Retinal nerve fiber layer measurements before and after photorefractive keratectomy

PURPOSE: To determine whether the nerve fiber analyzer GDx with the new variable corneal compensation (VCC) device allows measurements of retinal nerve fiber layer parameters independent from birefringence changes of the cornea after photorefractive keratectomy (PRK). METHODS: Retinal nerve fiber layers of 32 eyes in 32 myopic patients undergoing PRK were analyzed using the GDx-VCC. Photorefractive keratectomy was performed using a NIDEK EC-5000 excimer laser. Measurements were obtained before and after PRK without dilating the pupil. To evaluate birefringence changes, GDx-VCC macular images also were taken before and after PRK. RESULTS: After PRK, superior and inferior ratios, superior/nasal max modulation, and temporal-superior-nasal-inferior-temporal average changed significantly (P <.001) when corneal birefringence correction was not applied. When corneal birefringence correction was applied, only temporal-superior-nasal-inferior-temporal average and superior maximum improved significantly. Furthermore, a comparison of the postoperative data without polarization correction to the data with corrected corneal polarization demonstrated significant changes in superior ratio (P=.004), superior/nasal (P<.001), ellipse modulation (P=.007), and temporal-superior-nasal-inferior- temporal standard deviation (P=.009). CONCLUSIONS: Our results show the VCC algorithm is able to compensate for most of the changes in corneal birefringence induced by corneal refractive surgery if the polarization has been re-calculated. Because mild changes in GDx parameters could affect the interpretation of the results in some patients, a new postoperative baseline macular image should be acquired.