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Autoregulation of the retinal circulation in response to decrease of intraocular pressure below normal.

The autoregulatory response of the retinal circulation to a short-term reduction in intraocular pressure (IOP) to hypotonic levels was studied in 15 normal subjects by means of the blue-field entoptic phenomenon. This phenomenon allows the perception of the leukocytes flowing in one's own retinal macular capillaries. Subjects were asked to compare the leukocyte speed in one eye with that in the fellow eye while a scleral suction cup was used to raise the IOP in one eye to levels above 25 mm Hg for approximately 12 min. The release of the suction cup caused a drop in IOP to levels between 4 and 7 mm Hg, at which time all subjects reported a higher leukocyte speed (hyperemia) in this eye than in the fellow eye. After an average of 4 min the speed was observed to be equal in both eyes. The average IOP at which the equalization occurred was 6.8 +/- 1.3 mm Hg. The retina can therefore normalize leukocyte capillary speed and presumably blood flow at IOPs at least as low as 6.8 mm Hg. The results of 16 experiments on the same eye of one subject suggest that under these experimental conditions, the lowest IOP for which the retina can fully autoregulate is around 6 to 7 mm Hg.