Case

A 29-year-old man presented to the ophthalmic accident and emergency with a 3-day history of blurred vision and a dilated pupil in the left eye. He was previously healthy, reported no drug use or ocular trauma and had no relevant family history. He had no history to suggest inadvertent pharmacological dilatation.

His distance visual acuity was 20/20 OU unaided: near vision was N5 right, N24 left. The left pupil was regularly dilated, measuring 7 mm diameter; it was fixed, with no response to direct or consensual light stimulation or accommodation. The right pupil was 3 mm diameter and had normal responses (figure 1). There was no ptosis on the left, and extraocular movements were full. Convergence was normal, and there was no segmental iris constriction. Neither pupil constricted 25 min after instillation of 0.125% pilocarpine. Pharmacological dilatation was not considered to be likely; therefore, the 1%–2% pilocarpine test was not performed. A provisional diagnosis of acute Adie pupil was made and a review appointment arranged.

A week later, he developed intermittent double vision in down gaze. Two weeks after the initial presentation, the left pupil remained dilated and unreactive, there was no ptosis and his eyes were normally aligned in the primary position. An isolated –1 to 2 deficit of left inferior rectus function was documented. A partial left third nerve palsy was diagnosed and brain imaging arranged.

A CT brain scan demonstrated a high-density lesion on the left of the midbrain tegmentum at the level of the superior colliculus, in the region of the third …