Axillary nerve

Origin and course?

The axillary nerve arises from the posterior cord (fasciculus posterior) of the brachial plexus.

Topography and pathway of the nerve:

1. The nerve leaves the axillary fossa through the quadrilateral space (foramen quadrilaterum) on the posterior wall of the fossa.
2. In this space, it passes together with the posterior humeral circumflex artery (a. circumflexa humeri posterior), lying proximal to the vessel.
3. The nerve lies adjacent to the axillary recess (recessus axillaris) of the shoulder joint capsule.
4. Further, it winds 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 fascia of the deltoid muscle.

Skin projection: the neurovascular bundle is projected along a vertical line drawn from the posterolateral angle of the acromion to the intersection with the posterior border of the deltoid muscle. On average, this distance is 6 cm from the acromion, and when the shoulder is abducted at a right angle, it decreases to 2.5–3.0 cm.

What does the axillary nerve innervate?

• Deltoid muscle (m. deltoideus) — elevation, or abduction, of the arm to the side up to the horizontal level.
• Teres minor muscle (m. teres minor) — lateral rotation of the arm.
• Skin — cutaneous sensation disturbances in the nerve distribution area may occur when it is damaged.

Clinical Significance and Trauma

Isolated injuries of individual nerve branches are rare — 0.38%; axillary nerve lesions are among the most frequent isolated injuries. Young males are predominantly in the risk group.

Mechanisms of injury:

• Fractures of the surgical neck of the humerus: the nerve can be trapped by bone fragments or compressed by developing callus.
• Purulent omarthritis: the nerve is involved in the inflammatory process when pus breaks through the capsule of the shoulder joint via the axillary recess (recessus axillaris).

Clinical presentation of the lesion:

• Main symptom: paresis or paralysis of the deltoid muscle.
• Functional deficit: inability to abduct the shoulder to the horizontal level.
• Impairment of skin sensitivity in the area of nerve distribution.

To determine the severity of peripheral nerve damage and outline the recovery prognosis, instrumental methods are used: electromyography (EMG) and nerve conduction velocity (NCV) testing.

Developmental anomalies

In rare cases, a high origin of the superficial branch of the radial nerve is observed in the axilla. With this anomaly, this branch passes through the quadrilateral space along with the axillary nerve and posterior humeral circumflex artery. In stenosis of the quadrilateral space, the superficial branch of the radial nerve may be involved in the pathological process.

Class and exam questions

What else to study on this topic

• Quadrilateral space (foramen quadrilaterum)
• Brachial plexus (plexus brachialis)
• Deltoid muscle (m. deltoideus)
• Posterior humeral circumflex artery (a. circumflexa humeri posterior)

Other topics in the "Anatomical Structures" section

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