Purpose: We evaluated the supine 24-hour IOP rhythm reproducibility over 6 weeks in healthy humans.
Methods: Six healthy young male subjects underwent six 24-hour sessions of IOP measurements over a 6-week period. Subjects were housed in a sleep laboratory in a constant controlled supine position and in a strictly controlled environment. IOP was measured hourly using a pneumatonometer. A nonlinear least-squares dual harmonic regression analysis was used to model the 24-hour IOP rhythm. The intra- and intersubject variability of acrophase, bathyphase, amplitude, and IOP values were evaluated.
Results: A significant nyctohemeral IOP rhythm was noted in 30 of 36 (83%) sessions. Mean nocturnal IOP was significantly higher than diurnal IOP (20.1 0.2 MM HG [SD] VS. 18.8 0.1 MM HG, P 0.001) in all subjects. Amplitudes were not statistically different among subjects (P = 0.52). IN contrast, acrophase and bathyphase were statistically different (P 0.05). Intrasubject homogeneity of distribution over time of the acrophase and bathyphase was significant in 3 of 6 and 4 of 6 subjects, respectively. Intraclass correlation coefficients of midline estimating statistic of rhythm (MESOR) and IOP values AT 2: 00, 3:00, 4:00, 10:00, and 11:00 AM, and 2:00 PM showed fair to good agreement among sessions.
Conclusions: In a constant supine position, all subjects exhibited a nyctohemeral IOP rhythm present at an average rate of 80% of all sessions. With the currently available methods of tonometry, intrasubject reproducibility of rhythmic parameters and IOP values is limited. IOP values in the morning and IOP MESOR were the most reproducible parameters among the six visits.