Axillary Nerve Injury

During shoulder dislocation, the axillary nerve is the most commonly injured; occurring in about 5% of shoulder dislocations. After shoulder dislocation, the patient will be unable to abduct the shoulder, the deltoid will have no muscle tone. The supraspinatus muscle will be firing. There will be decreased sensation or absence of sensation on the area of the lateral shoulder. If the patient is older, the inability to abduct the shoulder may be attributed to tear of the rotator cuff, which is common in that age group with shoulder dislocation. This can be a confusing scenario. The physician will need to rule out a rotator cuff tear, and in order to do so, they should examine the shoulder sensation. The intact rotator cuff muscle may abduct the shoulder and confuse the examiner. It is important to check the sensation over the skin of the shoulder. It does not matter if the rotator cuff is intact or torn, the sensation over the skin of the lateral shoulder will tell you if there is axillary nerve palsy or not. Injury of the axillary nerve varies from neurapraxia to complete tear of the nerve.

The axillary nerve passes over the subscapularis and then curves backwards below it and underneath the shoulder joint capsule to enter the quadrangular space. The quadrangular space refers to the space bordered by the teres minor, the humerus, the long head of the triceps, and the teres major. Within the quadrangular space, the axillary nerve is accompanied by the posterior circumflex humeral artery. This artery is important because it is believed that this artery is the main blood supply to the humeral head. After passing through the quadrangular space, the axillary nerve divides into anterior and posterior divisions. The anterior division curves anterior under the deltoid muscle. The deltoid muscle is innervated by this anterior division of the axillary nerve. The posterior division supplies the teres minor muscle and the remaining posterior portion of the deltoid muscle as well as the skin over the shoulder. The anterior branch of the axillary nerve is located 5–7cm distal to the lateral edge of the acromion. Do not exceed deltoid splitting approach more than 5cm below the acromion or you risk injury to the axillary nerve. During surgery, adduction and external rotation moves the nerve away from the surgical field.

When the axillary nerve is injured, the patient should be given a sling for comfort and physical therapy should be initiated. It will be important for the physician to follow the patient’s progress clinically. The nerve will usually recover, but an EMG and further nerve studies may be necessary. Biphasic waveforms will be seen at 3–4 weeks if the nerve recovers. If the nerve does not recover, you will get fibrillation potentials and P-waves. Failure to abduct the shoulder after 4–6 months despite physical therapy means that the condition is permanent and the patient may not achieve abduction of the shoulder without recovery of the deltoid muscle. Since the intact rotator cuff muscle failed to abduct the shoulder and to take over the job of the deltoid muscle, then this condition is permanent. When the condition is permanent, the physician has two options. The physician can perform an exploration of the nerve for release, repair, or reconstruction of the nerve (reconstructed by a nerve graft). Otherwise, the physician could perform a tendon transfer, transferring the trapezius to the proximal humerus; however, the result of the transfer is usually poor.