Axillary Nerve Injury

Etiology

Axillary nerve injury typically presents with other brachial plexopathies. Axillary neuropathies are due to traumatic injuries, traction injuries, quadrilateral space syndrome, and brachial neuritis (also called neuralgic amyotrophy or Parsonage-Turner syndrome).[3]

An anterior dislocation and forced abduction of the shoulder joint appear to be common causes of axillary nerve injury in young people.

Epidemiology

An axillary nerve mononeuropathy is a rare event, occurring in about 0.3 to 6% of all brachial plexus injuries.[2] Axillary nerve injuries constitute 6% to 10% of all brachial plexus injuries during shoulder surgery.[7] Recent research showed that the risk of axillary nerve injury after glenohumeral dislocation increased with age. Approximately 65% of patients over 40 were diagnosed by electromyography (EMG) with an axillary nerve lesion.[8] Brachial neuritis has an estimated incidence of 1 to 3 per 100,000 annually with a female predilection.[9]

Pathophysiology

The axillary nerve injury may occur due to over-stretching of the axillary nerve during anterior should dislocation. The axillary nerve is most susceptible to injury at the origin from the posterior cord, in the quadrilateral space, the anteroinferior aspect of the shoulder capsule, and within the subfascial surface of the deltoid muscle.

History and Physical

The principle presenting symptoms are a weakness in glenohumeral abduction with or without numbness to the lateral shoulder area. Patients may also present with weakness in glenohumeral external rotation; however, this may not be apparent due to the ability of the infraspinatus. In patients presenting after dislocation or fracture, signs of trauma will be evident on physical exam. During external rotation and abduction, one may note the weakness of the deltoid and teres minor muscles.

Nevertheless, patients may not report muscle weakness or paresthesia due to acute pain and limited range of motion.[2] A compressive neuropathy may occur after blunt trauma to the deltoid muscle. Atraumatic neuropathy preceded by localized pain presents in brachial neuritis.[2][10] Patients with quadrilateral space syndrome (QSS) present with vascular symptoms such as cyanosis and pallor of the distal upper extremity and splinter hemorrhages in addition to neurologic symptoms.[11]

Evaluation

A proper history is essential in determining the correct intervention necessary. At the very least, the provider should obtain information on symptom onset, palliative and provoking factors, quality of the symptoms, radiation of symptoms to other regions, the severity of pain (if applicable), and timing. Key questions to consider are trauma, focal weakness, numbness and tingling, cyanosis, limitations in the range of motion, and pain.

The physical exam should begin with a visual inspection of the upper extremity, noting the presence or absence of trauma. The provider should palpate the ipsilateral neck and upper extremity for tenderness and muscle tone, followed by evaluating passive and active ranges of motion. Next, a neurologic exam focused on the axillary nerve and shoulder should also include assessing the spinal accessory, suprascapular, long thoracic, musculocutaneous, and radial nerves.[2] Several specialty clinical tests that evaluate the deltoid and teres minor muscles exist that may guide providers during assessment for axillary nerve injury in the absence of trauma, muscle strain, or known tendinopathies. The swallow-tail, deltoid extension lag, and Bertelli tests have been used to examine deltoid muscle weakness.[12][13] The external rotation lag, drop arm, and Patte tests evaluate the function of the teres minor muscle.[14]

Radiographs of the shoulder can help identify fractures involving the shoulder region after traumatic events. Shoulder magnetic resonance imaging (MRI) should be ordered if compressive neuropathy or inflammatory processes are suspected or in cases of chronic neuropathy that has resulted in muscle atrophy. The gold standard for diagnosis confirmation is EMG. Additionally, EMG findings can help establish a patient's baseline to evaluate recovery.[2]

Treatment / Management

Most axillary nerve injuries are treatable conservatively; however, select cases should require surgical management. For atraumatic injuries, a baseline EMG should occur within 1 month, and treatment with physical therapy (PT) should begin. In traumatic shoulder injury involving the axillary nerve, most patients may recover with non-operative treatment; however, there is the possibility of permanent paralysis. In many cases, then the shoulder is reduced, the need for surgical intervention is minimal. The reduced shoulder is immobilized for 4-6 weeks (young patients) or 7-10 days (elderly). This is then followed by extensive rehabilitation to regain muscle strength and shoulder mobility.

Surgical intervention can be considered in closed trauma if EMG shows no improvement after 3 months; however, acute nerve lesions warrant expedited surgical management. Neurorrhaphy, neurolysis, and nerve grafting are all possible surgical options. In patients with chronic muscle atrophy, muscle transfer may be a therapeutic option.[2]

For patients with neuropraxia, full recovery occurs over 6-12 months. The muscle weakness spontaneously recovers without any specific treatment.

For those with axonotmesis, the recovery rates are about 70% to 80% and often take many months. The patient should be followed with serial electromyogram (EMG) studies to assess for regeneration. If by 6-9 months, there are no signs of recovery, surgical exploration may be considered.

For those with neurotmesis, there is never any recovery without surgical intervention.

Differential Diagnosis

The following should be potential differentials when considering an axillary nerve lesion:[2][15]

• Cervical radiculopathy
• Thoracic outlet syndrome
• Rotator cuff tear
• Brachial plexopathy
• Quadrilateral space syndrome
• Brachial neuritis
• Glenohumeral fracture/dislocation
• Subacromial impingement syndrome
• Herpes zoster

Prognosis

Most injuries to the axillary nerve are considered low-grade following glenohumeral dislocation and will often result in complete recovery within 7 months.[8] A systematic review showed 85% of nerve graft patients and 79% of nerve transfer patients reached muscle strength grades 4/5 or greater at 18 to 24 months post-op.[16]

Complications

Chronic axillary nerve lesions result in permanent numbness to the lateral shoulder region, atrophy of the deltoid and teres minor muscles, and possibly chronic neuropathic pain.