Musculocutaneous neuropathy

MN can occur proximally where it painlessly affects biceps strength and sensation in the forearm or more distally near the elbow where is is purely a sensory syndrome accompanied by pain. Heavy physical activity with resistive movements of the upper extremity is the usual inciting factor, although it can occur with anterior shoulder dislocation and traumatically (gunshot or stab wound or secondary to surgery). Rest is the treatment and the prognosis is good.

The distal syndrome is due to compression of the lateral antebrachial cutaneous nerve near the biceps aponeurosis with strenuous activity with the arm hyper-extended or the forearm pronated. It is reported in racket sports, windsurfing and swimming. It also occurs after venipuncture, catheterization and placement of an av fistula. Surgery in the antecubital fossa and arm splinting can be responsible. So can pressure of a strap across the elbow or arm restraints. Clinical presentation is numbness and dysesthesias in radial-volar forearm and elbow pain that resembles lateral epicondylitis. Elbow flexion from biceps is absent although the brachioradialis muscle can compensate somewhat. Biceps tendon jerk is absent. Initial treatment is conservative and may involve local steroid injection although decompresive surgery is often necessary.

Nerve is often involved in combinations of axillary, radial, and suprascapular neuropathies or ABN (acute brachial neuritis). Differential-- in biceps rupture, there is not sensory loss, and the biceps muscle contracts into a small bal. With a C6 radiculopathy, there will be involvement of supra/infrapinatus, deltoid and brachioradialis. C6 sensory signs often extend into the hand, whereas lateral cutaneous nerve of forearm stops at the wrist. Surgical exploration can show the distal syndrome due to biceps aponeurosis in patients lacking another clear cause.