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

Topic

Posterior cutaneous nerve of forearm

The posterior cutaneous nerve of the forearm is a purely sensory branch of the radial nerve that supplies the skin over the posterior aspect of the forearm. It originates in the arm, just before the radial nerve enters the radial groove of the humerus, and descends posteriorly beneath the triceps before piercing the deep fascia to become cutaneous.

This nerve provides cutaneous sensation to the posterior and lateral regions of the forearm, extending from the elbow down to the wrist. It is clinically relevant in radial nerve injuries, lateral epicondyle trauma, and entrapment syndromes causing sensory disturbances without motor deficits.

Synonyms

Dorsal cutaneous branch of the radial nerve (older term)
Posterior antebrachial cutaneous nerve

Origin, Course, and Termination

Origin: Arises from the radial nerve in the posterior compartment of the arm, just proximal to or within the radial groove on the posterior surface of the humerus.
Course:
- Descends obliquely posterior to the lateral head of the triceps brachii.
- Pierces the lateral head of the triceps and the brachial fascia to emerge superficially near the lateral epicondyle of the humerus.
- Divides into upper and lower branches that continue distally along the posterior forearm.
Termination:
- Ends by supplying the skin over the posterior aspect of the wrist and lateral side of the dorsum of the forearm.

Branches

Upper branch: Supplies the skin over the lower part of the triceps region and the posterior surface near the lateral epicondyle.
Lower branch: Descends along the middle of the posterior forearm, supplying the skin as far as the wrist.

Relations

Proximally: Radial nerve in the posterior compartment of the arm, near the radial groove.
Laterally: Lateral head of triceps and deep fascia of the arm.
Anteriorly: Brachialis and lateral intermuscular septum (before piercing fascia).
Posteriorly: Deep fascia and skin of posterior arm and forearm.
Distally: Lies superficial on the posterior surface of the forearm with superficial veins and fascia.

Function

Sensory innervation: Supplies cutaneous sensation to the posterior and lateral regions of the forearm from the elbow to the wrist.
Protective feedback: Conveys sensory input related to touch, pressure, and pain from the dorsal forearm.
Clinical marker: Useful in identifying radial nerve lesions at or above the spiral groove, as sensation from this nerve is typically lost in such cases.

Clinical Significance

Nerve injury: May occur with fractures of the humeral shaft, direct trauma, or compression at the lateral arm.
Entrapment: Can be compressed as it pierces the fascia near the lateral epicondyle, causing dorsal forearm paresthesia.
Radial nerve palsy: Loss of posterior forearm sensation helps localize the lesion proximal to the nerve’s origin.
Iatrogenic injury: May occur during triceps surgery, lateral arm incisions, or venipuncture near the lateral epicondyle.
Sensory testing: Important in evaluating dorsal forearm anesthesia or altered sensation after trauma.

MRI Appearance

T1-weighted images:
- Nerve appears as a thin, intermediate signal tubular structure surrounded by bright subcutaneous fat.
- Best seen as it pierces fascia near the lateral epicondyle and travels distally along posterior forearm.
- Fat around the nerve enhances its visibility in the subcutaneous plane.
T2-weighted images:
- Normal nerve: intermediate-to-low signal, slightly brighter than muscle.
- Entrapment or neuritis: shows focal or diffuse hyperintense signal within or around the nerve.
- Edema in adjacent soft tissues or fascia may also appear bright.
STIR:
- Normal nerve: intermediate-to-dark signal with sharp borders.
- Pathologic changes: bright hyperintense signal in cases of neuritis, compression, or trauma.
Proton Density Fat-Saturated (PD FS):
- Normal: intermediate-to-dark linear signal within subcutaneous tissue.
- Inflamed or injured nerve: focal or segmental bright signal with possible perineural edema.
- Ideal for evaluating neuropathic changes or mild entrapment.
T1 Fat-Sat Post-Contrast:
- Normal nerve: minimal or no enhancement.
- Inflamed or entrapped segments: show linear or nodular enhancement along nerve course.
- Post-traumatic changes: may show surrounding soft-tissue enhancement from fibrosis or hematoma.

CT Appearance

Non-Contrast CT:

Nerve not directly visualized due to soft-tissue density.
Indirectly identified by its location relative to lateral epicondyle and surrounding fat planes.
Useful for detecting bony abnormalities, fractures, or calcific entrapments that may compress the nerve.

Post-Contrast CT (standard):

Normal nerve shows little or no contrast enhancement.
Inflamed or scarred tissue surrounding the nerve may enhance, indicating secondary entrapment.
Helpful in cases of traumatic compression, foreign bodies, or postoperative fibrosis near the lateral arm or forearm.

MRI image

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