Purpose: To determine in human eyes whether diurnal fluctuations in axial length are related to fluctuations in intraocular pressure (IOP) by studying these fluctuations in both eyes of individual subjects and by assessing the regularity of both rhythms on two separate study days.
Methods: Ten subjects, ages 18 to 24 years, underwent serial axial length and IOP measurements using highly precise, noncontact partial coherence interferometry and Goldmann applanation tonometry, respectively. Both eyes were measured at six 3-hour intervals during each of two study days, and significant fluctuations were modeled by sine curves.
Results: Of the 40 data sets, 29 had significant axial length high-low differences and 32 had significant IOP high-low differences (ANOVA, P < 0.05 for each). The magnitude of the significant high-low differences were 38 +/- 22 microm for axial length and 6.0 +/- 1.9 mm Hg for IOP (mean +/- SD). Neither axial length nor IOP fluctuations necessarily occurred bilaterally on the same day, and neither rhythm was regularly observed on two separate days in individual eyes. In eyes in which both parameters fluctuated on the same day, there were no correlations in the amplitude, period or phase of the two rhythms.
Conclusions: Both axial length and IOP fluctuate during the day much of the time in most subjects. However, diurnal IOP fluctuations do not appear to cause diurnal axial length fluctuations.