Stability of Nycthemeral Rhythm of Intraocular Pressure in Humans

Abstract

Abstract: : Purpose: To estimate the stability over time of the nycthemeral rhythm of intraocular pressure (IOP) in humans throughout a 24–hour period of continuous bedrest. Methods: Six healthy male subjects (21–29 years old) were studied during six 24–hour sessions over a period of six weeks. The rhythm of IOP was studied in association with that of temperature and heart rate. Subjects were housed in a sleep laboratory for 24 hours in a strictly controlled environment (stable posture, ligh cycle, temperature, fluid intakes, meals), with continuous monitoring of sleep. IOP was measured using the pneumatonometer Digilab Modular One® every hour during 24 hours in a supine position. The pneumatonometer was validated against the Goldmann tonometer (r = 0.96, p< 0.001). The cosine–fit 24–hour IOP rhythm was compared for each eye of each subject across the six sessions. Polysomnographic recording was made at night starting at 23:00. Sleep duration was on average 9 hours (23:00 – 08:00). Results: The duration of the IOP measurements, including the anesthetic application, was 59.3±0.7 (SEM) sec and the total awakening resulting from the IOP measurement averaged 123.1±1.7 sec. Our results confirm that there is a nycthemeral rhythm in IOP with mean IOP higher at night than during the day (mean difference: 1.25 mmHg, p< 0.01). In addition, there is variability of the IOP rhythm between individuals, for the mesor (p< 0.01, ANOVA), the amplitude (p< 0.01) and the acrophase (p< 0.05). In contrast, for an individual subject, there is no significant difference for the mesor, the amplitude and the acrophase between the experimental sessions. Conclusions: The nycthemeral rhythm of IOP is variable between healthy subjects but stable within an individual subject across successive experimental sessions. This suggests that a single 24 hour recording under controlled conditions is sufficient for characterization of the nycthemeral IOP rhythm in an individual. This result should be verified in glaucoma patients who exhibit a phase delay of the IOP rhythm.

Keywords: intraocular pressure • circadian rhythms