Autoregulation of human optic nerve head blood flow in response to acute changes in ocular perfusion pressure

Background: Studies in animals have demonstrated that optic nerve head (ONH) blood flow (F_onh) is autoregulated, but there is a lack of evidence for such a process in humans. Therefore, we investigated the relationship between F_onh and mean ocular perfusion pressure (PP_m) in normal volunteers when PP_m is decreased through elevation of the intraocular pressure (IOP). Methods: Laser Doppler flowmetry (LDF) was used to measure relative mean velocity (Vel_ohn), volume (Vol_onh) and F_onh of blood at sites of the ONH away from visible vessels, while PP_m was decreased in two ways: (1) rapidly, by IOP increments of 15 s duration, and (2) slowly, by IOP increments of 2 min duration, both by scleral suction cup in one eye of each of nine subjects. Results: A rapid and large decrease of PP_m of more than 100% induced a decrease of more than 80% in F_onh. With the slower decrease in PP_m F_onh remained constant down to a PP_m of 22 mm Hg (IOP=40 mm Hg) and then decreased, predominatly due to a decrease in Vel_ohn. Immediately after removal of the suction cup, F_onh increased transiently by 44% above baseline. Conclusions: This study demonstrates efficient blood flow autoregulation in the OHN, which is probably brought about by an increase in vascular capacitance. The magnitude of the reactive hyperaemia agrees with the compensatory decrease in ONH vascular resistance during IOP elevation. The time scale of the autoregulatory process and the dependence of the hyperaemia upon duration of IOP elevation suggest a metabolic mechanism of autoregulation.