Topographic steep central islands following excimer laser photorefractive keratectomy (PRK)

Background: The purpose of this study is to demonstrate that topographic irregularities in the form of central islands of higher refractive power can be seen following excimer laser refractive surgery. Methods: We reviewed the computerized corneal topographic maps of 35 patients undergoing excimer laser PRK for compound myopic astigmatism or anisometropia from 8/91 to 8/93 at the USC/Doheny Eye Institute. The topographic maps were generated by the Computed Anatomy Corneal Modeling System, and central islands were defined as topographic areas of steepening of at least 3 diopters and 3 mm in diameter. A grading system was developed based on the presence of central islands during the postoperative period as follows: Grade 0: absent, Grade 1: present at 1 week, Grade 2: present at 1 month, Grade 3: present at 3 months. Results: A total of 70%, 50% and 20% of patients had steep central island formation at 1 week, 1 month and 3 months, respectively. In the absence of nitrogen gas blowing, over 64% of patients had grade 2 or 3 central islands (present at 1 month and beyond) in comparison to only 20% when gas blowing was present. In those eyes with grade 2 islands, 80% had loss of best corrected visual acuity at 1 month with improvement of vision at 3 months when the island was no longer present. Conclusion: Visually significant topographic steep central islands may be seen in over 50% of patients at 1 month following excimer laser PRK, and persist at 3 months in up to 24% of patients without nitrogen gas blowing. Loss of best corrected visual acuity or ghosting is associated with island formation, and may prolong visual rehabilitation after excimer laser PRK.