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Abstract #13159 Published in IGR 7-3
Underestimate of tonometric readings after photorefractive keratectomy increases at higher intraocular pressure levels
Tamburrelli C; Giudiceandrea A; Vaiano AS; Caputo CG; Gulla F; Salgarello TInvestigative Ophthalmology and Visual Science 2005; 46: 3208-3213
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 mmHg, and during WDT was 18.32 ± 3.42 and 11.42 ± 3.10 mmHg at 10 minutes, 18.59 ± 2.99 and 11.54 ± 2.54 mmHg at 20 minutes, 17.80 ± 2.85 and 10.87 ± 2.22 mmHg at 30 minutes, 16.35 ± 3.02 and 10.26 ± 2.21 mmHg at 45 minutes, and 14.90 ± 2.52 and 9.81 ± 2.32 mmHg at 60 minutes, before and after PRK, respectively. The mean IOPC at baseline was 13.64 ± 2.33 and 13.05 ± 2.98 mmHg, and during WDT was 16.61 ± 2.77 and 15.08 ± 3.59 mmHg at 10 minutes, 16.96 ± 2.69 and 15.33 ± 2.96 mmHg at 20 minutes, 16.10 ± 2.50 and 14.42 ± 2.60 mmHg at 30 minutes, 14.92 ± 2.72 and 13.62 ± 2.65 mmHg at 45 minutes, 13.82 ± 2.27 and 13.05 ± 2.55 mmHg 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.
Dr. C. Tamburrelli, Institute of Ophthalmology, Sacred Heart Catholic University, Largo F. Vito 1, 00168 Rome, Italy. citam@tiscalinet.it
Classification:
8.4 Refractive surgical procedures (Part of: 8 Refractive errors in relation to glaucoma)