Corneal Topography, Keratoconus, Pellucid Marginal Degeneration, and Other Disease States
Corneal topography has become an essential diagnostic function in the era of modern eye care. It is used in evaluation of and planning for all refractive surgery, in planning cataract surgery, and in fitting contact lenses. It can be helpful in troubleshooting visual complaints (ghosting, distortion, etc.) both before and after any contemplated corrective care It is also helpful in monitoring certain medical conditions and diseases of the cornea.
Topography instruments take a video snapshot of the image formed by reflection of a multiple-ring light source off of the cornea. The image of the reflection off of the cornea is called a photokeratoscope. Computerized analysis of the diameter of the ring reflections can measure radius of curvature at specific points and across the entire corneal surface as a whole. The color maps created by this mapping are called topography maps.
Corneal topography mapping is helpful in assessing regularity of curvature and contour; and provides a visual display of corneal astigmatism. It can show whether astigmatism is regular or irregular, and whether the astigmatism is symmetric.
A normal corneal topography study is shown above. The smooth, round, concentric rings are characteristic of normal corneas. The even, symmetric curvature creates a topography map with very little variation across the surface. This is a cornea without astigmatism or topographic asymmetry.
Shown above is a topography study reflecting mild topographic asymmetry. Note the difference in pattern above and below the line of sight. The numeric values within the 4mm optical zone are 45.59 above and 46.28 below, so the difference is only 0.63 D. While this minimal amount of asymmetry is not significant enough to be considered a topographically abnormal cornea, it should guide the careful, conscientious practitioner toward a diagnosis of “form fruste keratoconus” or (“FFKC,” a “frustrated” or mild form of the disease). Patients with FFKC who seek laser vision correction should be dissuaded from having LASIK (sculpting under a thin flap of superficial tissue), but may be candidates for PRK, which renders teh sculpting on the outermost layer of corneal collagen.
The map above shows more significant topographic asymmetry. The numeric references reflect radius of curvature in diopters at specific points. Note that the numeric difference between steepest values in the lower cornea (49.03) and upper cornea (46.65) is ~2.5 diopters. There is some difference of professional opinion on exactly how to define the lower limit of acceptable asymmetry, but in general, there is consensus that if measurements differ by more than 1.4 D at the 4.0 mm optical zone (diameter) this defines an abnormal amount of asymmetry. Therefore, the cornea in the map shown above is an abnormal cornea.
Topography and photokeratoscope maps can disclose pathologic conditions such as keratoconus, in which there is distortion of surface curvature, significant topographic asymmetry, and abnormal steepening of the central area of the cornea (orange and red in the map). This causes the rings to appear egg-shaped. Keratoconus (often abbreviated “KC”) is a structural disorder of the cornea where abnormal, asymmetric thinning of tissue (usually below and nasal to the center), causes bulging or abnormal steepening of curvature over the thin area. This creates a non-uniform optical surface that creates distortion, ghosting, and blur that cannot be correctable by any lenses in glasses, or by soft contact lenses. Imagine the difference between looking through window glass and hand-blown glass, and you can begin to appreciate the visual challenge that patients have who are afflicted with KC, PMD, or other pathology states characterized by abnormal, asymmetric, distorted curvature.
There are three hallmarks of eyes with abnormal curvature: topographic asymmetry, abnormal thinning, and abnormal maximum steepness. Normal corneas have central radius measurements below 49 D, so those eyes with maximum curvature above this value are considered abnormal, or pathologic. Standardized color maps are designed to display curvature values within the normal range in shades of green or yellow, while values in the abnormal range display in orange or red.
Keratoconus and PMD in aggregate affect approximately 1 in 700 people in the US. People with these conditions may need specialty contact lens services or other measures (UV corneal cross-linking or CXL, see below)to treat mild or moderately severe conditions. Left untreated, keratoconus and PMD can often be progressive, showing greater asymmetry and more severe steepening over time. In cases of advanced KC, if contact lenses and CXL can’t can’t stabilize or can’t be tolerated, it may be of benefit to remove and replace the damaged, abnormal tissue with normal tissue from a donor. This is called corneal transplant care and is reserved only for the very most advanced cases.
Maximum central radius is in the range of 76 diopters. Keratoconus should be suspected whenever the central radius exceeds about 49 or 50 diopters.
Another type of abnormal corneal topography is called Pellucid Marginal Degeneration or PMD. Like keratoconus, PMD is characterized by profound topographic asymmetry and abnormal steepening. In PMD, the thin cornea is more peripheral and the map pattern is different. Some refer to the areas of red in the map as resembling a crab claw. Note the very distorted, non-circular ring reflections of this distorted surface. The photokeratoscope map shows much more ovoid or egg-shaped rings than circular.
Shown is an axial curvature map of an eye with advanced keratoconus. The numeric scale on the left is small, but shows that curvature ranges from 37 diopters on the low end to above 53 on the high end. Note also that the area of steepest curve (most red on the display) is not in the center of the cornea.
Keratoconus and PMD are forms of ectasia, or uncontrolled thinning of corneal tissue. In areas where the tissue is thinnest, the biomechanical stability is consequently weakened, and the tissue will bend more readily in this area than in an area of normal thickness. For this reason, areas of thinning are associated with topographic steepening of curvature. In keratoconus, the abnormally thin and steep tissue is in the central or paracentral area of the cornea. In PMD the areas of thinning are more peripheral, typically in the lower cornea.
Conventional laser vision correction treatment cannot correct topographic asymmetry. Furthermore, since corneas in keratoconus and PMD are abnormally thin, and laser sculpting would cause even more thinning, these are not conditions amenable to traditional laser therapy. However, newer modalities of treatment are evolving, including UV corneal collagen cross-linking (“CXL”), and topography-guided laser therapy with UV cross-linking.
To read more about abnormal topography and asymmetric topography, see Assessment of High Astimgatism and Keratoconus.