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John V. Lovasik, Helene Kergoat, Mireille Parent, Michael G. Quigley; Bloodflow Regulation In The Optic Nerve Head During Prolonged Elevation Of The Intraocular Pressure. Invest. Ophthalmol. Vis. Sci. 2012;53(14):6839.
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Vascular autoregulation (AR) for the eye refers to the ability of a vascular bed to maintain constant blood flow (BF) despite changes in the ocular perfusion pressure (OPP) that arise from variations in the BP or IOP. AR in the large retinal vessels occurs by modulation of vessel caliber. Perfusion of the optic nerve head (ONH) is entirely by a capillary network. Laser Doppler flowmetry (LDF) was introduced by Riva et al. for quantifying capillary blood flow, volume, and velocity in the ONH, but it’s sophistication, limited availability and measurements limited to short intervals have restricted its use to laboratory research. Consequently there is a remarkable paucity of needed clinical studies on AR in the ONH even though subnormal perfusion is implicated in low-tension glaucoma and standard therapy is directed at decreasing IOP for improving BF. The objective of this study was to determine if the effects of an acute reduction in the OPP could be measured continuously over extended periods using computer-based imaging.
Five healthy experienced adult volunteers (54.6 ±8.8 yrs of age) participated in this study. The pupil of the test eye was dilated. A suction cup, to increase the IOP, was placed on the temporal sclera ~2mm from the cornea after instillation of 2 drops of a topical anesthetic and 2 drops of a topical lubricant. Testing consisted of a 2-min baseline, 4-min increased IOP, and a 2-min recovery during which the ONH was imaged at 25fps through a broadband green filter (Imedos DVA) for better contrast of blood against tissue. Statistical analysis consisted of ANOVAs (alpha of 5%). Changes in ONH density during an increase in IOP were normalized to baseline. The IOP was increased to reduce the OPP between 10% to 40%. Subjects were prompted to blink fully to maintain clear corneas. Room lighting was minimal to eliminate scattering of ambient light into the test eye.
The abrupt increase in IOP caused a reduction of ~15% (p < 0.04) in the group-averaged normalized BF that peaked at ~35 sec. Thereafter, BF manifested low frequency undulations that slowly restored baseline BF in 135 sec. Surprisingly, BF continued to exceed baseline and was ~18% above baseline in ~110 sec that coincided with the end of the 4-min OPP reduction phase. When scleral suction was stopped abruptly, BF decreased linearly to be within 5% of baseline by the end of recovery.
The physiological stress localized to the eye, i.e. an abrupt increase in the IOP, elicited significant changes in the capillary BF in the ONH. None of our subjects demonstrated a rapid AR response. The hyper-perfusion trend midway through provocation may represent a regulatory-type response with diagnostic value.
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