A 59 year-old female with history of hemithyroidectomy for a benign lesion presented with 1 year of drooping of the right eyelid and anhidrosis of the right side of her face. She denied neck pain or trauma. Examination revealed ptosis of the right upper eyelid and miosis of the right pupil that was worse in dim light conditions (Fig. 1A). Thirty minutes after instillation of 0.5% apraclonidine in both eyes the ptosis resolved and the right pupil became larger than the left pupil. What is the diagnosis?