A 36-year-old man presented with a 4-year history of generalised muscle atrophy, mild tetraparesis and muscular cramps boosted after exposure. He mentioned a marked weight loss over the course of 3 years. The patient had a concomitant diagnosis of acute myeloid leukaemia. He was treated with chemotherapy. Five years ago, he had received allogeneic (haematopoietic) stem-cell transplantation from his cousin owing to a relapse.

The neurological examination showed a mild tetraparesis (Medical Research Council grade 4–5), accentuated at the distal arms and legs, and discrete generalised muscle atrophy. The facial muscles were paretic, leaving the impression of a myopathic face. Percussion myotonia at the arms could be provoked intermittently. Other signs of myotonia were absent. Muscle cramps were triggered by exercise. The patient showed a male-pattern frontal balding and had diabetes with insulin resistance. Testicular atrophy and cataract were absent. Laboratory findings included testosterone levels within the lower normal range and a slightly–raised serum creatine kinase level. The patient’s parents did not have any neuromuscular disorders.

The electromyogram showed mild myopathic changes and sparse spontaneous activity in the deltoid muscle, but no myotonic discharges. Nerve conduction studies showed a discrete decrease in the sensory nerve action potential amplitude in the sural nerve compatible …