Accuracy of schiotz tonometry in measuring the intraocular pressure after corneal refractive surgery

PURPOSE: To test the accuracy of Schiotz tonometry (ST) in patients that underwent photorefractive keratectomy (PRK). METHODS: 175 eyes previously treated with PRK for myopia, myo-pic astigmatism or mixed astigmatism, with a mean preoperative spherical equivalent (SE) of –4.27±2.83 diopters (range: -13 to –0.25 diopters), had their intraocular pressure (IOP) measured, at least 3 months after PRK, with Goldmann applanation tonometry (GAT) and ST with both 5.5-g and 10-g load and also utilizing the Friedenwald nomogram. These measurements were retrospectively compared to the IOP readings obtained with GAT before surgery. RESULTS: Before PRK the mean IOP, as measured with GAT, was 14.7±2.63 mmHg. After PRK, the mean IOP readings were 10.9± 2.85 mmHg (P<0.001) with GAT, 13.58±3.42 mmHg (P<0.001) with 5.5-g load ST; 12.32±3.67 mmHg (P<0.001) with 10-g load ST, and 16.19±5.63 mmHg (P<0.001) with the Friedenwald nomogram. The different methods showed a poor agreement, the worst one being between GAT before surgery and the Friedenwald nomogram (r2=0.05). Moreover, ST with both 5.5 g and 10 g load showed an underestimation (more pronounced for the 10-g than for the 5.5-g load) that increased with the amount of treatment. CONCLUSIONS: Although ST with a 5.5 g load seems to be more accurate to measure the true IOP in patients that have undergone PRK compared to the Goldmann technique, it cannot be conside-red to be accurate in a sufficient number of cases. IOP measurement after refractive surgery is still a problem, and further studies are needed to overcome it.

Purpose: To test the accuracy of Schiotz tonometry (ST) in patients that underwent photorefractive keratectomy (PRK). Methods: 175 eyes previously treated with PRK for myopia, myopic astigmatism or mixed astigmatism, with a mean preoperative spherical equivalent (SE) of-4.27±2.83 diopters (range:-13 to-0.25 diopters), had their intraocular pressure (IOP) measured, at least 3 months after PRK, with Goldmann applanation tonometry (GAT) and ST with both 5.5-g and 10-g load and also utilizing the Friedenwald nomogram. These measurements were retrospectively compared to the IOP readings obtained with GAT before surgery. Results: Before PRK the mean IOP, as measured with GAT, was 14.7 ±2.63 mmHg. After PRK, the mean IOP readings were 10.9 ±2.85 mmHg (P<0.001) with GAT, 13.58 ±3.42 mmHg (P<0.001) with 5.5-g load ST; 12.32 ±3.67 mmHg (P<0.001) with 10-g load ST, and 16.19 ±5.63 mmHg (P<0.001) with the Friedenwald nomogram. The different methods showed a poor agreement, the worst one being between GAT before surgery and the Friedenwald nomogram (r2=0.05). Moreover, ST with both 5.5 g and 10 g load showed an underestimation (more pronounced for the 10-g than for the 5.5-g load) that increased with the amount of treatment. Conclusions: Although ST with a 5.5 g load seems to be more accurate to measure the true IOP in patients that have undergone PRK compared to the Goldmann technique, it cannot be considered to be accurate in a sufficient number of cases. IOP measurement after refractive surgery is still a problem, and further studies are needed to overcome it. © 2008 Spanish Council of Optometry.