Tonographic studies done on 20 normal eyes and 23 eyes with open-angle glaucoma confirm that the main mechanism by which guanethidine sulfate lowers the intraocular pressure is by reducing aqueous humour production. This effect is probably the consequence of the sympathetic blockade produced by the drug. In addition, there has been also demonstrated an early and transient increase of outflow facility corresponding to the phase of peripheral sympathetic stimulation probably due to intraocular release of norepinephrine.
Bonomi L, Comite PD. Outflow Facility After Guanethidine Sulfate Administration. Arch Ophthalmol. 1967;78(3):337-340. doi:10.1001/archopht.1967.00980030339014