Effect of 24-hour corneal biomechanical changes on intraocular pressure measurement.

PURPOSE To study 24-hour changes of corneal biomechanical properties and their influences on measurement of intraocular pressure (IOP). METHODS Fifteen healthy young volunteers (age range, 20-25 years) were housed for 1 day in a sleep laboratory. Sitting and supine central corneal thickness (CCT) were measured every 2 hours with an ultrasonic pachymeter. Sitting IOP and corneal hysteresis, an indicator of viscoelasticity, were measured with a noncontact tonometer. RESULTS There were consistent 24-hour variations of CCT and IOP for the group. Nocturnal mean CCT and nocturnal mean IOP were significantly higher than the diurnal mean CCT and diurnal mean IOP, respectively. The peak CCT occurred at 1:30 to 5:30 AM and the trough CCT at 1:30 PM. The peak IOP occurred at 5:30 AM and the trough IOP at 9:30 PM. Cosine fits of each subject's 24-hour CCT and IOP data showed synchronized rhythms. The phase timing of 24-hour CCT rhythm was significantly earlier than the phase timing of 24-hour IOP rhythm. Twenty-four-hour variation of corneal hysteresis was inconsistent and cosine fits of 24-hour data of corneal hysteresis did not display a 24-hour rhythm. CONCLUSIONS In healthy young adults, CCT was thicker, and IOP was higher during the nocturnal period than during the diurnal period. Nocturnal peak CCT occurred a few hours earlier than did nocturnal peak IOP. The twenty-four-hour change in corneal viscoelasticity was not significant. There was no evidence that the 24-hour change in IOP was due to the change in corneal biomechanical properties.