The brachial plexus innervates the shoulder and the upper limb. It is formed by the ventral rami, or the anterior divisions, of the fifth to eighth cervical nerves (C5-8), along with the ventral ramus of the first thoracic nerve (T1). These five ventral rami are referred to as the five roots. These five roots merge to form three trunks, the superior (C5-6), middle (C7) and inferior (C8-T1). These three trunks then divide into anterior and posterior divisions to form six divisions in total. These three groups then reform to result in three cords, which are named according to their position with respect to the second part of the axillary artery, which lies posterior to the pectoralis minor muscle. The posterior cord is formed from the posterior divisions of the three roots (C5-8,T1), the lateral cord is a formation of the anterior division of the superior and middle trunks (C5-7) and the medial cord is a continuation of the inferior trunk (C8, T1). There are five terminal branches of the brachial plexus derived from these cords. One of these terminal branches is the axillary nerve, which is derived from the posterior cord (C5-6). This article will talk about the anatomical course and function of the axillary nerve, followed by any clinical pathology related to this nerve.
The axillary nerve branches from the posterior cord and descends in the axilla posterior to the axillary artery and anterior to the subscapularis. It emerges from the axilla at the level of the lower border of the subscapularis, by traversing the quadrangular space. This is a space in the posterior scapular region, which is bounded by the superior margin of the teres major inferiorly, the inferior margin of the teres minor superiorly, the lateral margin of the long head of the triceps brachii medially and the surgical neck of the humerus laterally. Here, it supplies a branch to the shoulder joint. The posterior circumflex humeral artery and vein also run posterior to this nerve in this space. The axillary nerve then passes medial to the surgical neck of the humerus before dividing into three terminal branches referred to as:
- The anterior branch
- The posterior branch
- The articular branch
The anterior branch winds around the surgical neck of the humerus, posteriorly, with the posterior circumflex humeral vessels and supplies the anterior aspect of the deltoid muscle.
The posterior branch supplies the posterior aspect of the deltoid muscle as well as the teres minor. Following this, it passes around the lower border of the deltoid and is then referred to as the upper lateral cutaneous nerve of the arm. This nerve supplies the posterior skin of the arm and the lateral head of the triceps brachii muscle.
The articular branch supplies the glenohumeral joint.
The axillary nerve has both a motor and a sensory distribution of innervation. It has motor fibres that innervate the deltoid muscle, acting as an abductor, flexor and extensor at the shoulder joint, as well as the teres minor muscle, allowing lateral rotation of the glenohumeral joint. As mentioned above, it has sensory innervation to the skin of the arm superficial to the deltoid muscle, as the upper lateral cutaneous nerve of the arm.
Isolated damage to the axillary nerve results in a mononeuropathy, a type of peripheral neuropathy where only one nerve is involved. Potential causes include direct trauma to the nerve, pressure on the nerve from impingement of surrounding anatomical structures or shoulder injury.
For athletes who participate in contact sports, damage to the axillary nerve is the most common peripheral nerve injury. An injury to the axillary nerve can be due to dislocation of the shoulder joint as the two are in close proximity with one another. Lesions to the nerve can also occur in a fracture of the surgical neck of the humerus. During shoulder operations, the axillary nerve is vulnerable to damage and this can be a complication of these surgeries. Injury to the axillary nerve can result in the following:
- Paralysis of the deltoid and teres minor muscles
- Weakness of arm abduction
- Wasting of the deltoid muscle
- Anaesthesia or loss in sensation of the area of the skin posterior to the deltoid muscle
Atrophy of the deltoid muscle results in a loss of the normal rounded appearance of the shoulder, which gives the shoulder a flattened appearance.
Damage to the 5th and 6th spinal cervical nerves during delivery of a neonate results in Erb’s palsy. Since the axillary nerve stems from the upper trunk of the brachial plexus (C5-C6), the infant cannot abduct nor externally rotate the arm at the shoulder joint.
Quadrangular Space Syndrome can cause weakness of the deltoid muscle but more commonly results in atrophy of the teres minor muscle. This syndrome occurs when the muscles of the quadrangular space hypertrophy and thus impinge on the axillary nerve in this space.