Atropine induces narrowing of the superior and inferior iridocorneal angle and ciliary cleft in normal horse eyes.

Atropine narrows the equine iridocorneal angle and should be used cautiously in horses with glaucoma.

This prospective experimental study investigated the effects of topical atropine 1% on the iridocorneal angle (ICA) and ciliary cleft (CC) in 12 normal adult horses using ultrasound biomicroscopy (UBM). Baseline measurements of intraocular pressure (IOP), pupil diameter, and UBM parameters—including pectinate ligament distance (PLD), ciliary cleft length (CCL), height of opening distance (HOD), ciliary cleft area (CCA), and pectinate ligament angle (PLA)—were obtained from all four quadrants (superior, inferior, nasal, temporal) bilaterally, then repeated two hours post-atropine administration. Key findings revealed a statistically significant decrease in mean IOP (14.17 to 12.67 mmHg) following atropine application. Regionally, the superior quadrant demonstrated significant reductions in CCL, CCA, HOD, and PLA, while the inferior quadrant showed significant decreases in PLD, HOD, and PLA. No significant changes were observed nasally or temporally. These results indicate that atropine-induced mydriasis causes measurable narrowing of the ICA and CC in the superior and inferior quadrants of normal equine eyes. The asymmetric regional response may reflect anatomical differences in the equine drainage angle. Clinically, these findings carry important implications: while atropine is commonly used in equine ophthalmology for uveitis management and ophthalmic examination, its capacity to narrow aqueous humor outflow structures suggests potential for elevating IOP in at-risk patients. The authors recommend exercising caution when administering atropine to horses with pre-existing elevated IOP or glaucoma.

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