Repetitive compound muscle action potential: a characteristic diagnostic clue

A 27-year-old woman complained of gradually deteriorating symmetrical weakness of finger extension over 18 months since the start of her first pregnancy. She had no other neurological symptoms but did recall a previously stable history of very mild and non-progressive hand weakness, first noticed in early childhood. Also, her mother had long-standing, although much milder, unexplained weakness affecting the hands. There was symmetrical grade 3 weakness of finger and wrist extension, and grade 4 weakness involving flexion and abduction of fingers and thumbs. More proximal upper limb, neck and lower limb strength was normal. Her deep tendon reflexes, coordination, sensation and cranial nerve examination were normal. Serum creatine kinase was normal. The differential diagnosis included a distal myopathy, upper-limb-predominant distal hereditary motor neuropathy (eg, type V) or multifocal motor neuropathy. We performed nerve conduction studies to evaluate these possibilities. All four limbs showed sensory and motor nerve responses that were within normal absolute …