Inferior Shoulder Dislocation: Axillary Nerve Injury, Symptoms, and Recovery

What nerve is injured in inferior shoulder dislocation?

In cases of inferior shoulder dislocation, the axillary nerve (also known as the circumflex nerve) is the nerve most commonly and significantly at risk of injury due to its anatomical proximity to the inferior aspect of the humeral head and glenoid.

Understanding Shoulder Dislocation

The shoulder, or glenohumeral joint, is a highly mobile ball-and-socket joint, formed by the head of the humerus (upper arm bone) and the glenoid fossa of the scapula (shoulder blade). Its remarkable range of motion comes at the cost of stability, making it the most frequently dislocated major joint in the body. A dislocation occurs when the humeral head completely separates from the glenoid fossa.

Types of Shoulder Dislocation:

• Anterior Dislocation: The most common type, where the humeral head moves forward and typically inferiorly.
• Posterior Dislocation: Less common, where the humeral head moves backward.
• Inferior Dislocation (Luxatio Erecta): The rarest type, accounting for less than 1% of all shoulder dislocations. In this severe injury, the humeral head is displaced directly inferiorly, often lodging below the glenoid, with the arm typically fixed in an abducted and externally rotated position, pointing upwards.

The Primary Nerve at Risk in Inferior Shoulder Dislocation

While any type of shoulder dislocation carries a risk of neurovascular injury, inferior shoulder dislocations pose a particular threat to specific structures. The axillary nerve is the nerve most frequently compromised in this scenario. Its vulnerability stems from its anatomical course, as it wraps around the surgical neck of the humerus, immediately inferior to the glenohumeral joint capsule.

When the humeral head is forcefully driven downwards and out of the glenoid in an inferior dislocation, it can stretch, compress, or directly contuse the axillary nerve against the bony structures, leading to neuropraxia (temporary nerve block), axonotmesis (axon damage), or, in severe cases, neurotmesis (complete nerve transection).

Anatomy and Function of the Axillary Nerve

To fully appreciate the implications of an axillary nerve injury, understanding its anatomy and function is crucial.

Origin and Course:

• The axillary nerve originates from the posterior cord of the brachial plexus (C5-C6 nerve roots).
• It passes through the quadrangular space, an anatomical landmark bounded by:
  • Superiorly: Teres minor muscle
  • Inferiorly: Teres major muscle
  • Medially: Long head of the triceps brachii
  • Laterally: Surgical neck of the humerus
• After exiting the quadrangular space, it divides into anterior and posterior branches.

Motor Innervation:

• Deltoid muscle: The primary abductor of the arm, responsible for lifting the arm away from the body in all planes (flexion, extension, abduction).
• Teres minor muscle: A rotator cuff muscle, assisting in external rotation of the arm.

Sensory Innervation:

• "Regimental Badge" area: The axillary nerve supplies sensory innervation to the skin over the lateral aspect of the shoulder, roughly where a military badge would be worn.

Clinical Manifestations of Axillary Nerve Injury

Injury to the axillary nerve following an inferior shoulder dislocation can result in a range of symptoms, from mild weakness to complete paralysis and sensory loss.

Key Signs and Symptoms:

• Motor Deficits:
  • Weakness or paralysis of shoulder abduction: Difficulty or inability to lift the arm away from the body, particularly between 15 and 90 degrees.
  • Shoulder flattening: Due to atrophy of the deltoid muscle over time, the normally rounded contour of the shoulder may appear flattened.
  • Impaired external rotation: Due to weakness of the teres minor.
• Sensory Deficits:
  • Numbness, tingling (paresthesia), or altered sensation over the "regimental badge" area on the lateral aspect of the shoulder.
• Pain: Localized pain around the shoulder may also be present.

It's important to note that swelling and pain immediately following a dislocation can mask some of these neurological signs, making thorough and repeated assessments critical.

Diagnosis and Management

Prompt diagnosis and appropriate management are crucial for optimizing recovery following an inferior shoulder dislocation and associated nerve injury.

Diagnostic Approach:

• Clinical Examination: A detailed neurological assessment is paramount, including:
  • Motor strength testing: Specifically assessing deltoid and teres minor function.
  • Sensory testing: Evaluating sensation over the lateral shoulder.
  • Observation: Looking for shoulder contour changes.
• Imaging Studies:
  • X-rays: To confirm the dislocation and rule out fractures of the humeral head or glenoid, which often accompany dislocations.
  • MRI: May be used to assess soft tissue damage, including the nerve itself, rotator cuff tears, or labral injuries.
• Electrophysiological Studies:
  • Electromyography (EMG) and Nerve Conduction Studies (NCS): These tests can confirm the presence and severity of nerve injury, differentiate between nerve damage types (e.g., neuropraxia vs. axonotmesis), and help monitor recovery over time.

Management Strategies:

• Immediate Reduction: The dislocated shoulder must be promptly reduced (relocated) by a trained medical professional to prevent further damage and alleviate pressure on the nerve.
• Immobilization: After reduction, the shoulder is typically immobilized in a sling for a short period to allow initial healing.
• Physical Therapy: A cornerstone of recovery. Rehabilitation focuses on:
  • Restoring pain-free range of motion.
  • Strengthening the surrounding shoulder musculature, including any recovering innervated muscles.
  • Proprioceptive training to improve joint stability.
• Pharmacological Management: Pain management and anti-inflammatory medications may be prescribed.
• Surgical Intervention: In rare cases of complete nerve transection or persistent, severe nerve compression that doesn't resolve spontaneously, surgical exploration and nerve repair or grafting may be considered.

Prognosis and Rehabilitation

The prognosis for axillary nerve injury following a shoulder dislocation varies widely depending on the severity of the injury.

• Neuropraxia: Most common and typically resolves spontaneously within weeks to months with full recovery of function.
• Axonotmesis: Recovery is slower, often taking several months to a year, as the nerve must regenerate. Full recovery is possible, but some residual weakness or sensory deficits may persist.
• Neurotmesis: Requires surgical intervention and has the poorest prognosis for full recovery.

Rehabilitation is crucial for all levels of injury:

• Early, controlled motion helps prevent stiffness.
• Progressive strengthening exercises are introduced as nerve function returns.
• Patient education regarding nerve recovery timelines and potential limitations is vital.

Conclusion

An inferior shoulder dislocation is a severe injury that places the axillary nerve at significant risk. Understanding the anatomy, function, and clinical manifestations of axillary nerve injury is paramount for fitness professionals and healthcare providers. Prompt diagnosis, immediate reduction, and a comprehensive rehabilitation program are essential to optimize recovery and minimize long-term functional deficits, allowing individuals to regain strength, mobility, and confidence in their shoulder function.