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Joel S. Schuman, Warren Chang, Nan Wang, Annelies W. de Kater, R. R. Allingham; Excimer Laser Effects on Outflow Facility and Outflow Pathway Morphology. Invest. Ophthalmol. Vis. Sci. 1999;40(8):1676-1680.
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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[
1 − αCo/Ce − (1 − 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.
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