There are several causes for sudden onset unilateral mydriasis, however impending transtentorial uncal herniation needs to be ruled out. This unique case highlights an uncommon adverse response to a common mode of treatment that leads to a diagnostic dilemma. A 3-year-old boy with a ventriculoperitoneal (VP) shunt for an obstructive hydrocephalus presented with an acute respiratory distress. He developed unilateral mydriasis with absent light reflex during treatment with nebulisers. An urgent CT scan of the brain did not show any new intracranial abnormality. A case of pharmacological anisocoria was diagnosed that resolved completely within 24 hours of discontinuation of ipratropium bromide. Although ipratropium-induced anisocoria has been reported in children, but to our knowledge none in a child with VP shunt for hydrocephalus. This emphasises the urgency in evaluating unilateral mydriasis to rule out life-threatening conditions. Clinicians should remember that ipratropium administered through ill-fitting face masks could cause this completely reversible adverse effect.

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