Nocturnal dip in the optic nerve head perfusion.

PURPOSE Many physiological parameters, including blood pressure, show circadian variations. Diurnal fluctuations of the optic nerve head (ONH) circulation have not yet been studied. The purpose of the present study was to determine the pattern of ONH blood flow variations over a 24-hour period in healthy subjects. METHODS The subject group comprised 15 healthy volunteers (6 women, 9 men) aged 22 to 43 years (mean +/- SEM: 28.2 +/- 1.3 years). Blood flow in the ONH was measured by laser Doppler flowmetry (LDF) over a 24-hour period at 08:00, 12:00, 16:00, 20:00, 24:00 hours, and at 08:00 hours the following morning. RESULTS ONH perfusion varied significantly over time. The mean LDF-flow during daytime ranged from 8.2 to 8.9 arbitrary units (AU) and fell at midnight to 7.1 AU (P =.0015). The mean LDF-volume during daytime was 0.23 to 0.24 AU and decreased at midnight to 0.20 AU (P =.04). The mean LDF-velocity ranged from 0.39 to 0.41 kHz with no significant differences at any time point. CONCLUSIONS Our study in normal subjects reveals small mean changes of ONH perfusion during daytime and a significant reduction at night. The physiological and clinical relevance of the nocturnal dip in the ONH perfusion needs to be determined in future studies.