Excimer laser effects on outflow facility and outflow pathway morphology

PURPOSE. To determine the relative contributions to aqueous outflow resista nce 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, u nroofing Schlemm's canal while leaving the inner wall intact. The laser bea m was masked to produce a beam 2 mm by 1 mm. The laser output was constant at a fluency of 75 mJ/cm(2) and 20 Hz. The excimer laser at a frequency of 1 Hz was used as the aiming beam. Photoablation was performed on human cada ver eyes at the limbus at an angle of 0 degrees to 45 degrees from the opti cal axis. As the excimer photoablations progressed, Schlemm's canal was vis ualized 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 me asured in 7 eyes. RESULTS. The facility of outflow increased in all eyes after the excimer si nusotomy, from a mean of 0.29 +/- 0.02 before the sinusotomy to 0.37 +/- 0. 03 mu l/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 r eduction of outflow resistance of 21.3%. Using the formula of Ellingsen and Grant (1972), percentage of resistance to outflow eliminated = 100 [1 - al pha Co/Ce - (1 - alpha)Co], where alpha = fraction of the circumference dis sected. Assuming that because of circumferential flow approximately 50% of Schlemm's canal is drained by the single opening made in the outer wall abl ation studies, this results in resistance to outflow eliminated of 35%, whi ch is consistent with the calculated eliminated resistance derived from the data of Rosenquist et al., 1989). Light and scanning electron microscopy c onfirmed the integrity of the inner wall Schlemm's canal underlying the are a of ablation. CONCLUSIONS. The results provide direct evidence indicating that approximat ely 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.