A parametric evaluation of retinal vascular perfusion pressure and visual neural function in man.

Body inversion and ophthalmodynamometry were used to alter the intraocular pressure/retinal vascular perfusion pressure (IOP/RVPP) relationship in 10 normotensive adults. Using scotopic and photopic flash electroretinograms (fERGs), a heightened susceptibility of the rod system relative to the cone system was identified for transient alterations of the RVPP. The scotopic fERG b-wave decreased by about 40% and the photopic fERG was relatively unchanged when the RVPP was reduced by 51%. When the RVPP was increased by some 90%, the scotopic b-wave decreased by about 11%, and the photopic fERG by 8%. The greater vulnerability of the rod system than the cone system to decreased RVPP was tentatively attributed to structural differences between the photoreceptor types, their retinal distribution, and the effectiveness of vascular autoregulatory mechanisms. The neural generators of the pattern evoked retinal potential (pERG) showed a large impairment in function with both an increase and a decrease in the RVPP. In contrast, pattern evoked cortical potentials (pVEPs) showed a significant reduction in amplitude only when the RVPP was increased. These latter results suggest that the generators of pERGs are dependent on some ideal and narrow range of RVPP for normal activity whereas the generators of pVEPs are more vulnerable to the direct pressure effects of vascular engorgement within the eye and intracranial structures.