Improvement of Tonographic Outflow Facility Following Selective Laser Trabeculoplasty (SLT) in Open Angle Glaucoma

Abstract

Purpose:: Selective Laser Trabeculoplasty (SLT) has been hypothesized to increase trabecular outflow although the precise mechanism for the decrease in IOP following SLT therapy is not yet known. This study evaluated the changes in tonographic aqueous outflow facility following SLT therapy in patients with open angle glaucoma (OAG) and ocular hypertension (OHT).

Methods:: A retrospective review of charts of 9 eyes of 7 subjects with either OAG or OHT who received SLT therapy was conducted. A 2-minute pneumotonography (Meditronic Solan Model 30) was performed at 1 week before and 6 weeks after SLT. The change in IOP, coefficient of facility of aqueous outflow (C-value, µl/min/mm Hg) and Po/C, where Po denotes resting baseline IOP, were determined.

Results:: Four eyes were diagnosed as OHT, three eyes as pseudoexfoliation glaucoma and two eyes as primary open angle glaucoma. There were 6 females and 1 male with a mean age of 69 ± 9.9 years. 6 eyes received SLT as a primary therapy without any additional therapy and 3 eyes received SLT as a secondary therapy without any changes in glaucoma medications. Five eyes received 360 degree and the four eyes received 180 degree of SLT. Mean baseline applanation IOP prior to SLT therapy was 24.8 ± 1.8 mm Hg which decreased to 16.8± 3.3 mm Hg (mean IOP reduction of 8.2 ± 4 mm Hg or 31% ± 14%) at 6 weeks after SLT therapy. The outflow facility (C value) improved in all eyes except one eye of OHT in which it remained unchanged despite reduction in IOP by 33%. Prior to SLT therapy the mean value of outflow facility (C value) was 0.24 ± 0.11 and the mean Po/C 103.9 ± 51. At 6 weeks following SLT therapy the mean C value improved to 0.35 ± 0.08 (or by 66%) and mean Po/C improved to 56.7 ± 47 (or by 45%) which were statistically significant (P value 0.04 and 0.01 respectively).

Conclusions:: SLT treatment significantly improved tonographic outflow facility in 8/9 eyes. This supports the hypothesis that SLT reduces IOP most likely by an effect on trabecular outflow. Given that one patient showed a decrease in IOP without a change in outflow facility suggests other mechanisms of IOP reduction may also occur, such as uveoscleral outflow, which was not measured in this study.

Keywords: intraocular pressure • laser • trabecular meshwork