Physostigmine increases aqueous humor production in human eyes

To investigate if part of the progressive reduction of intraocular pressure (IOP), seen when physostigmine is applied on alternate hours, is due to a reduced aqueous flow.

In a randomized, open study, one drop of physostigmine salicylate, 8 mg/ml, was instilled at 7 AM in one randomly assigned eye in each of twenty healthy volunteers. Instillations were repeated on alternate hours throughout the day. Each subject's untreated eye served as control. Fluorophotometry of the anterior segment was performed hourly between 7 Am and 8 PM and aqueous flow was calculated. Subsequently, the subjects underwent tomography and tonometry. The change in anterior chamber depth and volume induced by physostigmine was assessed separately.

The mean aqueous flow during the day was 25-28% higher in the physostigmine-treated eye than in the control eye. The difference was statistically significant from 9 AM (p < 0.05-p < 0.001). Each dose caused a further increase. The mean outflow facility increased by 0.14 microliters/min/mm Hg with 95% confidence interval (CI) 0.09-0.18. Although the increase in outflow facility was small, there was a marked reduction of IOP with a mean difference between treated and untreated eye of 3.2 mm Hg (95% CI: 2.3-4.0).

Repeated administrations of physostigmine increase the aqueous flow and outflow facility. The combined effect is a marked reduction of IOP.