Effect of single- and multiple-dose 0.5% timolol maleate on intraocular pressure and pupil size in female horses

Abstract

OBJECTIVE: To determine the effect of single and multiple-dose 0.5% timolol maleate on intraocular pressure (IOP) and pupil size between 8 AM and 8 PM. Animals Nine female horses with normotensive eyes. Procedure IOP, horizontal and vertical pupil size were measured on a single day, between 8 AM and 8 PM at hours 0, 0.5, 1, 2, 4, 6, 8, 10, and 12. A single dose of 0.5% timolol maleate was applied to both eyes immediately after the first measurement at 8 AM. IOP and pupil size were measured at 8 AM and 4 PM in a 5-day experiment of twice-daily application of 0.5% timolol maleate. RESULTS: A significant decrease in IOP from 24.9 +/- 4.2 mmHg prior to application of timolol maleate to 20.7 +/- 3.1 mmHg (4.2 mmHg = 17%) was observed 8 h after single-dose application. A significant decrease in horizontal pupil size (2.0 mm = 11%) was present 6 h after single-dose application. In the multiple-dose experiment, a significant decrease in IOP was present on days 4 and 5 as compared to IOP measured prior to application of timolol maleate. A significant decrease in horizontal and vertical pupil size was present throughout the 5-day study as compared to the values obtained prior to treatment. CONCLUSIONS: 0.5% timolol maleate significantly decreased IOP and pupil size in normo-tensive eyes of this group of female horses in both single and multiple twice daily applications.

Effect of topical ophthalmic dorzolamide(2%)–timolol(0.5%) solution and ointment on intraocular pressure in normal horses

“…We found that the compounded ointment and commercially available solution formulations of dorzolamide(2%)–timolol(0.5%) had similar effects in both treated and untreated eyes. The approximately 13% reduction in IOP of treated eyes in our study is comparable to previous reports evaluating topical beta‐adrenergic antagonists and carbonic anhydrase inhibitors in horses, which have described 10–17% reduction in IOP . In our study, the magnitude of IOP reduction was greater in treated eyes compared to untreated eyes, but this difference was not significant.…”

“…A baseline period of 2 days was selected for our study based on previous reports; notably, the mean IOP value obtained during our baseline period, 21.67 (3.47) mmHg, was within the range of previously reported normal values (17–28 mmHg), while those obtained during or after the treatment period approached the lower end of this range . Within the baseline period, IOP values on Day 2 were not significantly lower than those on Day 1; however, the descending trend in IOP values is readily observed (Figs and ).…”

“…Temporal influences on IOP should be considered as a possible explanation for the bilateral IOP reduction observed in our study as IOP values did not return to baseline within the follow‐up period . A protracted follow‐up period may have allowed differentiation between drug‐induced IOP reduction and horse acclimation to tonometry as the ocular hypotensive effect of topical ophthalmic carbonic anhydrase inhibitors may approach 2 days in normal horses .…”

“…Topical carbonic anhydrase and beta‐adrenergic inhibitors, such as the FDA‐approved dorzolamide(2%)–timolol(0.5%) ophthalmic solution, are widely used to lower intraocular pressure (IOP) in glaucomatous human and animal patients . Studies in glaucomatous horses are lacking, but topical carbonic anhydrase and beta‐adrenergic inhibitors are effective at reducing IOP in normal horses with application twice daily being more effective than once daily . Although the magnitude of this reduction is modest (10–17%), the combination of dorzolamide(2%)–timolol(0.5%) is proposed to be the most effective topical ocular hypotensive agent in the horse, and a 2009 survey of veterinary ophthalmologists suggests the product is the most commonly prescribed medication for treatment of equine glaucoma …”

Feasibility of aqueous shunts for reduction of intraocular pressure in horses

“…Current glaucoma therapies reduce IOP medically or surgically . Medical therapy consists of topical beta blockers and/or topical carbonic anhydrase inhibitors applied 2–3 times daily to reduce aqueous humour production, and concurrent systemic and topical anti‐inflammatory therapy to control any underlying equine recurrent uveitis . However, based on 2 retrospective case studies on equine glaucoma, the long‐term success of medical management appears poor .…”