The Effects of Trabeculectomy on the Fast and Slow Components of Visual Field Rates

Abstract

Purpose: To investigate the effects of trabeculectomy on the fast and slow components of visual field (VF) rates using two pointwise regression techniques.

Methods: Open-angle glaucoma patients with five or more reliable visual fields prior to and following trabeculectomy were included. Pointwise VF location rates were determined by using exponential and linear regressions. The visual field rates were partitioned into a faster and a slower component using a technique described recently. The rates (regression slope) of the identified locations within each of the two components were then analyzed for changes between the preoperative and postoperative periods. The "units" for Pointwise Exponential Regression (PER) rates prior to and post trabeculectomy were "%/year" and for linear regressions were "dB/year". The findings were then compared with the effects of surgery on the MD (using linear regression) of the visual fields.

Results: Seventy eight patients (96 eyes) were analyzed. The average follow up and number of visual fields (+SD) were 5.3 (± 3.01) years and 11.29 (± 8.67) prior to and 5.97 (± 2.82) years and 10.15 (± 5.46) post trabeculectomy, respectively. The average (± SD) fast rate with PER prior to surgery was 31.5 (± 32.8) %/year (-1.26 ± 1.42 dB/year with linear regression) and this slowed to 5.6 (± 35.8) %/year (-0.24± 1.10 dB/year with linear regression) (p <0.001) after trabeculectomy. The average (± SD) slow rate with PER before surgery was −8.6 (± 17.8) %/year (+0.54 ± 1.32 dB/yr; using linear regression) and this increased to 5.0 (± 18.8) %/year (−0.09 ± 1.02 dB/year; using linear regression) (p <0.001) after surgery. The MD rate of decay also showed a slowing down from −0.58 (±1.26) dB/year to 0.20±1.193 dB/year (p<0.001)

Conclusions: Trabeculectomy has a differential effect on the slow and fast components of VF’s. The progression rate of the fast component of VF decay is significantly slowed while the slow component rate is slightly increased. These differential effects of filtration surgery may be diminished or masked when global indices are used to assess outcomes, and could lead to an underestimation of the beneficial effects of surgical treatment of glaucoma.

Keywords: 758 visual fields