Posterior Cutaneous Nerve of Arm: Anatomy, Course, Function, Imaging

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Posterior cutaneous nerve of arm

The posterior cutaneous nerve of the arm is a superficial sensory branch of the radial nerve, providing cutaneous innervation to the posterior surface of the upper arm. It arises in the axillary region before the radial nerve enters the spiral groove of the humerus.

This small nerve plays an important role in conveying sensation from the posterior and lower lateral aspects of the arm, and although minor in size, it serves as an important landmark in evaluating radial nerve lesions or localized neuropathies.

Synonyms

Dorsal cutaneous nerve of the arm
Superior posterior brachial cutaneous nerve

Origin and Course

Origin: Arises from the radial nerve in the axilla, proximal to the origin of the nerve to the long head of the triceps.
Course:
- Passes posteriorly through the axilla with the radial nerve.
- Pierces the deep fascia near the posterior axillary fold.
- Descends superficially along the posterior surface of the arm.
- Ends by supplying the posterior skin of the upper arm, particularly over the long head of triceps and near the olecranon region.

Relations

Superiorly: Lies near the axillary fascia and posterior axillary fold.
Inferiorly: Continuous with branches of the inferior lateral cutaneous nerve of the arm.
Medially: Related to the long head of triceps and posterior aspect of the humerus.
Laterally: Lies close to the posterior border of the deltoid and triceps fascia.
Posteriorly: Covered by skin and superficial fascia.

Nerve Supply

Provides sensory innervation to:
- Posterior surface of the upper arm, particularly over the triceps region.
- May overlap with branches of the intercostobrachial nerve (T2 dermatome).
- Occasionally anastomoses with the lower lateral cutaneous nerve of the arm.

Function

Sensory role: Supplies the skin of the posterior and lower lateral arm, providing cutaneous sensation over the triceps region.
Protective sensation: Detects touch, pressure, pain, and temperature from the posterior arm surface.
Clinical relevance: Serves as a sensory marker for assessing radial nerve integrity proximal to the spiral groove.

Clinical Significance

Nerve injury: May be affected in axillary trauma, humeral fractures, or posterior surgical approaches to the arm.
Radial neuropathy: Loss of sensation over the posterior arm can indicate proximal radial nerve involvement.
Entrapment: Rare, but compression within axillary fascia may cause localized paresthesia or pain.
Injection site caution: Superficial location near posterior axillary fold makes it vulnerable during intramuscular injections or lymph node dissection.
Imaging relevance: Evaluated in the context of radial nerve neuropathy, soft-tissue trauma, or post-surgical scarring.

MRI Appearance

T1-weighted images:
- Nerve appears as a thin, low-to-intermediate signal linear structure surrounded by bright subcutaneous fat.
- Course can be traced from the posterior axilla along the posterior arm.
- Normal nerve shows smooth contour with no signal interruption.
- Pathology: Thickened nerve or focal discontinuity may indicate neuritis or injury.
T2-weighted images:
- Nerve: intermediate signal linear structure
- Background muscle (triceps): Intermediate-to-low signal.
- Pathology: Increased T2 signal in nerve or surrounding fat indicates edema or inflammation.
STIR:
- Normal nerve: Intermediate-to-dark signal.
- Pathologic nerve: Bright hyperintensity in neuritis, trauma, or compression.
- Useful for detecting subtle perineural edema or entrapment.
Proton Density Fat-Saturated (PD FS):
- Normal nerve: Intermediate signal with distinct fascicular pattern.
- Pathology: Bright hyperintense appearance with surrounding fat stranding in nerve inflammation or traction injury.
- Ideal for detecting post-traumatic changes or scarring along fascial planes.
T1 Fat-Sat Post-Contrast:
- Normal nerve: Minimal or no enhancement.
- Pathologic nerve: Focal or diffuse enhancement in cases of neuritis, neuroma, or post-surgical inflammation.
- Helps differentiate nerve inflammation from fibrosis or mass lesions.

CT Appearance

Non-Contrast CT:

Nerve not directly visualized but appears as a faint soft-tissue density in subcutaneous fat posterior to triceps.
Indirect signs: Surrounding fat stranding, hematoma, or edema may indicate injury.
Bone detail: Used to assess associated humeral fractures or callus formation compressing the radial nerve trunk.

Post-Contrast CT (standard):

Enhancement: Minimal for normal nerve; increased enhancement in inflamed or scarred tissue.
Soft-tissue windowing: May reveal perineural thickening or post-traumatic fibrosis.
Best used as an adjunct to MRI for evaluating radial nerve pathway and peri-neural pathology.

MRI image

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