Scanning laser ophthalmoscopy-based evaluation of epipapillary velocities: method and physiologic variability

Scanning laser ophthalmoscopy is capable of producing high-resolution fluorescein angiograms. Measurements of capillary blood velocities in the fovea are well established. In this study, we used an identical technique to measure particle velocities in the superficial layers of the optic nerve head. We compared these data with simultaneous velocity measurements in the retinal and macular vasculature.

Fluorescein angiograms were performed in 30 subjects (12 men, 18 women; mean age, 26 years; standard deviation [SD], 3 years). Off-line, the velocities of hypofluorescent particles through the microvasculature of epipapillary and macular vessels were measured by image-processing techniques. We also assessed the arteriovenous passage (AVP) time of the fluorescein dye front.

Epipapillary blood velocities ranged from 2.7 to 6.5 mm/sec (mean, 4.0 mm/sec; SD, 0.99 mm/sec) and differed significantly from macular capillary blood velocities (MCBVs), which ranged from 1.67 to 3.31 mm/sec (mean, 2.53 mm/sec; SD, 0.34 mm/sec). The AVP time in all subjects was 1.74 +/- 0.5 sec (mean +/- SD) and correlated with the MCBV (P < 0.05, r = -0.46). Epipapillary blood velocities showed no correlation with either AVP time or MCBV.

The scanning laser methodology, adapted to objectively assess velocities in the epipapillary vasculature, shows that these measurements are neither correlated with velocities in the perifoveal network, nor inversely correlated with overall retinal transit time. Epipapillary blood velocities were found to be substantially higher than those measured in macular capillaries. This result might be explained by the different anatomy of epipapillary vessels compared with macular capillaries.