Increased 24-hour variation of human intraocular pressure with short axial length.

PURPOSE To characterize 24-hour variation of intraocular pressure (IOP) in healthy young adults based on the axial length of the eye. METHODS Twenty-four-hour IOP data were collected from nine healthy young adults with hyperopia, age range 18 to 25 years, in a sleep laboratory. Every 2 hours, measurements of IOP were taken in the participants after 5 minutes in the supine position and 5 minutes in the sitting position during the 16-hour diurnal/wake period as well when supine in bed during the 8-hour nocturnal/sleep period. Variations in 24-hour IOP in this hyperopia group were analyzed, together with previously collected data under the same laboratory conditions from 32 age-matched subjects with emmetropia or mild myopia (emmetropia group) and 34 subjects with moderate to severe myopia (myopia group). RESULTS Average diurnal sitting IOP was lower in the hyperopia group than in the other two groups. The difference between the diurnal sitting and diurnal supine IOP was larger in the hyperopia group than in the myopia group. In all three groups, the nocturnal supine IOP was higher than the diurnal sitting IOP. This elevation in habitual IOP was most significant in the hyperopia group. The hyperopia group also presented a significant IOP elevation within the nocturnal period. Simulated 24-hour rhythms of supine IOP were detected in all groups with different phase timings, but simulated 24-hour IOP variations were not different. The 24-hour habitual IOP fluctuation (peak minus trough) was inversely correlated to axial length. CONCLUSIONS Shorter eyes had a larger 24-hour IOP variation than longer eyes in healthy young adults.