Regulation of intraocular pressure after water drinking

Purpose: Acute oral water loading transiently elevates intraocular pressure (IOP) via mechanisms that remain unexplained. We tested the possibilities that water drinking might elevate IOP by creating a blood-aqueous osmotic g radient, or that it might instead alter active ion pumping and the formatio n of aqueous humor. Methods: In the first series, 16 young, healthy individuals were studied du ring dehydration and for 1 hour after rehydration (14 mL H2O/kg body weight ). Hematocrit, total plasma osmolality, and plasma colloid osmotic pressure were determined simultaneously with measurements of IOP. In a second serie s (N = 16), rehydration occurred after pretreatment with either placebo or a topical carbonic anhydrase inhibitor (1 drop 2% dorzolamide in each eye, 12 and 2 hours before oral water loading). Results: In both series, mean IOP increased significantly 15 minutes after water ingestion and remained elevated above baseline for 45 minutes. In con trast, colloid osmotic pressure and hematocrit were unaltered by water drin king, and neither these variables nor total plasma osmolality correlated wi th IOP. In the second series, pretreatment with dorzolamide reduced baselin e IOP, but failed to alter the magnitude or time course of IOP elevations i nduced by water drinking. Conclusion: Because water drinking failed to create a blood-ocular osmotic pressure gradient, neither vitreous hydration nor increased aqueous ultrafi ltration can explain increases in IOP after acute hydration. Because the in crease in ocular tension apparently also is independent of active bicarbona te pumping, factors affecting aqueous drainage must explain the water drink ing effect.