Retinal capillary hemodynamics, visual-evoked potentials, and pressure tolerance in normal human eyes

Twenty-three normal adult volunteers underwent single eye visual-evoked potential (VEP) and blue field entoptic studies during suction cup-induced intraocular pressure (IOP) elevation, to determine whether IOP-induced changes in VEP are related to alterations in retinal capillary hemodynamics. VEP pressure tolerance testing through an ascending series of 6-8 IOP levels was carried out using a 7.1 Hz reversing checkerboard grating, with amplitudes averaged by Nicolet Pathfinder. Nineteen of the 23 subjects (83%) showed an increase from baseline in their VEP amplitude at IOP values approximating to central retinal diastolic pressures. All subjects underwent subsequent blue field entoptic hemodynamic studies at each of four IOP values related to their VEP pressure tolerance curve--at baseline IOP, at IOP corresponding to the VEP amplitude peak, and at IOPs corresponding to trough points either side of this peak. Blue field studies were conducted in a masked fashion with pressures generated in semirandom sequence. VEP amplitude pressure tolerance curves were found to vary in strong positive concordance with retinal leukocyte velocity pressure tolerance patterns (P less than 0.001). Leukocyte density initially varied inversely to velocity and VEP amplitude, increasing marginally with initial IOP elevation, but then fell in parallel with velocity at IOP levels exceeding the VEP amplitude peak (P = 0.009). These findings indicate a strong interrelationship between retinal hemodynamics and visual pathway activity as measured by VEP, suggesting that vascular autoregulation may account for the characteristic pattern of the normal VEP pressure tolerance curve.