Tonometry underestimates IOP after hyperopic LASIK

ZERIFIN, Israel — Applanation tonometry tends to underestimate true intraocular pressure in patients who have had LASIK for hyperopia, according to a study here.

David Zadok, MD, and colleagues here and in Jerusalem took IOP measurements in 108 eyes of 58 patients using Goldmann applanation tonometry at baseline and at 6 months or more postop. Patients were between 19 and 67 years of age with hyperopic spherical equivalents that ranged from +0.6 D to +10.25 D and no more than 3 D of astigmatism. LASIK was performed with either the Automatic Corneal Shaper or Hansatome (Bausch & Lomb) microkeratomes. Goldmann tonometry measurements were obtained before treatment and at each postop visit. Tonometry was performed between 2 p.m. and 7 p.m.

Mean spherical equivalent at the last visit was +0.47 D. Mean Goldmann IOP measurements dropped from 13.8 mm Hg before LASIK to 11.6 mm Hg at the last follow-up (P < .00001). Postoperative IOP measurements dropped by 2.5 mm Hg in the ACS group and by 1.9 mm Hg in the Hansatome group.

“We found a 2.2 mm Hg mean reduction in the tonometer reading, theoretically preserving central corneal thickness. Thus, the decrease in measured IOP is not the result of a change in central corneal thickness,” the study authors report in the September issue of Ophthalmology.

The researchers could not demonstrate a correlation between amount of spherical equivalent treatment and IOP change, indicating the complicity of factors. They said surgeons must gain a better understanding of the cause for variation in IOP measurement after hyperopic LASIK to better assess patients’ true IOPs.