Influence of the Water-Drinking Test on Intraocular Pressure, Schlemm's Canal, and Autonomic Nervous System Activity.

Purpose:To assess changes in Schlemm's canal (SC), intraocular pressure (IOP), and autonomic nervous system activity in healthy individuals after performing the water-drinking test (WDT). Methods:The SC area (SCAR), trabecular meshwork (TM) thickness, IOP, high frequency (HF) of heart rate variability (HRV), heart rate (HR), and systolic (SBP) and diastolic blood pressure (DBP) were measured in 22 young healthy participants before and after the WDT, which involved drinking a 1-liter water load in 5 minutes.The SC and TM profiles were captured using a Spectralis optical coherence tomography device (anterior segment module). HF was recorded using Kubios HRV Premium software to evaluate parasympathetic nervous system activity. Results:Compared with baseline values, IOP increased significantly (14.9 ± 2.7 mm Hg vs. 18.4 ± 3.3 mm Hg; P < 0.001), whereas HF (1587 ± 930 ms2 vs. 2193 ± 863 ms2; P < 0.001), mean SCAR (6521 ± 1360 μm2 vs. 5180 ± 1455 μm2; P < 0.001), and HR (69 ± 9.7 beats/min vs. 63 ± 8.9 beats/min; P < 0.001) values decreased significantly by 15 minutes after water-loading. Least significant difference pairwise comparison revealed significant fluctuations of all parameters (SCAR, IOP, HF, and HR) at 15 minutes and their recovery at 30-minutes post-WDT. TM thickness, SBP, and DBP post-WDT did not differ significantly. The increase in IOP (r = -0.4047; P = 0.010) and HF (r = -0.386; P = 0.014) correlated significantly with the decrease in SCAR. Conclusions:The WDT may cause parasympathetic nervous system stimulation, leading to the collapse of SC, which leads to increased IOP.