Giant vacuoles in the lining endothelium of the human Schlemm's canal after topical timolol maleate

The outflow system was studied by light microscopy in nine human eyes that were treated with topical timolol maleate (0.5%) prior to enucleation for malignant melanoma of the choroid. The frequency of 'giant vacuoles' in the lining endothelium of Schlemm's canal and the diameter of these structures was the same in a control group of eyes as in the timolol-treated series. There was no qualitative difference between the architecture of the meshwork in the control and treated groups. These morphologic findings suggested that timolol had no direct action on the outflow apparatus and did not reflect the intraocular pressure (IOP) lowering effect of the drug. The effects of anesthesia, surgical manipulations, and fixation by immersion may have masked any subtle drug effects.