A 61-year-old man presented with a 5 year history of progressive imbalance and clumsiness of the lower limbs, requiring him to look at his feet when walking. He complained of increased sensitivity of the soles of both feet, and occasional paraesthesiae in the toes, but no numbness. There were no upper limb symptoms. Sphincter function was normal. His only other symptom was weight loss of 18 kg over 3 years in the absence of significant gastrointestinal, respiratory or other systemic disease. There was no significant past medical or relevant family history.

On examination, there were no neurocutaneous stigmata. The gait was wide based and Romberg's sign positive. Sensory examination revealed diminished vibration sensation to the medial malleoli, and severe impairment of joint position …