A previously independent 64-year-old man presented to his local hospital with new-onset diplopia and dysarthria while watching television. Over the following days, he developed painless perioral and upper limb sensory disturbance, gait unsteadiness and limb ataxia, most pronounced in the upper limbs. He was initially admitted and treated for a cerebrovascular event, followed by a period of failed rehabilitation. At 6-week review, his symptoms had worsened, and he required the assistance of two people to walk short distances. He was then transferred to a regional neurology unit. It was uncertain whether his symptoms had stabilised or progressed over the preceding fortnight. He had no other neurological symptoms, normal bladder and bowel function, and no history of preceding illness. Table 1 provides the background history.
Assessment of eye movements identified a complex ophthalmoplegia, with complete failure of right eye abduction. Upgaze was bilaterally limited to 45° and…