Effects of mydriatics on intraocular pressure and pupil size in the normal feline eye

Abstract Objective To determine the effect of various mydriatics (1% atropine, 1% cyclopentolate, 0.5% tropicamide, 10% phenylephrine) on intraocular pressure (IOP) and pupil size (PS) in normal cats. Animals studied The mydriatics were tested in 10 adult ophthalmoscopically normal European Domestic Short‐haired cats. Procedure Single‐dose drug studies were divided into placebo (vehicle of phenylephrine), 10% phenylephrine, 0.5% tropicamide, 1% cyclopentolate and 1% atropine. After measurement of IOP and pupil size (PS) at 8 a.m. on the first day, one drop of the tested drug was applied to one randomly selected eye. The IOP and PS were measured for a minimum of 36 h until the pupil returned to pretest size. Results Ten per cent phenylephrine had no significant effect on IOP, and the effect on the pupil size was minimal (≤ 1 mm difference). One per cent atropine showed a maximal mean difference of 4.3 mmHg ± 4.1 mmHg in IOP compared to the untreated eye. These differences were statistically significant at 1, 3, 6, 12 and 16 h post‐treatment. At 1, 1.5 and 2 h after unilateral application of 1% cyclopentolate, the IOP in the treated eye was significantly higher than the IOP in the untreated eye with a maximal mean difference of 4.1 mmHg ± 4.5 mmHg. With 0.5% tropicamide, IOP of the treated eye was significantly higher than IOP of the untreated eye at 1 and 1.5 h after treatment with a maximal mean difference of 3.5 mmHg ± 3.3 mmHg. One per cent atropine caused the greatest increase in IOP, followed by 1% cyclopentolate and 0.5% tropicamide. The dilating effect of atropine, cyclopentolate and tropicamide lasted longer than the effect on intraocular pressure. Conclusions The parasympatholytics 1% atropine, 1% cyclopentolate and 0.5% tropicamide caused a statistically significant elevation of IOP and produced mydriasis in the treated eye in normal cats, while the sympathomimetic 10% phenylephrine had no significant effect on IOP or pupil size.