Effects of topical administration of timolol maleate on intraocular pressure and pupil size in dogs

Effects of topical administration of a single dose of timolol maleate on intraocular pressure (IOP) and pupil diameter were evaluated in normotensive eyes of 11 clinically normal dogs over 12 hours (7:00 AM to 7:00 PM). Mean (+/- SEM) normal IOP was 15.5 (+/- 1.1) mm of Hg and diurnal fluctuation was observed, with the highest IOP seen in the morning. Mean normal pupil diameter was 8.5 (+/- 0.3) mm. Topical treatment with 0.5% timolol resulted in reduction of IOP in the treated and nontreated eyes. Mean reduction of IOP in the treated eye was 2.5 mm of Hg, a reduction of 16.1%, with maximal reduction of 3.7 mm of Hg. Mean reduction of IOP in the nontreated eye was 1.4 mm of Hg, a reduction of 9.0%. The treated eye had reduced pupil diameter at 30 minutes after treatment, which persisted throughout the 12 hours of the study. Mean reduction of pupil diameter in the treated eye was 2.9 mm, a reduction of 34.1%. In addition, a contralateral effect on pupil diameter was seen in the nontreated eye, with mean reduction of 1.2 mm, a reduction of 14.1%. Topical administration of timolol maleate resulted in reduction of IOP and pupil diameter in treated and contralateral eyes, thus supporting the use of timolol for treatment of glaucoma in dogs. Miosis indicates possible beta-adrenergic inhibition or alpha-adrenergic activation of the sphincter muscle. beta-Adrenergic blockade would then result in miosis.