Excimer laser effects on outflow facility and outflow pathway morphology

Purpose. To determine the relative contributions to aqueous outflow resistance of the tissues distal to the inner wall of Schlemm's canal. Methods. While performing constant pressure perfusion at 10 mm Hg, a 193-nm excimer laser (Questek) was used to precisely remove portions of sclera, unroofing Schlemm's canal while leaving the inner wall intact. The laser beam was masked to produce a beam 2 mm by 1 mm. The laser output was constant at a fluency of 75 mJ/cm² and 20 Hz. The excimer laser at a frequency of 1 Hz was used as the aiming beam. Photoablation was performed on human cadaver eyes at the limbus at an angle of 0°to 45°from the optical axis. As the excimer photoablations progressed, Schlemm's canal was visualized by the fluorescence of the Barany's solution containing fluorescein dye. After perfusion fixation the eyes were immersion-fixed overnight. The facility of outflow before (Co) and after (Ce) the excimer ablation was measured in 7 eyes. Results. The facility of outflow increased in all eyes after the excimer sinusotomy, from a mean of 0.29 ± 0.02 before the sinusotomy to 0.37 ± 0.03 μl/min per mm Hg after (P < 0.05). The mean ratio of outflow facility after and before ablation (Ce/Co) was 1.27 ± 0.08 (range, 1.20-1.39), a reduction of outflow resistance of 21.3%. Using the formula of Ellingsen and Grant (1972), percentage of resistance to outflow eliminated = 100 [I - αCo/Ce - (I - α)Co], where α = fraction of the circumference dissected. Assuming that because of circumferential flow approximately 50% of Schlemm's canal is drained by the single opening made in the outer wall ablation studies, this results in resistance to outflow eliminated of 35%, which is consistent with the calculated eliminated resistance derived from the data of Rosenquist et al., 1989. Light and scanning electron microscopy confirmed the integrity of the inner wall Schlemm's canal underlying the area of ablation. Conclusions. The results provide direct evidence indicating that approximately one third of resistance to outflow in the human eye lies distal to the inner wall Schlemm's canal in an enucleated perfused human eye.