The axillary nerve is a major peripheral nerve of the upper limb. This article takes a look into the anatomy of this nerve, its motor and sensory functions, and any clinical correlations that are associated with it. The axillary nerve is formed within the axilla area of the upper limb, originating from the posterior cord of the brachial plexus and containing fibres from the C5 and C6 nerve roots. When examined in the axilla, it is located posterior to the axillary artery and anterior to the subscapularis muscle. It then continues on to exit the axilla via the quadrangular space at the inferior border of the subscapularis, often accompanied by the posterior circumflex humeral artery and vein.
The axillary nerve then winds around the surgical neck of the humerus, where it divides into three terminal branches. The posterior terminal branch provides innervation to the teres minor and deltoid muscles on the posterior aspect of the shoulder, as well as the skin on the lower part of the deltoid muscle as the upper lateral cutaneous nerve of the arm. The anterior terminal branch winds around the surgical neck of the humerus, providing motor innervation to the anterior portion of the deltoid muscle, and delivering cutaneous branches to the anterior and anterolateral shoulder. The articular branch supplies the glenohumeral joint.
The quadrangular space is an area bounded by the inferior aspect of teres minor, superior aspect of teres major, surgical neck of humerus, long head of triceps brachii, and the anterior subscapularis muscle. It is a pathway for neurovascular structures to move from the axilla anteriorly to the posterior shoulder and arm. It is through this space in which the axillary nerve, alongside the posterior circumflex humeral artery and vein, passes through. Unfortunately, the structures that pass through this space can become compressed due to trauma, muscle enlargement, or a space-occupying lesion - resulting in weakness in the deltoid and teres minor muscles, which is especially common in athletes who often do activities that involve overhead movements.
The sensory component of the axillary nerve is delivered via its posterior terminal branch, which is also responsible for providing motor innervation to the teres minor muscle. After the posterior terminal branch has performed this innervation, it continues on as the upper lateral cutaneous nerve of the arm. This nerve innervates the skin over the inferior part of the deltoid muscle, also known as the ‘regimental badge area’. Should a patient suffer axillary nerve damage, sensation at this area may be impaired or absent, and the patient may also experience paraesthesia (pins and needles) in the distribution of the axillary nerve.
The axillary nerve innervates two main muscles - the deltoid and teres minor. The deltoid is situated at the superior aspect of the shoulder and is responsible for performing abduction of the upper limb at the glenohumeral joint. It is innervated by the anterior terminal branch of the axillary nerve. The teres minor is a rotator cuff muscle which provides stability to the glenohumeral joint and acts to externally rotate the shoulder joint. It is innervated by the posterior terminal branch of the axillary nerve. Additionally, there is some evidence from research on cadavers to suggest that the axillary nerve may provide innervation to the lateral head of triceps brachii muscle.
To conclude, the axillary nerve is an important peripheral nerve of the upper limb, providing both motor and sensory innervation. Its terminal branches are responsible for innervation of the posterior deltoid and teres minor muscles, as well as the upper lateral cutaneous nerve of the arm, which innervates the skin over the lower part of the deltoid. Damage to the axillary nerve can cause disturbances in sensation at the ‘regimental badge area’, as well as paraesthesia in the distribution of the axillary nerve.
The axillary nerve is a branch of the brachial plexus that originates from the roots of C5 and C6. It passes medially to the surgical neck of the humerus, where it divides into three terminal branches.
The quadrangular space, also known as the quadrangular incisure, is a gap in the muscles of the posterior scapular region. It is bound by the inferior aspect of teres minor superiorly, the superior aspect of teres major inferiorly, the surgical neck of humerus laterally, the long head of triceps brachii medially, and the subscapularis anteriorly. This space serves as a pathway for neurovascular structures to move from the axilla anteriorly to the posterior shoulder and arm. These structures include the axillary nerve, as well as the posterior circumflex humeral artery and vein. Compression of these structures may occur as a result of trauma, muscle hypertrophy or a space-occupying lesion, resulting in weakness of the deltoid and teres minor. This is particularly common in athletes who perform overhead activities.
The sensory component of the axillary nerve is delivered via its posterior terminal branch. After the posterior terminal branch of the axillary nerve has innervated the teres minor, it continues as the upper lateral cutaneous nerve of the arm. It innervates the skin over the inferior portion of the deltoid, also known as the ‘regimental badge area’. Impairment or absence of sensation at this area is a key feature of axillary nerve damage.
The axillary nerve can be damaged through trauma to the proximal humerus or shoulder. It is often seen with other brachial plexus injuries. The most common mechanisms include fracture of the humeral surgical neck, shoulder dislocation or iatrogenic injury during shoulder surgery.
Motor functions may be affected; the patient will be unable to abduct the affected limb beyond 15 degrees due to paralysis of the deltoid and teres minor muscles. Sensory functions may also be affected; the patient will have loss of sensation over the inferior deltoid (‘regimental badge area’) as a result of the upper lateral cutaneous nerve of arm being affected.
Clinical tests to assess the severity of axillary nerve damage include deltoid extension lag and external rotation lag. Chronic lesions of the axillary nerve can result in permanent numbness at the lateral shoulder region, muscle atrophy, and neuropathic pain.
Erb's palsy is a condition caused by damage to the C5 and C6 roots of the brachial plexus, and as such, the axillary nerve is affected. The patient is usually unable to abduct or externally rotate at the shoulder. It is most commonly caused by excessive increase in the angle between the neck and shoulder, which stretches the nerve roots. The severity of the injury determines the prognosis, ranging from neuropraxia to avulsion.
