The shoulder is a complex and highly mobile joint, enabling a wide range of arm movements. Its intricate design relies on a network of muscles, bones, ligaments, and nerves. Nerves transmit signals between the brain and various parts of the body, orchestrating movement and sensation. The axillary nerve plays a significant part in proper shoulder function, providing insight into the mechanics of shoulder mobility and stability.
Anatomy and Role of the Axillary Nerve
The axillary nerve originates from the posterior cord of the brachial plexus, a complex network of nerves in the neck and shoulder region. It carries nerve fibers from the C5 and C6 spinal nerve roots.
The nerve then passes through a confined anatomical space known as the quadrangular space. This space is bordered by 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. Upon exiting this space, the axillary nerve divides into terminal branches.
These branches provide motor innervation to two shoulder muscles: the deltoid and the teres minor. The deltoid muscle is primarily responsible for abducting the arm, meaning lifting it away from the body. The teres minor, one of the rotator cuff muscles, contributes to the external rotation of the shoulder joint and helps stabilize the glenohumeral joint. The axillary nerve also carries sensory information, supplying sensation to the skin over the inferior part of the deltoid muscle, an area often referred to as the “regimental badge area”. This dual motor and sensory role highlights the nerve’s involvement in shoulder movement and feeling.
Common Causes of Axillary Nerve Injury
The axillary nerve is susceptible to injury from trauma, particularly around the shoulder joint. A frequent cause is a shoulder dislocation, especially an anterior-inferior dislocation, where the humeral head is displaced out of its socket. The nerve can become stretched or compressed during such an event.
Fractures of the surgical neck of the humerus, the upper arm bone just below the joint, also commonly lead to axillary nerve damage. The nerve passes close to this bone, making it vulnerable during a break. Direct trauma or compression to the shoulder area, such as from a fall or a blunt impact, can also injure the nerve.
The axillary nerve can be injured inadvertently during shoulder surgery. Its close proximity to the surgical field, particularly during procedures involving the inferior aspect of the shoulder, increases this risk. Conditions like quadrangular space syndrome, where structures within the quadrangular space compress the nerve, can also lead to injury, often seen in athletes performing overhead activities.
Recognizing Axillary Nerve Injury Symptoms
Damage to the axillary nerve results in motor and sensory deficits, reflecting its dual function. A primary motor symptom is difficulty or inability to abduct the arm, meaning lifting it sideways away from the body. This weakness stems from impaired function of the deltoid muscle, which the axillary nerve innervates. Patients may also experience weakness in externally rotating the shoulder, which is the movement of rotating the arm outward, due to compromised teres minor muscle function.
Sensory changes are also common. Individuals with an axillary nerve injury often report numbness, tingling, or a loss of sensation over the lateral aspect of the shoulder. This area, known as the “regimental badge area,” corresponds directly to the sensory distribution of the axillary nerve. These symptoms can appear suddenly or develop over time, depending on the nature and severity of the injury.
Diagnosis and General Treatment Approaches
Diagnosing an axillary nerve injury begins with a physical examination. A healthcare provider will assess muscle strength, particularly the ability to abduct and externally rotate the shoulder, and test sensation over the lateral shoulder. Weakness or atrophy of the deltoid muscle and numbness in the “regimental badge” area are often key indicators.
To confirm the diagnosis and determine the extent of nerve damage, electrodiagnostic studies such as nerve conduction studies (NCS) and electromyography (EMG) are employed. NCS measures how fast electrical signals travel through a nerve, while EMG evaluates the electrical activity of muscles at rest and during contraction. Imaging techniques, such as Magnetic Resonance Imaging (MRI), may also be used to rule out other shoulder pathologies or visualize the nerve itself.
Treatment approaches vary based on the injury’s severity. Many axillary nerve injuries, particularly milder ones, can heal spontaneously over several months. Conservative management is often the first line of treatment and involves rest, physical therapy, and sometimes pain medication. Physical therapy focuses on maintaining shoulder flexibility and strengthening surrounding muscles. For more severe injuries where significant improvement is not observed within 3 to 6 months, surgical intervention may be considered. This could include nerve repair, nerve grafting, or nerve transfer procedures to restore function. Medical consultation is necessary for proper diagnosis and treatment.