Purpose:
Recent research indicates that intraocular pressure (IOP) increases during the nocturnal period. However, the reason for this increase is unclear, as aqueous humor flow decreases at night. This study investigated whether or not changes in outflow facility at night could account for the higher nocturnal IOP.
Methods:
Forty-eight eyes of 24 healthy subjects (age 18-45 yrs, mean 31 yrs) were recruited from employees and students at the Mayo Clinic (Rochester, MN) and the local area. Subjects maintained a sleep log and wore a wrist actigraph for one week prior to the study to ensure a regular sleep schedule. Subjects were housed at the Mayo Clinic General Clinical Research Center over an 18-hour period covering the mid-diurnal and mid-nocturnal periods (approximately 1 PM to 7 AM). IOP, aqueous humor flow rate, and outflow facility were measured using pneumatonometry, anterior chamber fluorophotometry and Schiotz tonography respectively, in each eye during the mid-diurnal (2-4 PM) and mid-nocturnal (2-4 AM) periods. Nocturnal IOP, aqueous humor flow rate, and outflow facility were compared to the same variables during the diurnal period. Subgroup analysis was performed for 18-29 yrs (n=24) and 30-45 yrs (n=24). Generalized estimating equation models were used to control for inter-eye correlations.
Results:
Results are shown in Table 1. Flow rate change and facility change were not significantly correlated (p=0.26). Although facility decreased from the diurnal to nocturnal period, the change was not sufficient to compensate for the decrease in aqueous flow rate. Based on the Goldmann equation, a decrease in facility of about 50% would be required to produce the measured IOP if other parameters remained constant. There was no significant difference between the 2 subgroups or between the subgroups and all subjects.
Conclusions:
Outflow facility measured by tonography does not decrease enough during the nocturnal period to compensate for the decreased aqueous humor flow rate. Other possible factors are changes in episcleral venous pressure and/or uveoscleral flow rate. The complete mechanism of nocturnal IOP remains to be elucidated.
Keywords: intraocular pressure • aqueous • outflow: trabecular meshwork