Axial Length and Intraocular Pressure Fluctuations in Human Eyes

Abstract

Abstract: : Purpose:To determine if diurnal fluctuations in axial length (AL) in human eyes correlate with fluctuations in intraocular pressure (IOP) and to assess if fluctuations in AL and IOP are symmetric between fellow eyes and repeatable. Methods: Ten subjects, ages 18 to 24 years, underwent AL and IOP measurements, using partial coherence interferometry (a highly precise, non–contact method) and Goldmann applanation tonometry, respectively. Measurements were taken in both eyes at six 3–hour intervals during each of two study days, starting at 7AM and ending at 10PM. Results: Eighteen eyes (90%) had significant fluctuation in AL on at least one of the study days (ANOVA, p < 0.05). Eleven of these eyes had significant AL fluctuation on both study days. The mean magnitude of AL fluctuation was 37.5 ± 21.8 µm. Sine curve fitting on data sets with a significant fluctuation showed that AL fluctuation had a mean amplitude of 34.4 ± 16.1 µm, a mean period of 20.8 ± 5.1 hrs, and a mean peak time of 14:12 (SD 5 hr 6 min). When a significant fluctuation in AL was seen in one eye, a fluctuation tended to occur in the fellow eye with similar magnitude, period, and peak time. Eighteen eyes (90%) had significant fluctuation in IOP on at least one of the study days. Fifteen of those had IOP fluctuations on both days. The mean magnitude of IOP fluctuation was 6.0 ± 1.9 mmHg. Sine curve fitting for IOP resulted in a mean amplitude of 5.6 ± 1.5 mmHg, a mean period of 18.6 ± 7.1 hrs, and a mean peak time of 14:24 (SD 4 hr 54 min) were found. Also, when IOP fluctuations were seen in one eye, IOP fluctuations of similar magnitude, period, and peak time were likely to occur in the fellow eye. Conclusions: Both AL and IOP undergo diurnal oscillations that tend to peak in midday. The variations in both parameters tend to be symmetric between fellow eyes and repeatable between the two study days. Despite the temporal relationship between oscillations in AL and IOP, and AL, this study does not prove that IOP fluctuation causes AL fluctuation. Fluctuations in AL and IOP are similarly correlated in chicks, but it is experimentally possible to dissociate the two rhythms. Further research is necessary in humans to learn whether the relationship between IOP and AL is causal or whether other physiologic parameters might account for both AL and IOP fluctuations.

Keywords: myopia • intraocular pressure • circadian rhythms