A 69 year old woman was referred by her primary care doctor to the eye emergency department with a dilated left pupil and ptosis of her right upper lid. She had visited her primary care doctor because of an upper respiratory tract infection and was unconcerned about her unequal pupil size (anisocoria), stating that she “has had odd pupils and a droopy right lid since being a teenager.” She had no ocular or medical history and was not taking any oral or topical medication. She denied any history of trauma, neck pain, weight loss, diplopia, or anhydrosis. On examination she had a partial right upper lid ptosis (fig 1⇓) and her right pupil, which was the abnormal one, was constricted and showed a dilatation lag. Both pupils constricted to light and accommodation and there was no relative afferent pupillary defect. Iris colour was equal on both sides. Visual acuity was 6/9 in both eyes and intraocular pressures were 12 mm Hg and 14 mm Hg in the right and left, respectively. Ocular motility was full and funduscopy showed healthy optic discs. She had no other neurological signs. One drop of apraclonidine 1% was applied to both eyes. After one hour there was dilation of the constricted pupil and improvement in the ptosis. Fig 1 Photograph of patient in room light and distance fixation showing right upper lid ptosis ( A ) and right miosis ( B ) ### Short answer Right Horner’s syndrome. …