Applied Anatomy

The axillary nerve (also called the circumflex nerve) arises from the posterior cord of the brachial plexus near the shoulder joint. Its fibers originate from the C5 and C6 roots and travel through the upper trunk to the posterior cord. The nerve has a very short path. It courses posteriorly, inferior to the shoulder joint where it traverses the quadrilateral space, bounded superiorly by the teres minor muscle, inferiorly by the teres major muscle, medially by the long head of the triceps muscle, and laterally by the humeral neck. At this point, the axillary nerve gives branches to the teres minor muscle and then it curves around the neck of the humerus, under the deltoid muscle. There, it terminates into two branches, one posterior and the other, anterior; both innervate the deltoid muscle. The upper lateral cutaneous nerve of the arm, which innervates the skin overlying the deltoid muscle (Figure C10-1), originates from the posterior branch.

Figure C10-1 The axillary nerve, a terminal branch of the posterior cord.

(From Haymaker W and Woodhall B. Peripheral nerve injuries: principles of diagnosis. Philadelphia, PA: WB Saunders, 1953, with permission.)

Clinical Features

Injury of the axillary nerve is associated most often with shoulder trauma or surgery around the shoulder joint; or, it may be a component of idiopathic brachial plexitis (Table C10-1). Axillary mononeuropathy is a common complication of shoulder dislocation, especially among the elderly. Also, the axillary nerve may be injured individually during an episode of acute brachial plexitis (neuralgic amyotrophy); alternatively, this may occur with the suprascapular, long thoracic, or anterior interosseous nerve (refer to Case 16).

Table C10-1 Common Causes of Axillary Nerve Injury

Axillary nerve lesion causes weakness of shoulder abduction and extension, and a patch of sensory loss and paresthesias over the lateral deltoid (see Figure C10-1). Deltoid atrophy becomes predominant later resulting in flattening of the shoulder. Weakness of the teres minor is seldom clinically significant since the supraspinatus muscle performs similar functions.

Axillary mononeuropathy may mimic C5 or C6 radiculopathies, but biceps and brachioradialis muscle weakness and depressed reflexes are common findings in C5 and C6 radiculopathies. Lesions of the upper trunk or posterior cord of the brachial plexus result also in weakness of the deltoid muscle. However, there is weakness of the biceps and brachioradialis muscles in upper trunk lesions and with posterior cord lesion there is often associated weakness of radial innervated muscles including the triceps and wrist and finger extensors. Musculoskeletal conditions such as rotator cuff tear, adhesive capsulitis of the shoulder, and rupture of deltoid muscle tendon may present with apparent weakness of the deltoid muscle, but are associated with significant shoulder pain and restriction of passive shoulder abduction.

In general, the prognosis for axillary nerve lesions is relatively good because of the short distance required for reinnervation to the target muscle (deltoid). Unfortunately, adhesive capsulitis of the shoulder develop in many patients, which, by itself, has a negative effect on final functional outcome. Thus, an aggressive stretching and range-of-motion program should be initiated as soon after diagnosis as possible.

The quadrilateral space syndrome is a rare pain syndrome that implicates entrapment of the axillary nerve and accompanying posterior humeral circumflex artery as they pass this space. The reported patients have been young athletes such as professional volleyball players. The onset of symptoms is insidious and characterized by pain in the shoulder which worsens with shoulder abduction, external rotation, and flexion. Weakness of the deltoid is uncommon but can be difficult to examine in the presence of severe pain in the shoulder.

Electrodiagnosis

The main purpose of the EDX studies in patients with suspected axillary mononeuropathy is to confirm that the abnormalities are restricted to the axillary nerve distribution and to exclude a brachial plexopathy, particularly of the upper trunk or posterior cord, and a cervical radiculopathy, mostly of the C5 and C6 roots. The sensory nerve conduction studies are extremely useful in these cases because they usually are abnormal in relatively mild plexus lesions, assisting in the diagnosis of brachial plexopathy that sometimes could not be detected on clinical evaluation. Among various studies in the upper extremity, the following SNAPs should be normal in isolated axillary nerve lesions (Figure C10-2):

In cases of suspected axillary nerve lesions, the role of motor nerve conduction studies is twofold. First, they confirm what is seen on the needle EMG, such as normal musculocutaneous CMAP with normal needle EMG of the biceps muscle. The second, and perhaps more important, role is prognostic; the CMAP amplitude is the best semiquantitative and objective measure of axonal loss. Thus theoretically, and extrapolating from other proximal nerves for which only a single site of stimulation is possible (such as the femoral or facial nerve), a low axillary CMAP, stimulating Erb point and recording the deltoid muscle, is consistent with a severe axonal loss lesion and a protracted recovery. However, Erb point stimulation is supraclavicular and is likely to result in widespread stimulation of the brachial plexus, including the posterior cord and the proximal axillary nerve. Hence, it is conceivable that a demyelinative axillary nerve lesion around the head of the humerus (i.e., distal to the stimulation point) may result in distal conduction block, thus leading to a low axillary CMAP. Therefore, caution should be used when making definitive prognostication in cases of axillary nerve lesion.

On needle EMG, certain muscles are essential for accurate localization of the lesion (Table C10-2). These muscles are listed in order of their importance: