Purpose
The magnitude and duration of the intraocular pressure (IOP) increase after water drinking test have been used to assess the risk of developing glaucoma. However, the mechanism of the rise in IOP after water drinking is poorly understood. In this study we examined the effect of the water drinking test on aqueous humor dynamics (IOP, outflow facility, episcleral venous pressure (EVP), and aqueous humor flow) in healthy individuals.
Methods
Sixteen eyes of 8 healthy participants (age 27-61; mean 37.8 years) were studied. Variables were measured in two visits. During one study visit, IOP was measured in the sitting position by pneumatonometry, outflow facility was measured by digital Shiøtz tonography, and EVP was measured by using an automated venomanometer, based on the pressure-chamber method. After baseline measurements, participants drank 15 ml/kg of water within 10 minutes. Variables were re-measured at 10 min, 30 min, and 60 min after ingestion of water. During another study visit, baseline aqueous humor flow rate was determined during a one hour interval by fluorophotometry. Participants were then asked to drink same amount of water within 10 minutes. Aqueous humor flow was re-measured on the intervals 0-10 minutes, 10-30 minutes, and 30-60 minutes after water drinking. Aqueous humor flow rate, outflow facility, IOP, and EVP after drinking water were compared to the same variables at baseline by using generalized estimating equation (GEE) models. The increase in IOP after water drinking was compared to the change in EVP by using GEE models.
Results
EVP and IOP increased after water drinking by 3 mmHg at 30 minutes (Table 1, see p-values in table). Outflow facility did not change after water drinking. Aqueous humor flow decreased during the 10-minute interval after water drinking but was not different from baseline during any other interval. The rise in IOP was not different from the rise in EVP at 10 minutes and 30 minutes (p=0.06 and p=0.8 respectively). At 60 minutes, the difference in IOP from baseline was less than the difference in EVP from baseline (p=0.004).
Conclusions
IOP rises after the water drinking test primarily because of the increase in EVP. Changes in outflow facility and aqueous humor flow do not contribute to the rise in IOP. The elevation in IOP recovers earlier than the elevation in EVP after the water drinking test.
Keywords: 427 aqueous •
568 intraocular pressure •
467 clinical laboratory testing