Introduction
Corneal topography was developed as the result of several progressively more sophisticated technical innovations over the past century and a half. The roots of videokeratoscopy extend back to the first keratometer, described in 1854 by Herman van Helmholtz. His device was later improved upon by Javal, Schiotz, and others. The keratometer measures the corneal radius of curvature at two sets of opposed points approximately 3 mm apart based on the anterior cornea's resemblance to a convex mirror.
Keratoscopy was later developed as a means of obtaining qualitative information about the shape of the entire cornea. In the 1880s, Antonio Placido was the first to photograph the corneal reflections of a series of illuminated concentric rings. Gullstrand improved the development further in 1896 when he devised a method to quantify photokeratoscopic images of the cornea.
Corneal topography as we know it became available in 1984, when Klyce combined computer analysis and digital video into the videokeratoscope. Upgrades to the technology since then have made corneal topography an increasingly integral aspect of refractive surgery.
Color-Coded Scales
Many color-coded scales have been used to depict corneal topographic information. The normalized and absolute scales are the most popular.
A normalized scale is one in which the color intervals vary based on the cornea being examined. The computer selects the dioptric power based on the patient's cornea. The range is typically small for normal corneas (0.4 D to 0.6 D per color). An absolute scale assigns the same color and interval (typically 1 D to 1.5 D per color) for all patients.
An advantage of the absolute scale is that it allows rapid pattern recognition of topographic maps. For example, keratoconus, Terrien’s marginal degeneration, and regular or irregular astigmatism are more easily recognized with an absolute scale. A normative scale may be more sensitive and useful for research. For most work, the absolute scale should be used for routine clinical analysis to facilitate proper diagnosis. Warm colors — red, orange, and yellow — represent steeper portions of the cornea. Green denotes intermediate portions. Cool colors, such as blue, depict flatter portions.
Algorithms for Corneal Surface Power
Power is calculated from the reconstructed surface using various algorithms, including spherically based, axial, instantaneous-curvature, and refractive-power methods. Elevation maps of the height of the corneal surface are available on some instruments. Each of the available methods for reconstruction and presentation of corneal shape has advantages and limitations.
The most widely used topographic power maps use spherically based and axial algorithms. These maps may provide adequate representation of power in the central cornea. Refractive maps may provide a more accurate representation of the peripheral cornea. Power is calculated from the radius of curvature using the formula, P = (n – 1)/r. The axial solution to topography reconstruction disregards the incidence angle of rays at points distant from the corneal apex. Therefore, it provides a valid representation of corneal refractive power only for the central or paraxial corneal regions, with increasing error toward the periphery.
Another type of error introduced by underlying computational algorithms is a smoothing of the data between the measured points. This smoothing primarily affects topographic interpretation at areas of sharp transition, for example, at the edge of laser ablation zones.
Normal Corneal Topography
The normal cornea is aspheric and flattens from the center to the periphery. An aspheric surface that is flatter in the periphery is described as prolate. Reversal of this pattern is typical of all refractive surgical procedures for the correction of myopia. A cornea that is steeper peripherally and flatter centrally is oblate. Usually, the nasal cornea is flatter than the temporal cornea. Spherical corneas have circular central corneal domains of power. If the central power is at the transition of two color intervals, then the overall round pattern can be punctuated by islands that are themselves fairly regular.
A small degree of regular astigmatism is depicted by an oval pattern. Regular astigmatism is depicted as a fairly symmetrical bow-tie pattern. With-the-rule astigmatism is the most common, followed by oblique and against-the-rule astigmatism. The topographic patterns of both eyes of a patient are typically similar.
Quantitative Descriptors of Corneal Topography
Visual inspection of color-coded maps remains the gold standard for interpretation. Even so, quantitative algorithms provide useful indices for detecting abnormalities.
Simulated keratometry (SimK) provides the power and axis of the steepest and flattest meridian similar to values provided by the keratometer. They are calculated from rings 7 to 9 corresponding to the position on the cornea at which keratometry measurements are obtained.
The surface asymmetry index (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 for a perfectly radially symmetrical surface and increases as the corneal shape becomes more asymmetrical.
The surface regularity index (SRI) is calculated from a summation of local power fluctuations along 256 equally spaced meridians. The SRI rises with increasing irregular astigmatism and approaches zero for a smooth corneal surface. Numerous other indices have been developed such as potential visual acuity, average corneal power, coefficient of variation of corneal power, and algorithms for the detection of keratoconus.
Preoperative Considerations of Corneal Topography
Detection of Keratoconus
The detection of keratoconus has assumed greater significance in light of the
expansion of refractive surgery. It has been postulated that weakening of the
cornea after keratotomy and photorefractive keratectomy (PRK) may accelerate
the progression of the disease. Such concerns have been supported by case
reports of adverse complications following incisional keratorefractive surgery.
Keratoconus is a noninflammatory disease in which there is stromal thinning and corneal ectasia. This affects the central and inferior cornea. It is the most common corneal dystrophy, typically presenting in the late teens. The diagnosis relies on the history and assessment of clinical signs. Patients present with progressive myopia, oblique astigmatism, and reduced best-corrected visual acuity. In advanced cases, clinical features include thinning, scarring of the central and inferior cornea, Vogt striae, and a Fleischer ring.
Computerized videokeratoscopy has been shown to be the only reliable way of diagnosing keratoconus in early or mild cases. Topographic maps show a prolate shape of the cornea with an asymmetrical astigmatic bow-tie pattern. The cone lies with its apex inferior to the optical center of the cornea. Poor fixation on upgaze can cause the cone to be replaced with a symmetrical bow-tie pattern. Inferior corneal steepening can result from corneal warpage due to high-riding rigid contact lenses.
Detection of Contact Lens Warpage
Approximately 80% of patients presenting for refractive surgery wear contact
lenses. Topographic anomalies induced by contact lenses include irregular
astigmatism, loss of radial symmetry, and absence of the normal prolate pattern.
It is recommended that patients remove soft contact lenses for at least 1 week
before assessment, and rigid lenses should be removed at least 3 weeks before
an evaluation.
If topographic abnormalities are detected, contact lens wear is discontinued and videokeratoscopic examination is performed 1 month later. If at 1 month the topographic pattern is still abnormal, the patient should refrain from contact lens wear for a further period and return for examination at monthly intervals. It is not uncommon in patients with rigid lenses to have a 1 D to 2 D increase in myopia during this process. The mean time for contact lens warpage to resolve depends on the type of lens: PMMA (15 weeks), gas permeable (10 weeks), and soft (5 weeks).
Corneal Topography after Incisional Refractive Surgery
General Principles
The basic principle of myopic refractive surgery is to flatten the central
cornea. A single radial incision flattens the cornea. The most significant
flattening occurs in the meridian of the incision, and less flattening occurs
in the meridian, 90° from the incision.
Radial keratotomy (RK) involves placing multiple incisions in a symmetrical pattern. The anterior cornea curvature is flattened in a homogeneous pattern. The prolate shape compensates for spherical aberration because the peripheral corneal light rays are refracted less than in a spherical cornea. Therefore, night myopia is less common and less intense in a prolate cornea. In an oblate cornea, because the peripheral light rays are bent more steeply, night myopia and spherical aberration are increased. An oblate cornea also creates a multifocal effect with greater depth of field. Often, uncorrected vision is better than would be expected from the refractive error.
Astigmatic keratotomy (AK) involves placement of transverse or arcuate incisions on the cornea. AK cuts create flattening in the meridian incised and steepening in the meridian 90° from the incision. Coupling, that is, flattening the steep axis and steepening the flat axis, is usually in a 1:1 ratio until astigmatism is greater than 3 D.
Postoperative Management
After incisional surgery, the patient’s subjective opinion does not always
correlate with his/her Snellen acuity. A patient’s satisfaction relies less
on the amount of corneal flattening than on the smoothness of the corneal
contour. Corneal topographic analysis allows both qualitative and quantitative
assessments of the corneal surface.
In the immediate postoperative period, the central cornea (within 1 mm to 1.5 mm of the corneal light reflex) is steeper than the adjacent paracentral cornea. The greatest flattening occurs over RK incisions. The central cornea is still flatter than the preoperative values; however, it demonstrates a relative steepening compared with the paracentral cornea.
Over the ensuing weeks, a generalized flattening occurs. This flattening is greatest over the central and peripheral cornea. The central cornea becomes the flattest region and the cornea gradually becomes oblate. This pattern may not stabilize for 1 to 2 months.
Quantitative analysis of RK results have documented a lack of direct correlation between the changes in visual acuity, cycloplegic refraction, and keratometric measurement. The asphericity of the optical zone has been postulated to be the cause of this discrepancy.
McDonnell and Garbus identified three basic configurations for topography after RK. The most common configuration was round or oval. The second was band-like and was characterized as a large area of flattening that extended across almost the entire interpalpebral zone. The last pattern was split or dumbbell optical zones. In this pattern, there were two regions of flattening 180° apart and separated by relatively steeper areas. Patients with split optical zones almost always had some symptoms of visual disturbance. Inconsistent depth of the RK and AK incisions, as well as variable wound-healing between individual incisions, are common causes for an irregular corneal surface in the postoperative period.
After RK, approximately 25% of patients require enhancement surgery. Residual myopia is the most common indication. Because of a hyperopic shift, most surgeons target for a residual myopia of between 0.5 D and 0.75 D. Once the cornea is stable by serial topography measurements, a cycloplegic refraction should be obtained to determine the residual myopia. Residual myopia can be corrected by incut enhancement, optical zone reduction, or additional incisions.
Corneal Topographic Changes after Laser Refractive Surgery
Centration
Optical zone centration is one of the most important technical variables in
PRK. In most centers, the centration technique relies on patient fixation on
a point coaxial with the laser. PRK, as well as other refractive surgical
procedures, should be centered on the patient’s line of sight and entrance
pupil. The entrance pupil is the virtual image of the real pupil formed by
the cornea. It is located 0.5 mm anterior to the real pupil and is 14% larger.
The region of the cornea in which to change corneal refractive power overlies
the entrance pupil and is centered at its intersection with the line of sight.
In contrast, the center of topography rings represents the corneal vertex, which usually lies nasal to the pupil center. The corneal vertex is the point that intersects at a right angle a line connecting the fixation target with the first Purkinje image of the cornea fixation target. Therefore, the distance of the center of the ablation zone from the center of the entrance pupil should be measured. Decentered optical zones have been postulated to decrease visual acuity; induce astigmatism, glare, and halo; reduce contrast sensitivity; and cause monocular diplopia.
Corneal Topography after Laser Refractive Surgery for Myopia
Corneal topography following PRK for myopia generally reveals a central
circular area of flattening compared with the preoperative values. In reality,
the ablation zone is not necessarily homogeneously flattened. Instead,
well-defined topographic patterns have been described within the ablation
zone following PRK and LASIK. Some patterns of ablation-zone topography
include smooth or uniform, semicircular, keyhole, and central island.
Etiology of Central Islands
The cause of the various ablation-zone topography patterns is unknown.
There are many hypotheses. One possible explanation is that central islands
are a result of an irregularity in the excimer laser beam such that the laser
is not ablating as much centrally as peripherally. It has been hypothesized
that relatively hydrated corneas cause a decrease in excimer laser ablation
per pulse. A flat energy beam profile produces an acoustic shock wave that
tends to drive stromal fluid both toward the center of the ablation and
outside the treatment zone. The central stromal fluid accumulation results
in relatively less ablation occurring centrally.
Other theories include the effects of laser plume vortex, epithelial and stromal wound-healing, and postoperative medication exposure. Management of residual symptomatic islands has most commonly entailed focal excimer retreatment.
Topography of Transition Zone
The border between the ablated and nonablated cornea is referred to as the
transition zone. Optical aberrations occur when the transition zone lies
within the pupil border and with microirregularities of the transition zone.
Optical aberrations tend to occur with smaller optical zone sizes or large
pupils. Multizone PRK or LASIK are potential methods for decreasing the
abruptness of the junction between the ablated and nonablated cornea.
Conclusion
Corneal topography has become a vital tool in the preoperative and postoperative assessment of laser refractive surgery patients. Topography patterns tend to become more regular with time after surgery. Emphasis should be placed on continued attempts to identify and eliminate the causes of irregular topography following PRK and LASIK to avoid visual impairment.
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