Field use of N-butylscopolammonium bromide to facilitate thorough ophthalmic examination in horses

•N-Butylscopolammonium bromide assessed as mydriatic for normal equine eye exam.•NBB caused pupillary dilation/enabled complete ophthalmic exam in some horses.•Mydriatic effect of NBB may be more profound in horses with light irides. N-butylscopolammonium bromide (NBB), an anticholinergic muscarinic antagonist, was assessed as a mydriatic agent for field examination of equine eyes. Six adult horses were randomly assigned to four treatments with 2 weeks washout between treatments: (1) topical saline/IV saline (negative control); (2) topical tropicamide/IV saline (positive control); (3) topical NBB/IV saline; or (4) topical saline/IV NBB. Horizontal and vertical pupil diameters, temperature, pulse, respiration, pupillary light reflexes (PLRs) and mydriasis sufficient to perform complete fundic examination were recorded. Tropicamide induced mydriasis in all horses. Topical NBB induced mydriasis in one horse, and IV NBB enabled thorough fundic examination in two horses, delayed PLRs without allowing thorough examination in two horses and had no effect in two horses.