CORRECTION OF INTRAOCULAR-PRESSURE FOR CHANGES IN CENTRAL CORNEAL THICKNESS FOLLOWING PHOTOREFRACTIVE KERATECTOMY

Objective: To investigate the relation between measurements of intraoc ular pressure (IOP) and central corneal thickness (CCT) in myopic pati ents undergoing photorefractive keratectomy (PRK). Design: Descriptive study. Setting: University-affiliated eye care centre in Ottawa. Pati ents: A total of 481 consecutive eyes of 318 patients with a mean preo perative refractive error of -6.50 dioptres treated with excimer PRK b etween March 1993 and December 1996. Outcome measures: IOP measured by Goldmann applanation tonometry and CCT before and 3, 6, 12, 18 and 24 months after surgery. Results: CCT was a significant predictor of IOP only before PRK and 6 months after PRK (p less than or equal to 0.05) . The relation between IOP and CCT suggests corrections for CCT that a re not clinically significant (0.81 mm Hg [standard error (SE) 0.33 mm Hg] and 1.00 mm Hg [SE 0.38 mm Hg] per 100 mu m of corneal thinning p reoperatively and at 6 months respectively). On average, there was a s ignificant decrease in IOP after PRK (0.96 mm Hg and 1.24 mm Hg at 12 and 24 months respectively) (p < 0.05). There was a significant correl ation between change in IOP and change in CCT (decrease of 2.1 mm Hg p er 100 mu m of corneal thinning) (p < 0.05). For a given change in CCT , individual changes in IOP were variable, with increases or decreases of more than 5 mm Hg in some cases. Conclusions: There are individual differences in IOP changes following PRK, Until further data are avai lable we propose that the change in IOP between the preoperative visit (or the fellow eye, if healthy or untreated) and the 12-month visit b e used as an individual correction factor to be applied to IOP measure ments in the operated eye.