Argon laser trabeculoplasty and pilocarpine effects on outflow facility in the cynomolgus monkey

We sought to determine whether and how prior argon laser trabeculoplasty (ALT) alters outflow facility, the pilocarpine-induced facility increase, and the morphology of the trabecular meshwork in cynomolgus monkeys.

Eleven cynomolgus monkeys underwent unilateral 360 degrees argon laser trabeculoplasty, eight in two 180 degrees sessions (seven 3 months apart, one 11 days apart) and three in a single session. Intraocular pressure (IOP, Golmann ap-planation tonometry), aqueous humor flow rate (AHFR, fluorophotometry), total outflow facility (two-level constant pressure perfusion) and response to intracameral pilocarpine, and morphology (light and scanning/transmission electron microscopy) were determined at various times.

Resting total perfusion outflow facility 3 and 6 months after 360 degrees ALT was 30% higher in ALT-treated than in contralateral untreated eyes. There were no consistent or statistically significant differences between ALT-treated and contralateral untreated eyes in (a) IOP or AHFR before ALT, 5 weeks after initial 180 ALT, or 5 weeks after the second 180 ALT; (b) post-intracameral pilocarpine facility, or the dose-dependent facility response to pilocarpine 10 weeks after the initial 180 ALT or 3 and 6 months after the second 180 ALT; or (c) light or electron microscopic appearance of the trabecular meshwork or inner wall of Schlemm's canal 3-17 months after 360 ALT.

This study indicates that (a) ALT alters the biomechanical properties of the monkey trabecular meshwork so as to modestly increase its hydraulic conductivity without affecting the facility response to pilocarpine-induced ciliary muscle contraction; and (b) young healthy monkey outflow tissues exhibit remarkable reparative and homeostatic capabilities.