Fig. 23. Laminar flow sign. Nonpenetrating filtration surgery is a systematic layer by layer dismantling of the iridocorneal outflow system. A. Appearance of episclera before injecting balanced salt solution (BSS) into the ostium of Schlemm's canal. The episcleral venous system is easily seen, with particular reference to a large scleral collector (black arrows). B. Evidence of enhancement of outflow can be found by simultaneously observing the episclera vessels and carefully injecting BSS into the canal (white arrow). A rapid blanching of vessels is easily seen. This “white out” phenomena or laminar flow sign (blue arrows) is a sure sign of correct anatomic location. Observation of this phenomenon increases awareness of one possible mechanism of postoperative pressure reduction.