Understanding the mechanism of the water drinking test: the role of fluid challenge volume in patients with medically controlled primary open angle glaucoma

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Abstract
Background:  To determine whether a 500‐mL fluid challenge produces a different intraocular pressure (IOP) response profile compared with a 1000‐mL water drinking test (WDT).Methods:  Prospective, observer‐masked, cross‐over, observational study. Patients with primary open angle glaucoma were recruited from a private specialist glaucoma practice. One eye of each patient was included. Subjects were randomized to receive either a 500‐mL or 1000‐mL fluid challenge. Baseline IOP was recorded with a Goldmann applanation tonometer and then every 15 min for 1 h. A second WDT with the alternate volume of water was performed after a minimum washout period of 24 h. Primary statistical analysis was performed using a two‐way anova repeated measures of variance with a Bonferroni post‐hoc test.Results:  Fifteen patients were included in this study. The mean patient age was 67.0 ± 10.2 (SD) years, and 60% of patients were female. There was no statistically significant difference in baseline IOP between the 500‐mL and 1000‐mL WDTs (P = 0.11). Both fluid challenge volumes produced a statistically significant rise in IOP from baseline at 15, 30 and 45 min after water ingestion. However the mean maximum increase in IOP was less in the 500‐mL WDT (3.3 ± 1.8 mmHg [23.9%]) compared with the 1000‐mL WDT (4.9 ± 2.3 mmHg [32.5%]; P = 0.0095).Conclusion:  The 500‐mL WDT may provide an alternative for patients who are unable to tolerate drinking 1000 mL of water. However, because the maximum IOP obtained is less, a 500‐mL WDT cannot be used to directly estimate peak diurnal pressure.

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