Idiopathic Unilateral Transient Miosis in Dogs

To characterize the clinical features of idiopathic unilateral transient miosis in dogs.

Owner- and veterinarian-completed questionnaires were analyzed to identify and characterize dogs with idiopathic unilateral transient miosis.

Seventy-eight dogs met the inclusion criteria, showing no additional ocular or systemic abnormalities. Australian Shepherds (ASH), ASH mixes (n = 41), and Miniature Australian Shepherds (MASH) (n = 15) were markedly overrepresented, while other breeds were only sporadically affected. The merle phenotype was documented in 70% of cases, and 61% of affected dogs were female. The mean age at onset of the first episode was 6.5 months. Miosis was reported after sleep in 74% of cases and following physical activity in 22%. Blue eyes were affected in 76% of dogs, light-colored irises in 6%, heterochromatic eyes in 8%, and brown eyes in 13%. The median episode duration was 30 min. Recurrent episodes were reported in 71% of dogs (median: 2 episodes), with side alternation occurring in 62% of these cases. No significant interbreed differences were observed.

Idiopathic unilateral transient miosis, in the absence of other ocular or systemic disease, occurs predominantly in young dogs and is seen across various breeds and eye colors. Australian Shepherds, Miniature Australian Shepherds, the merle phenotype, blue eyes, and females were disproportionately represented in this cohort. Episodes were most commonly observed after sleep, typically lasted around 30 min, and recurred in more than half of affected dogs. To the authors' knowledge, this is the first report of idiopathic unilateral transient miosis in dogs.