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Underestimate of Tonometric Readings after Photorefractive Keratectomy Increases at Higher Intraocular Pressure Levels

From the Institute of Ophthalmology, Catholic University, Rome, Italy.

PURPOSE. To determine whether tonometric readings of increases in intraocular pressure (IOP) during the water-drinking test (WDT) are affected by variations in central corneal thickness (CCT) induced by photorefractive keratectomy (PRK).

METHODS. Data from 30 randomly selected eyes of 30 patients (18 men and 12 women; mean age, ± SD: 33.9 ± 7.6 years) undergoing bilateral PRK for myopia (–6.57 ± 2.39 D) were obtained. Objective refraction, anterior radius of corneal curvature (R), CCT, and IOP measurements at baseline and at different time intervals after ingestion of 1 L of water within 5 minutes, were performed before and 6 months after PRK. All measured IOPs were recalculated by a correction factor for R and CCT and expressed as corrected intraocular pressure (IOPC) measurements.

RESULTS. The mean R ± SD was 7.84 ± 0.20 and 8.76 ± 0.34 mm, and the mean CCT was 544.83 ± 19.69 and 453.97 ± 29.95 µm, before and after PRK, respectively. The mean IOP at baseline was 15.05 ± 2.78 and 9.83 ± 2.56 mm Hg, and during WDT was 18.32 ± 3.42 and 11.42 ± 3.10 mm Hg at 10 minutes, 18.59 ± 2.99 and 11.54 ± 2.54 mm Hg at 20 minutes, 17.80 ± 2.85 and 10.87 ± 2.22 mm Hg at 30 minutes, 16.35 ± 3.02 and 10.26 ± 2.21 mm Hg at 45 minutes, and 14.90 ± 2.52 and 9.81 ± 2.32 mm Hg at 60 minutes, before and after PRK, respectively. The mean IOPC at baseline was 13.64 ± 2.33 and 13.05 ± 2.98 mm Hg, and during WDT was 16.61 ± 2.77 and 15.08 ± 3.59 mm Hg at 10 minutes, 16.96 ± 2.69 and 15.33 ± 2.96 mm Hg at 20 minutes, 16.10 ± 2.50 and 14.42 ± 2.60 mm Hg at 30 minutes, 14.92 ± 2.72 and 13.62 ± 2.65 mm Hg at 45 minutes, 13.82 ± 2.27 and 13.05 ± 2.55 mm Hg at 60 minutes, before and after excimer laser treatment, respectively. Pre- and postoperative IOPs and percentages of IOP increase differed significantly (P < 0.05), in particular at the peak, as did IOPCs but not the percentages of increase in IOPC, apart from the highest values.

CONCLUSIONS. Corneal changes after PRK for myopia may induce an uneven underestimate of the IOP increases. The inadequacy of a correction factor to compensate for CCT and R at high IOP levels indicates that other biomechanical factors may play a role when the cornea is subjected to dynamic actual IOP variation. Such increase of the well-known underestimate of IOP after PRK at higher actual IOPs may have significant clinical implications in tonometric assessment of subjects at risk of glaucomatous damage.