The Effect of Trabeculectomy on Astigmatism

Purpose: To investigate the change in corneal astigmatism after trabeculectomy. Patients and Methods: Between January and April 2012, patients who underwent a primary trabeculectomy were enrolled in this prospective study. We measured the visual acuity, automated keratorefractometry, and the intraocular pressure preoperatively and postoperatively at 1, 3, and 6 months. Changes in astigmatism were quantified using the vector analysis described by Cravy. A Friedman test and a linear model for longitudinal measures were used to compare changes in the refraction and the intraocular pressure. A binomial test was used to compare the proportion of eyes with a shift in astigmatism with or against-the-rule. Results: A total of 47 eyes (47/48 patients) were included for analysis. The mean (+/- SD) intraocular pressure decreased from 17.5 +/- 5.4 mmHg preoperatively to 9.8 +/- 4.0 mmHg after 6 months (P < 0.001). At this time-point, 32/47 eyes showed a shift in astigmatism with-the-rule (P = 0.02), with a median difference in cylinder of + 0.50D (range, 0 to 4D) (P = 0.004). The mean (+/- SD) axis of the positive cylinder changed from 169 +/- 148 to 135 +/- 146 degrees after 6 months (P = 0.12) and the mean (+/- SD) spherical equivalent changed from -0.47 +/- 2.27 to -0.07 +/- 1.93 D (P = 0.15). The mean (+/- SD) logMAR was 0.17 +/- 0.22 preoperatively and 0.14 +/- 0.14 after 6 months (P = 0.9). Conclusions: Trabeculectomy induced a small but statistically significant shift in astigmatism with-the-rule after 6 months. The spherical equivalent did not change compared with the pre-operative value. Most often, glasses will not need to be changed after trabeculectomy.