The axillary nerve is located in the shoulder, controlling certain movements and providing sensation to part of the upper arm. Understanding this nerve is important for comprehending shoulder and arm mechanics.
Anatomical Position
The axillary nerve originates from the posterior cord of the brachial plexus, a network of nerves formed from spinal nerves C5 and C6. From its origin within the axilla, the nerve travels downward, positioned behind the axillary artery and in front of the subscapularis muscle. It exits the axilla by passing through the quadrangular space, accompanied by the posterior circumflex humeral artery and vein.
This quadrangular space is defined by anatomical boundaries: the teres minor muscle superiorly, the teres major muscle inferiorly, the long head of the triceps brachii medially, and the surgical neck of the humerus laterally. The axillary nerve then wraps around the surgical neck of the humerus. This close proximity to the shoulder joint and humerus makes it susceptible to injury. The nerve divides into anterior and posterior divisions.
Functional Role
The axillary nerve is a mixed nerve, containing both motor and sensory fibers. Its motor fibers innervate two muscles: the deltoid and the teres minor. The deltoid muscle is responsible for lifting the arm away from the body (abduction) and assists in shoulder flexion and extension.
The teres minor, a rotator cuff muscle, receives innervation from the posterior branch of the axillary nerve and contributes to external rotation and stabilization of the shoulder joint. The sensory component of the axillary nerve provides sensation to the skin over the lower part of the deltoid muscle, often called the “regimental badge” area. It also innervates the glenohumeral joint capsule, providing sensory feedback from the shoulder joint.
Common Injuries and Symptoms
The axillary nerve’s anatomical course makes it vulnerable to injury from traumas affecting the shoulder. Common causes include anterior shoulder dislocation, where the nerve can be stretched or compressed. Fractures of the surgical neck of the humerus, the upper part of the arm bone, also pose a risk due to the nerve’s close proximity. Direct trauma to the shoulder can similarly lead to axillary nerve damage.
Symptoms of an axillary nerve injury involve both motor and sensory deficits. Individuals may experience weakness or difficulty lifting their arm away from the body (abduction) due to impaired deltoid function. Weakness in externally rotating the arm can also occur if the teres minor muscle is affected. Sensory symptoms include numbness, tingling, or reduced sensation over the lateral aspect of the shoulder, corresponding to the “regimental badge” area. Muscle atrophy, or wasting, of the deltoid muscle may become visible over time.
Diagnosis and Management
Diagnosing an axillary nerve injury begins with a physical examination. A healthcare provider assesses muscle strength, particularly testing the deltoid and teres minor, and evaluates sensation in the affected skin area. Observing for visible muscle atrophy can also provide clues. Electrodiagnostic studies are used to confirm the diagnosis and determine the extent of nerve damage.
Nerve conduction studies (NCS) and electromyography (EMG) measure the electrical activity of nerves and muscles. These studies help identify the location and severity of the nerve injury, distinguishing it from other issues. Management approaches vary depending on the severity and cause of the injury. Axillary nerve injuries, especially those from stretching, can heal spontaneously over several months with conservative measures.
Conservative treatment includes rest, physical therapy to maintain shoulder flexibility and strength, and pain management. If no improvement occurs after several months, or if the injury is severe, surgical intervention may be considered. Surgical options range from neurolysis, which frees the nerve from scar tissue, to nerve grafting or transfer in complex cases. Recovery can be a lengthy process, sometimes extending over a year, and regular medical consultation is important for guidance throughout treatment.