Review Question - QID 216490

This young man with a history of shoulder trauma who presents with pain, visible shoulder deformity, and an arm held abducted and externally rotated likely has an anterior shoulder dislocation. The most likely nerve to be injured in this type of injury is the axillary nerve.

Anterior shoulder dislocations are characterized by a complete displacement of the humeral head out of the glenoid and are often due to trauma during contact sports or falls. Patients with an anterior shoulder dislocation present with pain and visible deformity of the affected shoulder. They may hold their arm abducted and externally rotated to relieve the pain. Axillary nerve injury is often present since the nerve passes directly beneath the glenohumeral joint capsule, making a thorough neurological exam an important part of the workup for any shoulder dislocation. Symptoms of axillary nerve injury include loss of sensation over the superior lateral arm and deltoid weakness, manifested as weak arm abduction. Most cases of axillary nerve dysfunction resolve with reduction.

Avis and Power provide a review of the diagnosis and treatment of axillary nerve injuries associated with shoulder dislocation. The authors find that injury to the axillary nerve is present in many shoulder dislocations. Risk factors associated with a higher-grade axillary injury include concurrent fracture to the humerus or scapula, vascular injury, a delay in shoulder reduction longer than 2 hours, and neurological deficit or neurologic pain in the axillary nerve distribution. The authors recommend early identification of patients with high-grade injuries and prompt referral to peripheral nerve injury centers.

Incorrect Answers:
Answer 2: Long thoracic nerve injury classically occurs after breast surgery or trauma to the lateral chest wall. Damage to the long thoracic nerve causes motor impairment of the serratus anterior muscle, which manifests as winging of the scapula. Long thoracic nerve injury is not classically associated with shoulder dislocation.

Answer 3: Musculocutaneous nerve injury is usually an iatrogenic injury during invasive procedures. An injury to this nerve manifests as a sensory deficit over the lateral part of the forearm or weakness in the coracobrachialis, brachioradialis, or biceps brachii (responsible for glenohumeral flexion/adduction, elbow flexion, and glenohumeral flexion/abduction, respectively). Injury to the musculocutaneous nerve during shoulder dislocation is rare.

Answer 4: Radial nerve injury is classically caused by “Saturday night palsy” due to prolonged compression of the nerve over the spiral groove of the humerus (as in sleeping with an arm draped over the back of a chair). Radial nerve injuries classically present with wrist drop (extensor carpi radialis longus and brevis), weakness in finger extension (extensor digitorum), weakness in elbow extension (triceps), and sensory loss over the dorsal aspect of the hand. This patient's preserved wrist and finger extension makes radial nerve injury unlikely.

Answer 5: Spinal accessory nerve injuries manifest as weakness of the trapezius muscle (as during shrugging against resistance) as well as weakness or paralysis of the sternocleidomastoid muscle. These injuries are not associated with sensory loss. This patient with preserved sternocleidomastoid strength (evidenced by his ability to shrug) is unlikely to have a spinal accessory nerve injury.

Bullet Summary:
The axillary nerve is the most likely nerve to be injured in an anterior shoulder dislocation and may present with loss of sensation over the superior lateral arm and deltoid weakness.