Diurnal variations in axial length, choroidal thickness, intraocular pressure, and ocular biometrics

Purpose. To investigate the pattern of diurnal variations in axial length (AL), choroidal thickness, intraocular pressure (IOP), and ocular biometrics over 2 consecutive days. Methods. Measurements of ocular biometrics and IOP were collected for 30 young adult subjects (15 myopes, 15 emmetropes) at 10 different times over 2 consecutive days. Five sets of measurements were collected each day at approximately 3-hour intervals, with the first measurement taken at ~9 AM and final measurement at ~9 PM. Results. AL underwent significant diurnal variation (P < 0.0001) that was consistently observed across the 2 measurement days. The longest AL was typically observed at the second measurement session (mean time, 12:26) and the shortest AL at the final session of each day (mean time, 21:06). The mean diurnal change in AL was 0.032 ± 0.018 mm. Choroidal thickness underwent significant diurnal variation (mean change, 0.029 ± 0.016 mm; P < 0.001) and varied approximately in antiphase to the AL changes. Significant diurnal variations were also found in vitreous chamber depth (VCD; mean change, 0.06 ± 0.029 mm; P < 0.0001) and IOP (mean change, 3.54 ± 0.84 mm Hg; P < 0.0001). A positive association was found between the variations of AL and IOP (r ² = 0.17, P < 0.0001) and AL and VCD (r ² = 0.31, P < 0.0001) and a negative association between AL and choroidal thickness (r ² = 0.13, P < 0.0001). There were no significant differences in the magnitude and timing of diurnal variations associated with refractive error. Conclusions. Significant diurnal variations in AL, choroidal thickness, and IOP were consistently observed over 2 consecutive days of testing.