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Correction of intraocular pressure for changes in central corneal thickness following photorefractive keratectomy
R Munger1, W G Hodge, G Mintsioulis
1University of Ottawa Eye Institute, Ottawa General Hospital, Ont.
Objective:
To investigate the relation between measurements of intraocular pressure (IOP) and central corneal thickness (CCT) in myopic patients undergoing photorefractive keratectomy (PRK).
Design:
Descriptive study.
Setting:
University-affiliated eye care centre in Ottawa.
Patients:
A total of 481 consecutive eyes of 318 patients with a mean preoperative refractive error of -6.50 dioptres treated with excimer PRK between 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 < or = 0.05). The relation between IOP and CCT suggests corrections for CCT that are 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 microns of corneal thinning preoperatively and at 6 months respectively). On average, there was a significant 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 correlation between change in IOP and change in CCT (decrease of 2.1 mm Hg per 100 microns 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 available we propose that the change in IOP between the preoperative visit (or the fellow eye, if healthy or untreated) and the 12-month visit be used as an individual correction factor to be applied to IOP measurements in the operated eye.