Axillary Nerve: Anatomy, Course, Branches, Function, Imaging

Topic

Axillary nerve

The axillary nerve (also called the circumflex nerve) is a major branch of the posterior cord of the brachial plexus. It carries fibers from the C5 and C6 spinal roots and provides motor innervation to the deltoid and teres minor muscles, and sensory innervation to the skin over the lower part of the deltoid (regimental badge area).

It passes through the quadrangular space with the posterior circumflex humeral artery, wrapping around the surgical neck of the humerus. Because of this close relationship, it is vulnerable to injury in shoulder dislocation, proximal humeral fractures, and surgical interventions.

Synonyms

Circumflex nerve
Superior lateral cutaneous nerve of the arm (sensory branch)

Origin, Course, and Branches

Origin: Arises from the posterior cord of the brachial plexus (fibers from C5 and C6).
Course:
- Passes posteriorly through the quadrangular space along with the posterior circumflex humeral artery.
- Winds around the surgical neck of the humerus, lying deep to the deltoid.
- Divides into anterior and posterior terminal branches beneath the deltoid.
Branches:
- Anterior branch: Supplies the anterior and middle portions of the deltoid.
- Posterior branch: Supplies the posterior deltoid and teres minor; gives rise to the upper lateral cutaneous nerve of the arm.
- Articular branch: To the shoulder joint (glenohumeral capsule).

Relations

Anteriorly: Subscapularis muscle and shoulder joint capsule.
Posteriorly: Teres minor and long head of triceps.
Superiorly: Teres minor and capsule of shoulder joint.
Inferiorly: Teres major.
Lateral to: Long head of triceps within the quadrangular space.

Nerve Supply

Motor:
- Deltoid muscle: Abducts arm (beyond 15°).
- Teres minor muscle: External rotation of shoulder.
Sensory:
- Upper lateral cutaneous nerve of the arm: Skin over the inferior portion of the deltoid (“regimental badge area”).

Function

Motor function:
- Initiates and maintains abduction of the arm.
- Assists in external rotation of the shoulder.
Sensory function:
- Provides cutaneous sensation over the lateral shoulder region.
Joint proprioception:
- Articular branches to the shoulder capsule aid in proprioceptive feedback and joint stability.

Clinical Significance

Injury mechanisms:
- Anterior shoulder dislocation — stretches or compresses the nerve.
- Proximal humeral fractures — especially at the surgical neck.
- Iatrogenic trauma — during deltoid splitting or shoulder surgery.
Symptoms:
- Weakness or paralysis of deltoid and teres minor → impaired abduction and external rotation.
- Sensory loss over the lateral shoulder.
Chronic compression:
- Seen in overhead athletes (“quadrilateral space syndrome”).
Clinical tests:
- Deltoid muscle atrophy and loss of shoulder contour.
- Nerve conduction studies show delayed latency in injury.
Imaging importance:
- MRI and CT neurography are vital for assessing entrapment, trauma, and postoperative nerve integrity.

MRI Appearance

T1-weighted images:
- Normal nerve: Thin, continuous low-to-intermediate signal band with surrounding bright fat.
- Deltoid and teres minor: Normal intermediate muscle signal, with bright subcutaneous fat outlining.
- Pathology: Loss of perineural fat plane or focal thickening; chronic denervation shows fatty replacement (bright T1) in muscles.
T2-weighted images:
- Normal nerve: Intermediate-to-low signal, slightly darker than surrounding fat.
- Pathology: Increased T2 hyperintensity in nerve from edema, trauma, or entrapment.
- Muscles: Acute denervation → bright T2 hyperintensity due to edema; chronic denervation → dark signal from fibrosis.
STIR:
- Normal nerve and muscle: Intermediate-to-dark signal.
- Pathologic: Bright hyperintensity in affected nerve and muscle (indicative of neuritis or denervation edema).
- Excellent for detecting subtle nerve inflammation or post-traumatic edema.
Proton Density Fat-Saturated (PD FS):
- Normal nerve: Low-to-intermediate signal, surrounded by dark fat-suppressed tissue.
- Abnormal: Bright signal along the course of the axillary nerve within the quadrangular space in entrapment or trauma.
- Detects early denervation changes in the deltoid and teres minor before fatty atrophy develops.
T1 Fat-Sat Post-Contrast:
- Normal nerve: Minimal or no enhancement.
- Inflamed or injured nerve: Focal or diffuse enhancement along nerve trajectory.
- Chronic cases: Enhancing fibrotic sheath or neuroma formation.

CT Appearance

Non-Contrast CT:

The axillary nerve itself is not well visualized.
Indirect signs: Deltoid or teres minor atrophy and fatty infiltration suggest chronic denervation.
Fracture correlation: Identifies bony lesions at the humeral neck that can compress the nerve.
Quadrangular space: May show soft-tissue swelling or space-occupying lesions.

MRI image

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