Depth of penetration of scanning laser Doppler flowmetry in the primate optic nerve.

OBJECTIVES: To estimate the measuring depth of the blood flow and to establish the vascular contributions to these measurements with scanning laser Doppler flowmetry (SLDF) of the primate anterior optic nerve. METHODS: Optic nerve blood flow in each eye of 8 monkeys was measured using SLDF before and following surgical occlusion of the central retinal artery (n = 4) or posterior ciliary arteries (n = 4). The regional blood flow in both eyes was determined using a nonradioactive microsphere method. RESULTS: The blood flow in the nerve fiber layer (NFL), including the prelaminar region, was measured with microspheres after central retinal artery occlusion; it was significantly reduced (-83%) with no significant change in the combined laminar and retrolaminar regions. The blood flow measured with SLDF had a 51% reduction. After posterior ciliary artery occlusion, the blood flow in the NFL was measured with microspheres and was not significantly affected (+2%); neither was that measured with SLDF (-12%). However, there was a 51% reduction in the laminar and retrolaminar regions when microspheres were used. The mean +/- SD tissue thickness of the NFL was 359 +/- 16 microm and 353 +/- 54 microm in each group. CONCLUSIONS: Scanning laser Doppler flowmetry measures blood flow principally in the NFL of the anterior optic nerve, which is primarily supplied by the central retinal artery. Blood flow in the laminar and retrolaminar regions makes a small contribution to the SLDF measurement, with an NFL thickness between 300 and 400 microm. CLINICAL RELEVANCE: Scanning laser Doppler flowmetry is used for the noninvasive evaluation of ocular microcirculation in diseases such as glaucoma. Because of the dual blood flow supply in the optic nerve and the limited penetration power of the laser, the instrument primarily measures the microcirculation on the surface of the optic nerve, which is largely supplied by the central retinal artery rather than the ciliary arteries.