73 Fortunately, successful pterygium surgery can improve topographic abnormalities. Topographic astigmatism has been shown to decrease from an average of 6 D to almost 2 D, and the SRI and SAI significantly improve after surgery.67 However, the topographic and refractive shifts do not stabilize until approximately 1 month after surgery; therefore, if any cataract or refractive surgery is considered, it is best performed at least 1 month after pterygium removal. The effect of an aging cataract has also been found to influence CVK results. A cataractous lens in a dilated pupil has been shown to cause large inconsistencies between keratometric and videokeratoscopic curvature values, with

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CVK values as much as 10 D steeper than keratometric values.74 With the pupil constricted, corneal curvature readings were consistent with those obtained by keratometry. The reflections from a cataractous lens may combine with those from the cornea, adversely affecting computer calculations. Therefore, during routine cataract surgery evaluation, topography should be performed on an undilated pupil, and keratometric values should be recorded for comparison. Modern cataract surgery can also influence the corneal topography. In sutureless cataract surgery, clear corneal incisions produce much greater regular and irregular corneal astigmatism, with delayed topographic stabilization compared to scleral incisions.75-79 Furthermore, in limbal tunnel and scleral pocket sutureless cataract surgery, incision location has been shown to influence the corneal shape. Specifically, superior and nasal incisions produce statistically significant changes in corneal astigmatism post-operatively, compared to temporal incisions.80,81 G. Surgical Influences

Changes in refractive error are frequently reported in patients after retinal detachment surgery and, less commonly, after extraocular muscle surgery.82-86 In extraocular muscle surgery, refractive changes have presumably occurred via corneal surface changes secondary to altered extraocular muscle tension.84-86 Qualitative examinations of corneal topographic maps do not easily demonstrate these changes in the corneal surface. However, quantitative analysis of corneal topography clearly demonstrates that muscle tension has a direct effect on corneal curvature. For example, CVK was used in a quantitative analysis of corneal shape after inferior rectus muscle recession in patients with Grave’s disease.86 Corneas steepened in the quadrant adjacent to the involved muscle (inferiorly and inferotemporally) after operation, whereas reciprocal flattening was found superiorly and superotemporally. Conversely, quantitative CVK analysis after superior rectus recession (in rabbits) demonstrated flattening in the quadrant adjacent to the involved muscle.85 Yet, others have reported global rather than localized corneal changes after extraocular muscle surgery. Hainsworth found significant changes in total corneal power after various muscle recession, resection, and combined recession and resection procedures, which were not isolated to a specific corneal quadrant or adjacent to the altered muscle. He concluded that this illustrates the interaction and interdependence all corneal positions have with respect to each other; i.e., a change in tension of one muscle does not produce a change in an adjacent quadrant as much as it produces a significant change in the entire corneal surface. However, comparison of the calculated changes from preoperative to postoperative corneal power showed mean differences of less than 0.25 diopters; therefore, these corneal power changes may not be clinically significant. Retinal detachment surgery is another well-known cause of ocular refractive shifts. With the improving sur196

gical success of scleral buckling procedures for retinal detachment, growing attention is now directed toward visual rehabilitation and quality of postoperative vision. The refractive changes induced are not limited simply to increases in myopia. Corneal topography is the best way to assess these changes in a transitory or permanent fashion. After placement of a full encircling band to repair a detached retina, the globe is increased in its anteroposterior axial length and myopic shifts may be as much as 3 D.87 Although these myopic shifts are caused by the elongation of axial length, both regular and irregular astigmatism can also be induced due to secondary effects on the corneal shape.82,83,88-90 Induced astigmatism after retinal detachment surgery is most often associated with radial, rather than circumferential, scleral buckles.82,83,88-90 Sutured segmental buckles induce greater astigmatism than widely spanned or encircling buckles, because the indentation of the sclera causes isolated corneal distortion and induced astigmatism. Fourier harmonic analysis has been used to elucidate the influence of buckling procedures on corneal irregularity and visual function after reattachment surgery.82 Fourier analysis provides a quantitative analysis of irregular astigmatism by mathematically modeling CVK data into spherical, regular astigmatism, decentration, and higher order irregular astigmatic components.91 With use of this technique, it was found that induced regular and irregular astigmatism is typically greater in eyes that had scleral buckles extending less than or equal to 180 degrees, but not in eyes with buckles extending greater than 180 degrees. Additionally, the astigmatism is transient and returns to preoperative levels approximately one month after reattachment surgery. However, cases in which a high degree of irregular astigmatism persisted, necessitating the removal of the scleral buckle, have been reported.89 In addition to the degree of encircling, it has also been noted that higher and/or more anteriorly located buckles produce greater changes in regular corneal astigmatism83,88 Okada reported that the shorter the distance between the limbus and the buckle, the greater the degree of induced corneal astigmatism.83 One month after local buckling surgery, induced corneal astigmatism can average approximately 2.25 D. This induced corneal astigmatism decreases with time and may stabilize at approximately 1.50 D.83,92 With circular buckle operations, the central cornea can initially steepen by 2 D and gradually flatten 3-6 months after surgery.93,94 In addition to a transient myopic shift, corneal astigmatism has been reported with encircling bands. Regular astigmatism can increase by as much as 23 D in the first postoperative month, gradually decreasing to baseline by 6 months.93 Transient irregular astigmatism of approximately 0.3 D can also occur (Figure 9). Therefore, in the first 6-month postoperative period, visual function is likely affected by these transient corneal changes. Even after a well-planned and carefully executed surgical procedure, visual acuity in the first 6 postoperative months may be disappointing. Careful topographic assessment and

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refractions may reveal these transient and induced astigmatic and myopic shifts and should be routinely performed in cases of unexpected postoperative visual acuity results. Conventional pars plana vitrectomy surgery, without a scleral buckling procedure, can also influence corneal curvature and is an important factor in the visual outcome of surgery. Regular, asymmetric, and irregular corneal changes have been noted to persist up to 6 months after pars plana vitrectomy, and these will have direct, clinical consequences on the patient’s visual acuity in the postoperative period.9597 Even with small sclerotomies at a location relatively posterior to the central cornea, significant alterations can be produced in the cornea. The corneal surface dynamics are affected by several factors, which are related primarily to the peripheral wound healing response.95 Factors such as incision size, position, configuration, wound edema, cautery, different tensile strengths of suture materials, suture placement, knot tying technique, tension, and number of sutures affect the wound healing and respective contracture of the scleral tissue.98,99 For example, scleral cautery near the incision leads to thermal contracture of the treated tissue, with secondary corneal steepening.100 Conversely, as scleral incisions heal and wound edema clears, sutures are absorbed, tension is released, and collagen contracture from cautery dissipates. The relaxation of the wound site will then produce corneal flattening. Interestingly, additional gas injections during pars plana vitrectomy induce more surgically-induced corneal cylinder, likely related to stronger compression of the sutures intraoperatively to avoid loss of gas.95 Total induced astigmatism captured by CVK can be dramatic. During the first postoperative week, it can approximate 3 to 4.5 D, with maximum changes detected as high as 9 D.95,96 Qualitatively, corneal topography changes from a homogeneous contour preoperatively to an asymmetric bowtie or irregular astigmatic pattern, postoperatively. Additionally, surface regularity and asymmetry (SRI and SAI) have been shown to deteriorate after vitreoretinal surgery.97 Although most of these surgically induced changes are transient, in some patients, visually disabling astigmatism can persist longer, requiring intervention for visual rehabilitation. CVK has been used to document disabling astigmatism at 4 months after surgery.96 The astigmatism is typically symmetric, and the axis of the astigmatism is in the axis of the superonasal and inferotemporal sclerotomies. These patients can be treated with interventional suture lysis if they are unable to visually rehabilitate with spectacles. CVK readily detects this characteristic astigmatism, and cutting the scleral sutures in this steep axis can reduce it, improving visual acuities significantly. Therefore, because of the possible significant curvature changes after pars plana vitrectomy, corneal topography should be performed in cases of surprisingly reduced visual acuity results. This may reveal significant astigmatism as the cause of inadequate postoperative vision, preventing unnecessary prescription for glasses with poor tolerance, either in the transient or longer postoperative period.

IV. SUMMARY Modern CVK is a proven technology to measure and analyze the corneal shape. It can detect subtle corneal changes secondary to everyday ocular surface influences, and those related to tear film abnormalities. CVK can also detect corneal surface changes related to systemic and pharmacologic influences. Although corneal pathology and surgery clearly alter the corneal shape, other surgeries and pathologies external to the cornea also do so. CVK is a technology that should be routinely employed to investigate any unexplained visual loss due to probable changes that can develop in the corneal topography after a variety of ocular surface changes and interventions. REFERENCES 1. Smith TW. Corneal topography. Doc Ophthalmol 1977; 43:249-76 2. Arffa RC, Warnicki JW, Rehkopf PG. Corneal topography using rasterstereography. Refract Corneal Surg 1989;5:414-7 3. Belin MW, Cambier JL, Nabors JR, Ratliff CD. PAR corneal topography system (PAR CTS): the clinical application of closerange photogrammetry. Optom Vis Sci 1995;72:828-37 4. Naufal SC, Hess JS, Friedlander MH, Granet NS. Rasterstereography-based classification of normal corneas. J Cataract Refract Surg 1997;23:222-30 5. Szczotka-Flynn L. Computerized corneal topography in gas permeable lens fitting, in Bennett M (ed): Manual of gas permeable contact lenses. St Louis, Elsevier Science, 2004, pp 117-37 6. McMann MA, Parmley VC, Brady SM, et al. Analysis of anterior and posterior corneal curvature changes using Orbscan technology in radial keratotomy eyes exposed to hypoxia. J Cataract Refract Surg 2002;28:289-94 7. Liu Z, Pflugfelder SC. Corneal thickness is reduced in dry eye. Cornea 1999;18: 403-7 8. Roberts C. Corneal topography: a review of terms and concepts. J Cataract Refract Surg 1996;22:624-9 9. Klein SA, Mandell RB. Shape and refractive powers in corneal topography. Invest Ophthalmol Vis Sci 1995;36:2096-109 10. Chan JS, Mandell RB. Alignment effects in videokeratography of keratoconus. CLAO J 1997;23:23-8 11. Sano Y, Carr JD, Takei K, et al. Videokeratography after excimer laser in situ keratomileusis for myopia. Ophthalmology 2000;107: 674-84 12. Lim-Bon-Siong R, Carr JD, Takei K, et al. Screening of myopic photorefractive keratectomy in eye bank eyes by computerized videokeratography. Arch Ophthalmol 1998;116:617-23 13. Chan JS, Mandell RB, Berger DS, Fusaro RE: Accuracy of videokeratography for instantaneous radius in keratoconus. Optom Vis Sci 1995;72:793-9 14. Rabinowitz YS. Tangential vs sagittal videokeratographs in the “early” detection of keratoconus. Am J Ophthalmol 1996;122:887-9 15. Szczotka LB, Thomas J. Comparison of axial and instantaneous videokeratographic data in keratoconus and utility in contact lens curvature prediction. CLAO J, 1998;24:22-8 16. Roberts C. The accuracy of ‘power’ maps to display curvature data in corneal topography systems. Invest Ophthalmol Vis

THE OCULAR SURFACE / JULY 2004, VOL. 2, NO. 3 / www.theocularsurface.com

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OCULAR SURFACE INFLUENCES ON CORNEAL TOPOGRAPHY / Szczotka-Flynn Sci 1994;35:3525-32 17. Roberts C. Characterization of the inherent error in a spherically-biased corneal topography system in mapping a radially aspheric surface. J Refract Corneal Surg 1994;10:103-11; discussion 112-6 18. Szczotka LB. Corneal topography and contact lenses. Ophthalmol Clin North Am 2003;16:433-53 19. Mandell RB, Chiang CS, Klein SA. Location of the major corneal reference points. Optom Vis Sci 1995;72:776-84 20. Bogan SJ, Waring GO 3rd, Ibrahim O, et al. Classification of normal corneal topography based on computer-assisted videokeratography. Arch Ophthalmol 1990;108:945-9 21. Alvi NP, McMahon TT, Devulapally J, et al. Characteristics of normal corneal topography using the EyeSys corneal analysis system. J Cataract Refract Surg 1997; 23:849-55 22.Wilson SE, Klyce SD. Quantitative descriptors of corneal topography. A clinical study. Arch Ophthalmol 1991;109:349-53 23. Goto E, Yagi Y, Matsumoto Y, Tsubota K. Impaired functional visual acuity of dry eye patients. Am J Ophthalmol 2002;133:181-6 24. Dingeldein SA, Klyce SD, Wilson SE. Quantitative descriptors of corneal shape derived from computer-assisted analysis of photokeratographs. Refract Corneal Surg 1989;5:372-8 25. Wilson G, Bell C, Chotai S. The effect of lifting the lids on corneal astigmatism. Am J Optom Physiol Opt 1982;59:670-4 26. Grey C, Yap M. Influence of lid position on astigmatism. Am J Optom Physiol Opt 1986;63:966-9 27. Buehren T, Collins MJ, Iskander DR, et al., The stability of corneal topography in the post-blink interval. Cornea 2001;20:826-33 28. Swarbrick HA, Wong G, O’Leary DJ. Corneal response to orthokeratology. Optom Vis Sci 1998;75:791-9 29. Cosar CB, Rapuano CJ, Cohen EJ, Laibson PR. Chalazion as a cause of decreased vision after LASIK. Cornea 2001;20:890-2 30. Nisted M, Hofstetter HW. Effect of chalazion on astigmatism. Am J Optom Physiol Opt 1974;51:579-82 31. Santa Cruz CS, Culotta T, Cohen EJ, Rapuano CJ. Chalazioninduced hyperopia as a cause of decreased vision. Ophthalmic Surg Lasers 1997;28:683-4 32. Brown MS, Siegel IM, Lisman RD. Prospective analysis of changes in corneal topography after upper eyelid surgery. Ophthal Plast Reconstr Surg 1999;15:378-83 33. Goldhahn A, Schrom T, Berghaus A, et al. [Corneal astigmatism as a special complication after lid-loading in patients with lagophthalmos]. Ophthalmologe 1999;96:494-7 34. Kartush JM, Linstrom CJ, McCann PM, Graham MD. Early gold weight eyelid implantation for facial paralysis. Otolaryngol Head Neck Surg 1990;103:1016-23 35. Holck DE, Dutton JJ, Wehrly SR. Changes in astigmatism after ptosis surgery measured by corneal topography. Ophthal Plast Reconstr Surg 1998;14:151-8 36. Lieberman DM, Grierson JW. The lids influence on corneal shape. Cornea 2000;19:336-42 37. Evinger C, Shaw MD, Peck CK, et al. Blinking and associated eye movements in humans, guinea pigs, and rabbits. J Neurophysiol 1984;52:323-39 38. Mandell RB. Bilateral monocular diplopia following near work.

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Am J Optom Arch Am Acad Optom 1966;43:500-4 39. Knoll HA. Letter: bilateral monocular diplopia after near work. Am J Optom Physiol Opt 1975;52:139-40 40. Bowman KJ, Smith G, Carney LG. Corneal topography and monocular diplopia following near work. Am J Optom Physiol Opt 1978;55:818-23 41. Campbell C. Corneal aberrations, monocular diplopia, and ghost images: analysis using corneal topographical data. Optom Vis Sci 1998;75:197-207 42. Ford JG, Davis RM, Reed JW, et al. Bilateral monocular diplopia associated with lid position during near work. Cornea 1997;16:525-30 43. Kommerell G. [Monocular diplopia caused by pressure of the upper eyelid on the cornea. Diagnosis based on the “Venetian blind phenomenon” in streak retinoscopy]. Klin Monatsbl Augenheilkd 1993;203:384-9 44. Carney LG, Liubinas J, Bowman KJ. The role of corneal distortion in the occurrence of monocular diplopia. Acta Ophthalmol (Copenh) 1981;59:271-4 45. Golnik KC, Eggenberger E. Symptomatic corneal topographic change induced by reading in downgaze. J Neuroophthalmol 2001;21:199-204 46. Buehren T, Collins MJ, Carney L. Corneal aberrations and reading. Optom Vis Sci 2003;80:159-66. Erratum in: Optom Vis Sci 2003;80:720 47. Nemeth J, Erdelyi B, Csakany B, et al. High-speed videotopographic measurement of tear film build-up time. Invest Ophthalmol Vis Sci 2002;43:1783-90 48. Nemeth J, Erdelyi B, Csakany B. Corneal topography changes after a 15 second pause in blinking. J Cataract Refract Surg 2001;27:589-92 49. Dursun D, Monroy D, Knighton R, et al. The effects of experimental tear film removal on corneal surface regularity and barrier function. Ophthalmology 2000;107:1754-60 50. Liu Z, Pflugfelder SC. Corneal surface regularity and the effect of artificial tears in aqueous tear deficiency. Ophthalmology 1999;106:939-43 51. Dursun D, Piniella AM, Pflugfelder SC. Pseudokeratoconus caused by rosacea. Cornea 2001;20:668-9 52. De Paiva CS, Harris LD, Pflugfelder SC. Keratoconus-like topographic changes in keratoconjunctivitis sicca. Cornea 2003;22:22-4 53. Shimmura S, Goto E, Shimazaki J, Tsubota K. Viscosity-dependent fluid dynamics of eyedrops on the ocular surface. Am J Ophthalmol 1998;125:386-8. Erratum in Am J Ophthalmol 1998;125:743 54. Novak KD, Kohnen T, Chang-Godinich A, et al. Changes in computerized videokeratography induced by artificial tears. J Cataract Refract Surg 1997;23:1023-8 55. Huang FC, Tseng SH, Shih MH, Chen FK. Effect of artificial tears on corneal surface regularity, contrast sensitivity, and glare disability in dry eyes. Ophthalmology 2002;109:1934-40 56. Pavlopoulos GP, Horn J, Feldman ST. The effect of artificial tears on computer-assisted corneal topography in normal eyes and after penetrating keratoplasty. Am J Ophthalmol 1995;119:712-22 57. Mansour AM, Haddad RS. Corneal topography after ocular

THE OCULAR SURFACE / JULY 2004, VOL. 2, NO. 3 / www.theocularsurface.com

OCULAR SURFACE INFLUENCES ON CORNEAL TOPOGRAPHY / Szczotka-Flynn rubbing. Cornea 2002;21:756-8 58. Sun R, Beldavs RA, Gimbel HV, Ferensowicz M. Effect of pharmacological dilation and constriction of pupil on corneal topography. Cornea 1996;15:245-7 59. Gao L, Zhuo X, Kwok A, et al. The change in ocular refractive components after cycloplegia in children. Jpn J Ophthalmol 2002;46:293-8 60. Oliver KM, Walsh G, Tomlinson A, et al. Effect of the menstrual cycle on corneal curvature. Ophthalmic Physiol Opt 1996:16:467-73 61. Handa T, Mukuno K, Niida T, et al. Diurnal variation of human corneal curvature in young adults. J Refract Surg 2002;18:58-62 62. Goto T, Klyce SD, Zheng X, et al. Gender-and age-related differences in corneal topography. Cornea 2001;20:270-6 63. Guirao A, Redondo M, Artal P. Optical aberrations of the human cornea as a function of age. J Opt Soc Am A Opt Image Sci Vis 2000;17:1697-702 64. Oshika T, Klyce SD, Applegate RA, Howland HC. Changes in corneal wavefront aberrations with aging. Invest Ophthalmol Vis Sci 1999;40:1351-5 65. Artal P, Berrio E, Guirao A, Piers P. Contribution of the cornea and internal surfaces to the change of ocular aberrations with age. J Opt Soc Am A Opt Image Sci Vis 2002;19:137-43 66. Lin A, Stern G. Correlation between pterygium size and induced corneal astigmatism. Cornea 1998;17:28-30 67. Stern GA, Lin A. Effect of pterygium excision on induced corneal topographic abnormalities. Cornea 1998;17:23-7 68. Seitz B, Gutay A, Kuchle M, et al. [Impact of pterygium size on corneal topography and visual acuity - a prospective clinical cross-sectional study]. Klin Monatsbl Augenheilkd 2001;218:609-15 69. Ashaye AO, Refractive astigmatism and pterygium. Afr J Med Med Sci 1990;19:225-8 70. Hansen A, Norn M. Astigmatism and surface phenomena in pterygium. Acta Ophthalmol (Copenh) 1980;58:174-81 71. Ibechukwu BI. Astigmatism and visual impairment in pterygium: affected eyes in Jos, Nigeria. East Afr Med J 1990;67:912-7 72. Oldenburg JB, Gabus J, McDonnell JM, McDonnell PJ. Conjunctival pterygia. Mechanism of corneal topographic changes. Cornea 1990;9:200-4 73. Tomidokoro A, Miyata K, Sakaguchi T, et al. Effects of pterygium on corneal spherical power and astigmatism. Ophthalmology 2000;107:1568-71 74. Singh D. Effect of cataract on corneal topography results. J Cataract Refract Surg 1996;22:1506-8 75. Huang FC, Tseng SH. Comparison of surgically induced astigmatism after sutureless temporal clear corneal and scleral frown incisions. J Cataract Refract Surg 1998;24:47781 76. Beltrame G, Salvetat ML, Chizzolini M, Driussi G. Corneal topographic changes induced by different oblique cataract incisions. J Cataract Refract Surg 2001;27:720-7 77. Olsen T, Dam-Johansen M, Bek T, Hjortdal JO. Corneal versus scleral tunnel incision in cataract surgery: a randomized study. J Cataract Refract Surg 1997;23:337-41

78. Roman SJ, Auclin FX, Chong-Sit DA, Ullern MM. Surgically induced astigmatism with superior and temporal incisions in cases of with-the-rule preoperative astigmatism. J Cataract Refract Surg 1998;24:1636-41 79. Simsek S, Yasar T, Demirok A, et al. Effect of superior and temporal clear corneal incisions on astigmatism after sutureless phacoemulsification. J Cataract Refract Surg 1998;24:515-8 80. Kohnen S, Neuber R, Kohnen T. Effect of temporal and nasal unsutured limbal tunnel incisions on induced astigmatism after phacoemulsification. J Cataract Refract Surg 2002;28:821-5 81. Kohnen T, Mann PM, Husain SE, et al. Corneal topographic changes and induced astigmatism resulting from superior and temporal scleral pocket incisions. Ophthalmic Surg Lasers 1996;27:263-9 82. Tomidokoro A, Oshika T, Kojima T. Corneal astigmatism after scleral buckling surgery assessed by Fourier analysis of videokeratography data. Cornea 1998;17:517-21 83. Okada Y, Nakamura Y, Kubo E, et al. Analysis of changes in corneal shape and refraction following scleral buckling surgery. Jpn J Ophthalmol, 2000;44:132-8 84. Hainsworth DP, Bierly JR, Schmeisser ET, Baker RS. Corneal topographic changes after extraocular muscle surgery. J AAPOS 1999;3: 80-6 85. Kwito S, Sawusch MR, McDonnell PJ, et al. Effect of extraocular muscle surgery on corneal topography. Arch Ophthalmol 1991;109: 873-8 86. Kwitko S, Feldon S, McDonnell PJ. Corneal topographic changes following strabismus surgery in Grave’s disease. Cornea 1992;11:36-40 87. Smiddy WE, Loupe DN, Michels RG, et al. Refractive changes after scleral buckling surgery. Arch Ophthalmol 1989;107:1469-71 88. Goel R, Crewdson J, Chignell AH. Astigmatism following retinal detachment surgery. Br J Ophthalmol 1983;67:327-9 89. Burton TC, Herron BE, Ossoinig KC. Axial length changes after retinal detachment surgery. Am J Ophthalmol 1977;83: 59-62 90. Watanabe K, Emi K, Hamano T, et al. [Corneal topographic evaluation of retinal detachment surgery]. Nippon Ganka Gakkai Zasshi 1988;92: 367-71 91. Raasch TW. Corneal topography and irregular astigmatism. Optom Vis Sci 1995;72: 809-15 92. Hayashi H, Hayashi K, Nakao F, Hayashi F. Corneal shape changes after scleral buckling surgery. Ophthalmology 1997;104:831-7 93. Ornek K, Yalcindag FN, Kanpolat A, Gunalp I. Corneal topographic changes after retinal detachment surgery. Cornea 2002;21:803-6 94. Weinberger D, Lichter H, Loya N, et al. Corneal topographic changes after retinal and vitreous surgery. Ophthalmology 1999;106:1521-4 95. Wirbelauer C, Hoerauf H, Roider J, Laqua H. Corneal shape changes after pars plana vitrectomy. Graefes Arch Clin Exp Ophthalmol 1998;236:822-8 96. Slusher MM, Ford JG, Busbee B. Clinically significant corneal astigmatism and pars plana vitrectomy. Ophthalmic Surg Lasers 2002;33:5-8

THE OCULAR SURFACE / JULY 2004, VOL. 2, NO. 3 / www.theocularsurface.com

199

OCULAR SURFACE INFLUENCES ON CORNEAL TOPOGRAPHY / Szczotka-Flynn 97. Azar-Arevalo O, Arevalo JF. Corneal topography changes after vitreoretinal surgery. Ophthalmic Surg Lasers 2001;32:168-72 98. Arciniegas A, Amaya LE. Experimental modification of the corneal curvature by means of scleral surgery. Ann Ophthalmol 1984;16:1155-66

99. Armeniades CD, Boriek A, Knolle GE Jr. Effect of incision length, location, and shape on local corneoscleral deformation during cataract surgery. J Cataract Refract Surg 1990;16: 83-7 100. Bergmann MT, Koch DD, Zeiter JH. The effect of scleral cautery on corneal astigmatism in cadaver eyes. Ophthalmic Surg 1988;19:259-62

Procedures for Initiating and Submitting Reviews to The Ocular Surface The purpose of articles in this journal is to help clinicians and researchers stay abreast of developments in the increasingly complex and diverse areas involving the ocular surface. Articles should follow a review format and should be based on representative literature reports. The authors’ own findings may be cited in the context of findings published in the literature, but original work should not be the focus of the review. The purview of The Ocular Surface ranges from molecular biology to surgery, encompassing lacrimal, lid and ocular surface physiology, pathology, pharmacology and medical/ surgical therapeutic interventions. Reviews in this journal are usually not general overviews of topics, but, rather, are indepth, critical reviews that focus on specific areas of the topic. Unsolicited proposals for review articles are welcomed. To propose a review article, please email the following to the Editor-in-Chief, Michael A. Lemp, MD ([email protected].) with a copy to the Managing Editor, Susan Erickson ([email protected]): 1. A narrative statement describing the need for the proposed review and identifying specific controversies, questions, new developments, etc., that you intend to emphasize. 2. A detailed outline of the proposed review. As you formulate the outline for your review, bear in mind that the readership of The Ocular Surface is comprised of clinicians, clinical scientists, and laboratory scientists. Readers

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will have a high level of knowledge in some area(s) related to the ocular surface, but they will not all have detailed knowledge in all areas. Thus, the review should provide in-depth, critical discussion at a level that is understandable and beneficial to basic scientists, as well as more clinically oriented readers. Authors are encouraged to focus on aspects of the topic in which they have special interest, emphasizing particular ideas, controversies, or questions that they feel are exciting and contribute to the understanding of the ocular surface. A review written with this approach will be an original work that reflects the special knowledge and expertise of the authors. Outlines are first reviewed by the editors to determine that they do not overlap substantially with other reviews in development. They then undergo peer review by outside reviewers, who may offer suggestions for revision. All manuscripts are subject to editorial review and revision. Manuscripts will be considered with the understanding that they have not been previously published and are not under consideration for publication in any other journal, book or publicly available electronic source. It may be acceptable to publish portions of the manuscript elsewhere, but this must be approved in advance. Copies of previously published or to-be-considered-for-publication portions of the manuscript should be submitted to Dr. Lemp for evaluation, along with information regarding the other source of dissemination.

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