The tonic pupil is characterised by poor reactivity to light, a slow tonic constriction and redilation to a near target, and supersensitivity to topical dilute pilocarpine.1 Most instances do not have an identifiable cause; however, a variety of conditions including herpes zoster, orbital trauma including surgery, and paraneoplastic syndromes may result in tonic pupils. The mechanism(s) which produce tonic pupils are not known, although axonal loss within the ciliary ganglion is generally believed to result in supersensitivity of the iris sphincter muscle. Giant cell arteritis (GCA) has been reported as an uncommon cause of tonic pupils.2 Furthermore, the cause of tonic pupils in GCA is unknown and direct evidence for an ischaemic cause is not convincing. We used orbital colour Doppler imaging to study orbital and ocular blood flow in a patient with GCA, unilateral visual loss, and bilateral tonic pupils. A 58 year old woman presented with visual loss of the left eye for 2 days. She had had neck discomfort and intermittent jaw claudication over the previous 2 weeks. She had no past medical or ocular problems. Visual acuity was 20/20 in the right eye and no light perception in the left eye. The right pupil reacted briskly to light, with a normal consensual response, the left was amaurotic. She identified all of the Ishihara pseudoisochromatic colour plates with the right eye, and automated perimetry of the right eye was normal. Extraocular motility was normal. Funduscopy of the right eye (Fig 1A) was normal, and the left optic disc was pale and swollen, with retinal cotton wool infarcts within the retina, chiefly within the left …