The effects of topical parasympatholytic drugs on pupil diameter and intraocular pressure in healthy dogs treated with 0.005% latanoprost

Purpose: Prostaglandin analogs induce miosis and lower intraocular pressure (IOP). As pupils of latanoprost-treated eyes may have to be dilated for ophthalmoscopy or intraocular surgery, we studied whether 0.5% tropicamide or 1% atropine alter the effects of 0.005% latanoprost on pupil diameter (PD) and IOP in healthy dogs. Methods: IOP and PD were measured hourly, 8 AM–4 PM, with the right and left eyes serving as control (CE) and treated (TE) eyes, respectively. Measurements were conducted in ten Labrador retrievers with one-week washout: (i) baseline values, (ii) latanoprost at 8 AM, (iii) tropicamide at 8 AM, (iv) latanoprost at 8 AM and tropicamide at 11 AM, and (v) latanoprost at 8 AM and atropine at 11 AM (n = 4). Results: At 4 PM, TE PD was 5.88 ± 0.59, 3.62 ± 0.66, 6.33 ± 1.00, 5.42 ± 0.57, and 8.12 ± 1.24 mm in sessions 1–5, respectively. TE PD was significantly different between treatment sessions 2, 4, and 5 (P = 0.018, Friedman), being most mydriatic in session 5. At 4 PM, TE IOP was 11.27 ± 2.07, 7.10 ± 1.07, 11.1 ± 2.21, 7.70 ± 1.85, and 8.87 ± 1.42 mm Hg in sessions 1–5, respectively, with no differences between treatment sessions 2, 4, and 5 (P = 0.105, Friedman). Conclusions: Tropicamide and atropine counteracted latanoprost's miotic effect, with atropine causing significantly larger mydriasis, sufficient for indirect ophthalmoscopy. Neither drug counteracted the hypotensive effect of latanoprost during this study period. Further studies are necessary to evaluate the potential risks in glaucomatous dogs.