One of the most serious topographical complications of excimer laser PRK and LASIK is a form of irregular astigmatism known as a central island. Although the condition is relatively uncommon, its potential for increasing optical aberrations can be very serious. The differential diagnosis of central islands includes irregular epithelial healing, increased corneal prolateness, and progressive ectasia. There is a greater potential for a central island to form following laser treatment with broad beam lasers than with scanning lasers, as well as in patients with large ablation diameters and greater dioptric corrections. The difference between central islands and increased postoperative corneal prolateness may be very subtle; it is likely that a central island may be desirable after surgery if it is negligible, as it is able to preserve negative asphericity.
Visually significant central islands have been reported more often following PRK, but they may be more likely to persist after LASIK. A recent review showed a 5.7% incidence of central islands following LASIK, and only 25% of those resolved over a 6-month period. Because of the limited regression that may occur in these patients, surgical intervention is often required. The following information illustrates the approach and surgical treatment of central islands after LASIK.
Rule out other possibilities
Prior to attempting surgical intervention, it may be valuable to monitor the patient for 3 to 6 months to rule out progressive corneal ectasia, and determine whether the central island has regressed or progressed. Corneal ectasia may masquerade as a central island, and is more likely to progress or show minimal improvement over time. It also may worsen if retreated. If during this follow-up period there is improvement in both symptoms and the appearance of the central island, is it more than likely that the patient has a true central island as opposed to a pseudocentral island caused by corneal ectasia.
Surgical Intervention
If a true central island does not regress to a satisfactory level, it may require surgical intervention.1 Prior to treating a central island, the surgeon must determine the power (in diopters) and the diameter (in millimeters) of the island, as well as the patient's manifest refraction. The power of the central island can be determined by placing the cursor at the apex, and then comparing this point to the average at the base of the central island. By moving the cursor to the desired locations, or by simply printing out a numeric map and/or a profile map (which are available on most systems), the central island power can be calculated. The size of the central island can be determined using a grid overlay in which each box represents 1 millimeter.
Compare the power of the central island to the refraction, and use this to determine the desired correction. If the central island power matches the refraction, you can use Munnerlyn's equation to determine the number of laser pulses necessary for treatment: Ablation depth = S2 • D/3, where S = the diameter of central island in millimeters and D = the desired correction in diopters. Therefore, if a central island that is 3 mm in diameter and 4 D in height has resulted in 4 D of induced myopia, it will require 12 mm (32 • [4/3]) of tissue ablation. For most lasers, each laser pulse removes approximately 0.25 mm of tissue, and therefore 48 pulses would be required.
If the power and diameter of the central island predict a significantly different number of laser pulses as compared to the manifest refraction, then the treatment may require a two-step approach. The central island should be treated based upon the refractive data, ignoring the diopteric height (step 1). The prescription is monitored during follow-up visits. If this process topographically eliminates the central island, yet a refractive error still exists, then further ablation may be required after stabilization is established 3 to 6 months later.
Topography-linked lasers and wavefront analysis will become more important in the future when assessing central island position and optical effect. Currently, a patient with a central island is monitored based upon subjective symptoms and elevation-based topographical analysis. As wavefront technology becomes more commonplace, we will be able to better understand the amount of distortion that a specific central island is creating. Such an objective measurement of the regression of a central island may allow us to improve how we monitor these complications.
Dimitri T. Azar, MD, is Associate Professor of Ophthalmology at Harvard Medical School. He also serves as Director, Corneal and Refractive Surgery Services, and Associate Chief of Ophthalmology at the Massachusetts Eye and Ear Infirmary in Boston. Dr. Azar may be reached at (617) 573-4440; dazar@meei.harvard.eduThe author would like to thank Jeffrey D. Johnson, OD, for his assistance with data collection.
1. Johnson JD, Azar DT: Surgically induced topographical abnormalities after LASIK: Management of central islands, corneal ectasia, decentration, and irregular astigmatism. Curr Opin Ophthalmol 12:309-317, 2001
