Ординатура / Офтальмология / Английские материалы / Wavefront Analysis Aberrometers and Corneal Topography_Boyd_2003

Renato Ambrósio Jr, MD

Marcelo V. Netto, MD

Steven E. Wilson, MD

INTRODUCTION

Since its introduction in the mid 1980’s, computerized videokeratography or corneal topography has provided a superior modality for the diagnosis and treatment of corneal diseases.1 Stephen D. Klyce, Ph.D. deserves credit for pioneering corneal topography and color-coded maps derived from quantitative analysis of numerous points on the mires obtained from videophotokeratoscopy.2 Videokeratography allows for pattern and color recognition facilitating distinction between normal and abnormal corneal topography.

Corneal topography is an essential diagnostic modality for screening and evaluating refractive surgery patients.3-6 Even with the advent of new technologies designed to identify lower and higher order aberrations of the eye (wavefront analysis), corneal topography will remain indispensable. It will always be crucial to have detailed corneal topographic information in order to apply custom alteration of the anterior corneal surface, especially for corneas with irregular astigmatism and other difficult to treat anomalies. 7

This chapter will review the clinical applications of corneal topography for evaluating the corneal surface and diagnosing abnormalities and disease.

COLOR-CODED MAPS AND THE IMPORTANCE OF

STANDARDIZED COLOR SCALES

The raw data used to generate the colorcoded map must be reliable if the resulting colorcoded maps are to be meaningful. When using Placido disc–based instruments it is important to simultaneously examine the videokeratoscope image along with the color-coded topographic map. In this way, the clinician can ascertain whether or not the map is based on a reliable videokeratoscope image that was appropriately processed. For example, both the technician and surgeon can easily detect tear film break up during acquisition of the videokeratoscope image and repeat the study to obtain more reliable information. It is much more difficult to identify unreliable corneal maps with scanning slit technology since these systems do not provide information regarding unreliable analyses.

Corneal topography instruments should leave blank spaces on color-coded maps in areas with mire distortions that cannot be reliably processed. Some available instruments extrapolate missing data from adjacent regions. In our opinion, the latter strategy can often lead to erroneous diagnoses, especially for irregular corneas. Technicians who obtain corneal topography exams must be trained to recognize tear film breakup and other problems that can lead to

erroneous topographic maps. The surgeon should be made aware of rapid tear breakup since this could be important in identification of dry eye and other ocular surface abnormalities.

After confirming the trustworthiness of the photokeratoscope image the surgeon should insure that an appropriate color scale is used. A fixed or absolute color-coded scale facilitates the interpretation of corneal topography (Figure 1). A welldesigned absolute scale provides all the clinically significant information to the surgeon and also includes a broad range that allows most corneas to be

Fig. 1. Axial corneal topography maps showing the importance of the color-coded scales to facilitate the interpretation. The top map is an absolute scale with 1.5 diopter intervals. It shows a cornea with inferior steepening that could represent contact lensinduced warpage or early keratoconus. It could also be normal asymmetry since the overall curvature is in the normal range. With the normalized scale with 0.2 diopter intervals shown on the bottom it appears distinctly abnormal.

Section III: Clinical Applications of Topography

imaged.7 If an optimal absolute scale is used the surgeon becomes comfortable with the meaning of the colors. This color recognition provides immediate information about the cornea being abnormally flat or steep and, therefore, alerts the clinician to corneas that require further analysis.

Most videokeratography systems can also be programmed to provide normalized scales. The computer automatically adjusts the normalized scale to the range of keratometric powers of the particular cornea that is being studied. 7,8 This will lead to loss of color recognition since even normal corneas typically have red colors associated with steep areas and blue colors associated with flat areas (Figure 1). In addition, the normalized scales typically utilize small intervals that highlight clinically insignificant topographic features. Thus, normal corneas frequently appear abnormal when normalized scales are used for analysis. Normalized scales should be used as an adjuvant to compliment analyses obtained with an absolute scale. It should not be used alone.7,8 Popular absolute color-coded scales have include the Klyce/Wilson 1.5 diopter scale and the Waring/Maguire 1.0 diopter scale.7 The consistent use of a particular absolute scale facilitates longitudinal analysis of changes in topography that occur with progression of disease, regression of the effect of refractive surgery, or ectasia. In addition, use of these scales will facilitate communication between clinicians.

In 1999, the American National Standards Institute (ANSI) issued a report entitled "Corneal Topography Systems—Standard Terminology, Requirements" (ANSI Z-80.23 -1999) to encourage use of scales that would allow better comparison topographic maps produced by different manufacturers.9 The use of a single color palette and specific fixed intervals was suggested. However, this report did not specify a single, well-defined color palette, but rather suggested three numeric and color scale combinations: 0.5, 1.0, and 1.5 D. A Universal Standard Scale (USS) was proposed by Smolek, Klyce and Hovis.10 We believe that communication between scientists and clinicians would be facilitated

if all manufacturers of corneal topography instruments would adopt this scale or at least provide it as an option.

Advanced software programs, such as the CTView (Server & Associates, Celebration, FL), allow the clinician to load the raw data files from different corneal topography systems and generate standard maps. This approach allows for comparison of topographic information from different instruments. Other software developed by Server provides a powerful choice of analysis and graphical representations for color maps, point spread functions, and simulated images. This analysis software can be accessed at http://www.sarverassociates.com/.

QUANTITATIVE DESCRIPTORS AND ARTIFICIAL INTELLIGENCE PROGRAMS

Quantitative descriptors of corneal topography consist of indices generated from artificial intelligence programs that facilitate interpretation of topographic information. Different manufacturers offer alternative programs designed to increase the utility of these instruments for clinical and research analyses. The most common index available is the simulated keratometry value (SimK). SimK provides the power and axis of the steepest and flattest meridian similar to values provided by the standard keratometer. For example, with the Nidek-Tomey algorithm they are calculated from data obtained from the rings 7 to 9 of the videokeratoscope image. This generally corresponds to the position on the cornea at which keratometry measurements are obtained.

There are several other programs available from different manufactures that are useful for screening and detecting corneal abnormalities. The surface regularity index (SRI) first proposed and validated by Klyce and Wilson is among the most useful.11 This index is a measure of the regularity of the corneal surface that correlates with best spectaclecorrected visual acuity if the other components of the eye (i.e. the lens, vitreous, macula, and nervous system) are normal. In other words, these measurements

estimate the vision of the eye assuming the cornea is the only limiting factor.11,12 The SRI value increases with increasing irregularity of the corneal surface, with normal corneas having a value less than approximately 1.0. The CIM value provided by the Humphrey Atlas Topographer is a similar measure of regularity of the corneal surface, but its correlation with best spectacle-corrected visual acuity has not been established. Another descriptor is the surface asymmetry index (SAI).11 SAI is a centrally weighted summation of differences in corneal power between corresponding points 180° apart on 128 equally spaced meridians. The SAI approaches zero in the case of a perfectly radial symmetric surface and increases as the cornea becomes more asymmetric. Numerous other indices have been developed. Examples include the asphericity index, shape factor, potential visual acuity, average corneal power, coefficient of variation of corneal power, as well as artificial intelligence algorithms for the detection of keratoconus. These indexes differ significantly from system to system. The clinician should evaluate the utility of the information provided by the specific topographer he or she is using before relying on the information provided. Some indices have been tested in peer-reviewed literature and others have not.

Several automated analyses have been developed to detect keratoconus . Leading examples are the Klyce/Maeda, Smolek/Klyce and Rabinowitz algorithms.13-16 These systems provide useful additional information to augment interpretation of the color-coded map that the clinician can use to decide whether a particular cornea is likely to have keratoconus. Care should be taken when using these automated programs, however, since false positives and false negatives can and do occur. For example, one study compared the sensitivity and specificity of Rabinowitz/McDonnell and Klyce/Maeda methods for diagnosing keratoconus.14 Sensitivity and specificity were 96% and 85% respectively with the Rabinowitz/McDonnell method, and 98% and 99% with Maeda/Klyce method. It is important that the clinician also examine the corneal topography map itself for abnormalities and use other clinical information such as slit lamp examination and pachyme-

try to determine the likelihood that keratoconus or other abnormalities are present. This is particularly important when screening for candidates for keratorefractive surgery. A major concern is that most of these programs were developed to detect keratoconus and may, therefore fail to identify other ectatic corneal diseases such as pellucid marginal degeneration. Thus, if the clinician relies too heavily on automated screening programs, the decision could be made to proceed with surgery when the cornea actually has an abnormality such as pellucid marginal degeneration.17

Some systems offer software packages that include several different programs. For example, the Holladay Diagnostic Summary (HDS) provides 4 maps and 15 corneal parameters that include 2 refractive power maps on standard and normalized scales, a profile difference map for determining the corneal shape relative to normal asphericity, and a distortion map to display the optical quality of the cornea.18 The 15 corneal parameters provide quantitative information about the cornea for a 3.0 mm pupil, such as the effective refractive power, regular astigmatism, asphericity, and predicted corneal acuity. Other instruments provide a different array of computerized algorithms to evaluate corneal topography. The clinician should carefully evaluate the corneal descriptor and analysis programs provided with the model of corneal topographer he or she selects and determine the usefulness and reliability

THE NORMAL PROLATE CORNEA

The ability to detect abnormal topography is based on the interpretation of what is normal. This is true regardless of the particular instrument used for the analysis. The best way for the individual clinician to develop expertise is to routinely examine topographic maps of normal preoperative patients. There are, however, some typical features that are considered "normal."

Normal corneas tend to be steeper in the center than in the periphery. This is referred to as a pro-

Section III: Clinical Applications of Topography

late shape. The asphericity of the prolate corneal shape surface is slightly negative (negative Q value which averages around -0.26).19 Other algorithms, such as the shape factor, measure corneal asphericity in different ways. The Shape Factor (SF) is a measure of corneal asphericity as a derivative of eccentricity. Prolate corneas have a positive Shape Factor. Oblate shapes are those that are flatter in the center and have a negative SF. Refractive surgical procedures that correct myopia typically convert the normal prolate corneal shape to one that is oblate. Thus, the cornea is steeper in the periphery and flatter in the center and is said to be an oblate asphere.

There is progressive flattening from the center to the periphery in the normal cornea. The nasal cornea is typically flatter than the temporal cornea.

Normal corneas have a mean central corneal curvature somewhere in the range of 41 to 46 diopters. Corneas that fall outside this range are not necessarily abnormal, but measurements outside this range should alert the clinician to take care in evaluating the corneal topography. There are, however, many patients with corneas steeper or flatter than this range that appear to be normal by other criteria such as the absence of clinical signs of disease with the slit lamp and normal corneal thickness measured with ultrasonic pachymetry. In many cases there appears to be a genetic component to the mean corneal curvature with several family members having corneas that are steeper or flatter than the range of 41 to 46 diopters.

Most normal corneas have a high degree of radial symmetry over the central and paracentral cornea. A small degree of regular astigmatism is depicted by an oval pattern, while moderate to high levels of regular astigmatism are represented as symmetrical bow-tie patterns. With-the-rule astigmatism is the most common type of astigmatism (vertical bow-tie, Figure 2), followed by oblique astigmatism (oblique bow-tie) and against-the-rule astigmatism (horizontal bow-tie, Figure 3). It is important for clinicians to understand, however, that some normal corneas have considerable asymmetry. A small

degree of inferior steepening relative to the superior cornea is the most common asymmetry. It can be difficult to distinguish this type of mild inferior steepening from early keratoconus.

The topographic patterns of both eyes of a single patient typically have non-superimposable mirror-image symmetry with respect to the vertical midline plane of the body. This is referred to as enan-

tiomorphism.19-21 Thus, if astigmatism is present in one cornea, it is also usually present in the other cornea (Figure 4). When there is asymmetry between the patterns in the two eyes, the clinician must be more vigilant in considering corneal diseases, contact lens warpage, prior corneal surgery, or other possibilities that can generate this asymmetry.

Fig. 4. Symmetry of the right (A) and left (B) corneas of a patient. A normalized scale with 0.25 diopter intervals was used to generate these maps. Note the irregular appearance of the topography for these normal corneas when a scale with such fine resolution is used.

ECTATIC DISEASES OF

THE CORNEA AND CONTACT LENS-INDUCED WARPAGE

Corneal topography is one of the most sensitive methods for the detection of early ectatic corneal disease and for following progression over time. It is also very useful for detecting and evaluating contact lens-induced warpage. These are among the most

Section III: Clinical Applications of Topography

important applications of modern computerized topography.

KERATOCONUS AND PELLUCID MARGINAL CORNEAL DEGENERATION

The topographic patterns of keratoconus (Figures 5 and 6) and pellucid marginal corneal degeneration (Figure 7) are very different.

Although both diseases are ecstatic corneal dystrophies, the difference is relevant in planning therapy. For example, the steep perilimbal area in pellucid marginal degeneration is a factor in fitting rigid gaspermeable contact lenses or planning for corneal transplantation.25 It is not yet clear whether the pathogenesis of keratoconus and pellucid marginal degeneration are different.26-28 Future molecular genetic studies should establish the relationship, if any, between the two diseases. There is evidence for a genetic component to keratoconus.

Keratoconus has also been associated with accelerated levels of keratocyte apoptosis and mitosis.30,31 It is unknown whether or not the latter association is present in corneas with pellucid marginal degeneration.

Computerized corneal topography often provides evidence of ectatic disease before there is significant thinning of the cornea, signs on slit lamp biomicroscopy (Fleischer's corneal epithelial iron ring, Munson's sign, Rizzuti's sign, or Vogt's striae), or scissoring of the retinoscope reflex. In some cases, it is difficult for a clinician to distinguish between true early keratoconus and an asymmetric bowtie or inferior steepening due to contact lens–induced warpage when using corneal topography. Regional pachymetry measurements may yield characteristic clues in some cases. In other cases, only repeated analysis over a period of many years will allow the clinician to reliably distinguish disease from normal.

Scanning slit-based videokeratography instruments, like the Orbscan (Bausch & Lomb,

Orbtek, Inc., Salt Lake City, UT) generate a pachymetry map by calculating the difference in elevation between the anterior and posterior corneal surfaces of the cornea. This could be extremely helpful in imaging regional thinning on the cornea. However, the posterior elevation measurements have been shown to be unreliable.32, 33 A few studies have addressed the accuracy of optical pachymetric data obtained from Orbscan compared with ultrasound. 34-36 These studies have found significant differences between corneal thicknesses measured with ultrasonic pachymetry and scanning slit-based topographers. The data derived from scanning slit technology may be more reliable in normal corneas than in corneas with haze and irregularity (i.e. in corneas where this information would be more useful). Thus, we recommend that clinicians not rely on posterior surface information provided by the Orbscan. Anterior surface information is comparable to other

instruments since a videokeratoscope was introduced into the Orbscan instrument.

The term "keratoconus suspect'", originally suggested by George Waring, M.D. is an appropriate designation for corneas with inferior steepening without corneal thinning or slit lamp signs of keratoconus. One of the most important hallmarks of true keratoconus is progression and in some cases progression over time will be the only conclusive evidence that the disease is present. The best course of action in cases in which there is a suggestion of keratoconus, but no signs that provide for definitive diagnosis, is to follow the patient to see if there is any change in the topographic pattern or corneal thickness. Keratoconus could still be present, however, even if there is no change over a year or two. This follows from studies that have shown that keratoconus often has periods of progression separated by periods of stability.