Chronic Musculocutaneous Nerve Injury: An Important Differential in Progressive Arm Atrophy

An isolated musculocutaneous nerve (MCN) injury is a rare condition that can be easily missed if it presents late. A 28-year-old man reported painless and progressive wasting of the right arm for 6 months. On examination, there was visible wasting of the right biceps brachii muscle along with its slight weakness, depressed biceps jerk, and an impaired pinprick sensation in the lateral antebrachial cutaneous nerve distribution. He described a history of a road traffic accident 14 months beforehand. Based on the history and clinical examination, the differential diagnosis included an isolated MCN injury, upper trunk plexopathy, lateral cord plexopathy, C5/6 radiculopathy, and monomelic amyotrophy involving the C5/6 myotomes on the right side. The results of nerve conduction studies and electromyography were consistent with chronic proximal MCN neuropathy (right). In cases of arm wasting without pain or numbness, MCN injury should be included in the differential diagnosis, even in the presence of good power of the elbow flexors. The importance of detailed history-taking and clinical correlation cannot be over-emphasized in such cases.