Autoregulation of retinal circulation in response to decrease of perfusion pressure.

Autoregulation of the retinal circulation in response to an acute elevation of intraocular pressure was investigated in 17 subjects (23 eyes) with no ocular abnormalities, by means of the blue field entoptic phenomenon. This phenomenon allows a person to observe leukocytes flowing in his own macular capillaries. Subjects were instructed to compare the speed of their leukocytes in one eye with that in the other eye. All subjects perceived equal speed in both eyes. During their observation of the leukocytes, the intraocular pressure (IOP) was rapidly raised in one eye to a level at which autoregulation was not sufficient to maintain normal blood flow. At that level of IOP, subjects described the leukocytes moving slower in this eye than in the fellow eye. The IOP was then decreased in steps of 2 to 3 mm Hg until the subjects reported observing equal leukocyte speeds in both eyes. The IOP at which this occurred, IOPmax, represents the highest IOP (lowest mean perfusion pressure P min) at which the retina is able to maintain normal blood average IOPmax was 29.6 +/- 2.0 mm Hg, corresponding to an average of P min of 27 +/- 6 min Hg and demonstrating that a decrease of 36% or less in perfusion pressure is adequately compensated by retinal vascular autoregulation.