A unilaterally dilated pupil is often viewed as an ominous sign. However, a majority of patients with a neurologically isolated unilateral mydriasis have a benign process.1 Detailed history and examination can help avoid referral for an expensive neurological work-up. We present a patient with intermittent dilation of the pupil with no apparent cause.

Case report

A 39-year-old lady presented to the casualty with a traumatic corneal abrasion to the left eye. She was systemically well, except the occasional classical migraine. Vision was 6/12 in the right eye and 6/36 in the left. Examination revealed bilateral congenital cataracts. No other abnormality was found. The abrasion healed in 2 days with vision improving to 6/18. The right pupil, however, remained dilated (Figure 1a). This was interpreted as an abnormally prolonged response to tropicamide drops. The patient was discharged. Six months later, she was referred to us with a dilated left pupil. A detailed history revealed no trauma and no possibility of pharmacological dilation. Her vision was unchanged from her last visit to the department. The anisocoria was more marked in light. There was no ptosis and full ocular motility. There was no other ocular abnormality, except for the previously noted cataracts. It was observed that 0.125% Pilocarpine did not constrict the pupil, whereas 1% Pilocarpine constricted both pupils well. The anisocoria spontaneously disappeared in 3 days. Over the next 2 years, this patient presented four times with similar episodes of unilateral mydriasis, twice affecting the left eye (Figure 1b). Three of these episodes were accompanied by headache and two by ocular pain. Each time there were no other significant findings and pharmacological tests were negative.

Figure 1
figure 1

Episodic unilateral mydriasis. Either pupil may be dilated during an episode.


The features of our patient were consistent with a rare but innocuous condition termed ‘benign episodic unilateral mydriasis.’2 The affected individuals, usually women, often have a history of migraine.2 The episodes may be accompanied by blurred vision, orbital pain, headache, or photosensitivity.2 The dilated pupil is the only ocular finding. The cataracts in our patient were an incidental finding.

Anisocoria is often viewed as a worrying sign. A systematic approach is required to examine and investigate this condition.3 In the absence of any other ocular abnormality, unilateral mydriasis is rarely due to an intracranial cause.1 We found only one report of an intracranial aneurysm causing internal ophthalmoplegia without extraocular muscle involvement.4 Pharmacological blockade is the most common cause of such a presentation. These pupils can be identified by their refusal to constrict with 1% Pilocarpine.1 Adies pupil and trauma are other common causes. Once these are systematically excluded, benign episodic unilateral mydriasis should be considered a possibility.