orthokeratology today
Central Island Topography Patterns with Orthokeratology
BY JOHN MARK JACKSON, OD, MS, FAAO

Corneal topography is an essential tool for evaluating corneal changes during orthokeratology treatment. The typical desired end result of therapy shows a well-centered, circular zone of central flattening surrounded by a ring of steepening. Other, undesirable patterns can result if the ortho-k lens does not fit properly.

One type of anomalous pattern is a central island. This type of map looks similar to a good treatment map except for a small area of central steepening. Clinical significance varies depending on the severity of the steepening. In mild cases it simply limits treatment and undercorrects the patient; in more dramatic cases, the patient's myopia may actually increase.

Figure 1. Central island pattern with ortho-k indicating a poor fit.

Identifying Central Islands

A difference or subtractive map best evaluates any type of ortho-k topography. Set the topo-grapher to run a differ-
ence map that compares baseline and post-treatment data. The difference map will show how much change actually occurred, and it is a better diagnostic tool than the post-treatment map in isolation. What may appear to be a central island on an individual map may disappear in the difference map.

Although the exact cause of central island patterns remains unknown, they may result from a lens that has slightly excessive sagittal height, which causes the lens to bridge slightly over the apex of the cornea. This may cause tissue to migrate centrally rather than peripherally and result in the central steepening. How you correct the problem will depend on the lens design, but it will entail decreasing the sag depth of the lens, usually by flattening the reverse curve, the alignment curve or both. Your laboratory can guide you in these changes.

	Figure 2. Normal post-treatment map with central flattening.

Central Island Effects

This case shows how dramatically a central island can affect a patient. This patient's baseline Rx was ­4.50 ­1.00 x 075 20/20 OD. Figure 1 shows the difference map for this eye after two nights of lens wear. The central island is evident. The refractive error had increased to ­7.75 ­1.00 x 075 20/20 during this time.

The patient discontinued lens wear, and the eye returned to baseline within three days. We then refit a lens with reduced sag depth. Figure 2 shows a typical ortho-k pattern, which resulted after one month of wear. The final Rx was +0.50 ­0.50 x 060 20/20, with 20/20 uncorrected acuity. This is a dramatic case, but it illustrates what can happen with an improper fit.

Dr. Jackson is an assistant professor at Southern College of Optometry where he works in the Advanced Contact Lens Service, teaches courses in contact lenses and performs clinical research.