Origin and Course
The axillary nerve arises from the posterior cord (fasciculus posterior) of the brachial plexus.
Topography and Course:
- The nerve leaves the axilla by passing through the quadrangular space (foramen quadrilaterum) in the posterior wall of the axillary cavity.
- In this space, it travels alongside the posterior circumflex humeral artery (a. circumflexa humeri posterior), lying proximal to the vessel.
- The nerve lies in close proximity to the axillary recess (recessus axillaris) of the glenohumeral joint capsule.
- It then wraps around the surgical neck of the humerus (collum chirurgicum humeri) from posterior to anterior.
The fascial sheath of the nerve is connected with the deep layer of the deltoid muscle fascia.
Surface Anatomy (Projection): The neurovascular bundle is projected along a vertical line dropped from the posterolateral angle of the acromion to its intersection with the posterior border of the deltoid muscle. On average, this distance is 6 cm from the acromion, reducing to 2.5–3.0 cm when the arm is abducted to 90 degrees.
Innervation
- Deltoid muscle (m. deltoideus) — elevates and abducts the arm horizontally.
- Teres minor muscle (m. teres minor) — laterally rotates the arm.
- Skin — cutaneous sensory deficits may occur in the innervation area (over the lower part of the deltoid, known as the 'sergeant's patch') if the nerve is damaged.
Clinical Significance and Injuries
Isolated injuries to individual nerve branches are rare (~0.38%), with axillary nerve lesions being among the most frequent isolated peripheral nerve injuries in this category. Young males are predominantly at risk.
Mechanisms of Injury:
- Fractures of the surgical neck of the humerus: The nerve can be stretched or torn by bone fragments or compressed by developing callus.
- Suppurative glenohumeral arthritis: The nerve can become involved in inflammation when pus ruptures from the shoulder joint capsule through the axillary recess (recessus axillaris).
Clinical Presentation:
- Primary symptom: Paresis or paralysis of the deltoid muscle.
- Functional deficit: Inability to abduct the arm to the horizontal level.
- Cutaneous sensory loss over the lateral shoulder.
To determine the severity of peripheral nerve injury and prognosis for recovery, instrumental diagnostics are used: electromyography (EMG) and nerve conduction studies (NCS).
Developmental Variations
In rare cases, a high origin of the superficial branch of the radial nerve is observed in the axilla. With this variation, this branch passes through the quadrangular space alongside the axillary nerve and the posterior circumflex humeral artery. In cases of quadrangular space syndrome, this superficial radial nerve branch may become involved in the pathological process.