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Today’s topographers help clinicians detect, prevent potential postsurgical issues
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As refractive surgeons and clinicians require more information about the cornea for effective surgical and treatment outcomes, corneal topographers have evolved to include measurements of corneal curvature and elevation as well as pachymetric and wavefront data.
Today’s topographers can help eye care practitioners identify preoperative risk factors for ectasia as well as postoperative subjective complaints.
Tomography
Placido disc topographers project rings onto the cornea and then analyze them for curvature data. Elevation and corneal wavefront may be calculated as well. While this modality excels at measuring corneal curvature, evaluation is limited to the anterior corneal surface.
Elevation-based tomographers create three-dimensional images from two-dimensional slit images or ocular coherence tomography/placido combinations. Elevation mapping of the anterior and posterior surfaces, as well as pachymetry measurement, is possible.
Note the regular appearance of the anterior elevation (upper left) with a suspicious posterior elevation (upper right). The enhanced elevation maps are shown in the middle row. The anterior map shows a mild elevation while the posterior shows a focal area more elevated than the posterior, indicating forme fruste keratoconus. The fellow eye is fully keratoconic.
Image: Tullo W
Such 3D modeling demonstrates that the posterior surface does not mirror the anterior in cases of keratoconus or keratoectasia. The posterior surface shows elevation prior to changes in curvature on the anterior surface. In rare cases, the anterior surface may appear normal despite early ectasia. Should this occur in a patient seeking keratorefractive surgical correction, it may result in keratoectasia postoperatively.
Computerized analysis of elevation maps also aids in screening for early ectasia, focusing on the skewing of the best fit sphere (BFS) by the cone. The purpose of the BFS is to enable the clinician to identify suspicious areas of the map.
A pachymetry profile for a keratoconus suspect. The keratometry values are 41.4/42.3 @ 45.0º, an oblique angle. The thickness map (lower right) shows the thinnest point of 467 µm, below average. The red line denotes the data for the eye and should fall within the inner and outer lines for the normal population. Should the line fall near or outside the bottom line in either the spatial profile or the percentage thickness increase, ectasia is suspected. In this case, the thickness profile falls off in the periphery, and the percentage is far below expected. .
Image: Tullo W
In keratoconus, the cone’s local elevation will cause the BFS power to increase, masking local differences caused by the cone, as the BFS incorporates the whole surface. If the cone is ignored in the BFS calculation, the suspicious area is highlighted, enabling abnormalities to be easily identified.
Pachymetry
We have learned that patterns of pachymetry may indicate disease prior to other clinical manifestations. In a normal eye, the thinnest point is relatively centered. In a keratoconic eye, it is typically below center and asymmetrical. Indices can indicate the location of the thinnest point, corneal volume, peripheral corneal thickness, corneal thickness distribution and the percentage of thickness increase.
A combination of the enhanced elevation display and the pachymetric assessment in a patient with subjective visual complaints after LASIK. As expected, the keratometry values are reduced secondary to myopic ablation (39.00/40.50@ 58.4º), as is the increased corneal shape factor (Q-value of 0.69). The elevation maps shown on the left depict a mild elevation inferiorly, which appears more significant using the enhanced BFS modeling [middle row, elevation (back)]. The pachymetric thickness increase shows significant variation from normal, suggesting midperipheral ectasia. This would likely be the reason for the patient’s subjective complaint: early ectasia.
Recent investigations by Ambrósio into normal and keratoconic eyes found that significant differences in central corneal thickness, thinnest point, position of the thinnest point and various pachymetric indices existed between groups. Significant differences in pachymetry between eyes are rare and should be considered suspicious. Progression of corneal thickness from the thinnest point toward the corneal periphery and comparison of the pachymetry between the two eyes may also indicate risk for keratoconus and keratoectasia. Falavarjani and colleagues found that a 29.6-micron difference in minimum corneal thickness occurs in less than 0.5% of the population.
Corneal wavefront
Corneal wavefront also enables clinicians to better understand corneal disease, particularly when biomicroscopy fails to identify the etiology. In some cases, the cornea appears normal, but the patient reports significant visual disturbance. This may be due to increased higher-order aberrations (HOAs).
This patient was a suspect for pellucid marginal degeneration. His keratometry values were flat, and the anterior segment was clear. However, topography found mild steepening with horizontal astigmatism and coma on the corneal wavefront. Treatment of these conditions should improve the irregular astigmatism, lessen the higher order aberrations and, subsequently, the visual disturbance.
Many topographic and tomographic systems now create wavefront maps to identify visual problems related to irregular astigmatism. These problems may include ocular surface disease, keratoconus, pellucid marginal degeneration, previous keratorefractive surgery and contact lens distortion. Studies have found an association between the presence of HOAs and corneal disease. The presence of coma is associated with keratoconus and keratoectasia, and trefoil or increased HOAs are associated with pellucid marginal degeneration.
The advanced technology also facilitates corneal disease care.
References:
Ambrósio R, et al. J Refract Surg. 2011;doi:10.3928/1081597X-20110721-01.Falavarjani KG, et al. Clin Exp Optom. 2010;93(1):26-30.
For more information:
Tracy Schroeder Swartz, OD, MS, FAAO, is the center director at Vision America of Huntsville, Ala. She can be reached at tracysswartz@hotmail.com.Disclosure: Swartz has no financial interests to disclose.