Musculocutaneous neuropathy

Background

Musculocutaneous neuropathy is a rare peripheral nerve injury involving the musculocutaneous nerve, which arises from the lateral cord of the brachial plexus (C5–C7). It provides motor innervation to the biceps brachii, brachialis, and coracobrachialis muscles, and sensory innervation to the lateral forearm via its terminal branch—the lateral antebrachial cutaneous nerve. Injury to this nerve may result from direct trauma, shoulder dislocation, surgical complications, or repetitive overhead activity. Recognition in the ED is critical to guide appropriate workup, limit unnecessary imaging, and arrange early neurology or rehabilitation referral when needed.

Clinical Features

Motor deficits:

• Weakness of elbow flexion (biceps, brachialis)
• Weakness of forearm supination (biceps)

Sensory loss:

• Numbness or paresthesia over the lateral aspect of the forearm
• Atrophy of the biceps may be seen in delayed or chronic cases

Reflexes:

• Biceps reflex may be diminished or absent

Usually unilateral

Differential Diagnosis

• Cervical radiculopathy (C5–C6)
• Brachial plexopathy (upper trunk/lateral cord lesions)
• Median or radial nerve injury
• Axillary nerve injury (especially post-shoulder dislocation)
• Neuralgic amyotrophy (Parsonage-Turner syndrome)
• Humeral fracture with nerve involvement
• Supraclavicular trauma or compression
• Iatrogenic injury (e.g., from shoulder surgery or traction during anesthesia)

Evaluation

Workup

• Detailed neurologic exam focusing on motor strength (biceps, brachialis), forearm supination, and sensory testing of the lateral forearm
• Reflex testing (biceps and brachioradialis)
• Cervical spine exam to assess for radiculopathy
• Consider:
  • Cervical spine imaging (X-ray or MRI) if radicular symptoms or trauma present
  • Shoulder X-ray if trauma or suspected dislocation
  • MRI brachial plexus if concern for plexopathy
  • Electrodiagnostic studies (EMG/NCS)—best for confirming diagnosis, typically performed 2–3 weeks after symptom onset

Diagnosis

Primarily clinical, based on focal motor and sensory deficits in the musculocutaneous distribution. Confirmatory diagnosis is often made with electromyography and nerve conduction studies, which help localize the lesion and exclude more proximal plexus or root pathology.

Management

• Supportive care for most cases:
  • Activity modification (especially avoiding aggravating overhead motions)
  • Pain control (NSAIDs, acetaminophen)
  • Physical therapy to maintain joint range of motion and reduce disuse atrophy
• In cases of traumatic injury or surgical complication, refer early to neurology or orthopedic surgery
• No role for steroids unless part of an inflammatory neuropathy (e.g., Parsonage-Turner syndrome)

Disposition

• Outpatient follow-up is appropriate for stable patients with isolated deficits
• Refer to neurology for persistent weakness, unclear diagnosis, or if EMG is indicated
  • Urgent imaging or consultation if:
  • Rapid progression of symptoms
  • Suspicion for cervical spine or plexus injury
  • Associated trauma (e.g., fracture, shoulder dislocation)

See Also

Brachial Plexus Injury

Radial Nerve Palsy

Cervical Radiculopathy

Neuralgic Amyotrophy