A 59-year-old man presented with a three-week history of double vision, worse on looking to the left. He also had non-insulin dependent diabetes mellitus, hypertension and hypercholesterolaemia. For the previous six months he had been experiencing daily left-sided frontal headaches, present on awakening but which resolved during the day; they were not associated with nausea and did not have any meningitic or migrainous features. He had had an acute episode of vertigo four months previously, following which he was investigated at another hospital for a possible transient ischaemic attack. A computerised tomography (CT) head scan at that time had been reported as normal. He had occasionally felt vertiginous since then. He was an ex-smoker and drank alcohol only occasionally. Originally from the Philippines, he had lived in the UK for the last 26 years.

On examination his visual acuity was 6/6 in each eye with normal colour vision, visual fields, pupillary reactions and fundi. Spontaneous venous pulsation was visible bilaterally. There was no ptosis and his extra-ocular movements were normal apart from a partial left lateral rectus palsy, which did not fatigue. The rest of his neurological and systemic examination was unremarkable; in particular, his temporal arteries were pulsatile and non-tender. Given his vascular risk factors, a microvascular sixth nerve palsy was the working diagnosis, but there were atypical features—headache, a stuttering onset to the diplopia, and the partial nature of the lateral rectus palsy—and so further investigation was necessary. The full blood count, erythrocyte sedimentation rate (ESR) and C reactive protein (CRP) were all normal, and MRI of the brain was reported as normal.

His spectacles were fitted with a prism which corrected the diplopia for three months, although his headaches persisted. He then felt that his diplopia worsened and the prism no longer helped. He took to wearing …