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Medial cutaneous nerve of forearm (posterior branch)

The posterior branch of the medial cutaneous nerve of the forearm (also known as the posterior antebrachial branch) is one of the two terminal branches of the medial cutaneous nerve of the forearm. It provides cutaneous (sensory) innervation to the posterior aspect of the forearm, particularly the skin overlying the ulnar and posterior surfaces of the distal arm and proximal to mid forearm.

This nerve plays a vital role in conveying sensory input from the skin of the posteromedial forearm. Though small, its course is clinically significant during elbow and forearm surgeries, trauma, or venipuncture procedures, as injury can lead to localized numbness or paresthesia.

Synonyms

  • Posterior branch of the medial antebrachial cutaneous nerve

  • Posterior antebrachial cutaneous branch of the medial cutaneous nerve

  • Posterior forearm branch of medial cutaneous nerve

Origin and Course

  • Origin: Arises from the medial cutaneous nerve of the forearm, which is a branch of the medial cord of the brachial plexus (C8–T1).

  • Course:

    • The parent nerve descends along the medial side of the arm, initially close to the basilic vein.

    • Near the middle of the arm, it pierces the deep fascia along with the basilic vein to enter the superficial fascia of the forearm.

    • At this level, the nerve divides into anterior and posterior branches.

    • The posterior branch curves posteriorly around the medial epicondyle of the humerus, passes beneath the fascia, and then descends on the posterior surface of the forearm.

    • It travels superficial to the flexor carpi ulnaris and ulnar head of the pronator teres, lying subcutaneously in the distal half of the forearm.

    • Terminates by supplying the skin over the dorsomedial aspect of the forearm, reaching nearly to the wrist.

Relations

  • Proximally: Related to the basilic vein and deep fascia of the arm

  • At the elbow: Winds posterior to the medial epicondyle, superficial to the ulnar collateral ligament

  • In the forearm: Lies superficial to the flexor carpi ulnaris and deep to the superficial fascia

  • Distally: Accompanies small superficial veins on the posterior ulnar aspect of the forearm

Distribution

  • Cutaneous supply:

    • Provides sensation to the posterior and ulnar aspect of the forearm

    • Extends from just below the elbow to approximately two-thirds of the way to the wrist

  • No motor supply: Purely sensory nerve

Function

  • Sensory innervation: Conveys touch, pain, and temperature sensations from the posteromedial surface of the forearm

  • Protective role: Provides cutaneous feedback important for spatial awareness and forearm positioning

  • Clinical marker: Used for sensory testing in brachial plexus or medial cord lesions

Clinical Significance

  • Nerve injury:

    • May occur during venipuncture, medial elbow incisions, or trauma to the medial epicondyle

    • Results in paresthesia, numbness, or burning pain over the posterior ulnar forearm

  • Entrapment or compression: Rare, but may present in soft tissue fibrosis or postoperative scarring

  • Iatrogenic causes: Common in cubital tunnel surgeries or ulnar nerve decompressions

  • Diagnostic importance: Differentiating from ulnar nerve sensory loss helps localize lesion to the medial cord or forearm cutaneous branch

  • Peripheral neuropathy: Can be affected in systemic neuropathies or diabetic polyneuropathy

MRI Appearance

  • T1-weighted images:

    • Normal nerve: intermediate-to-low signal (slightly brighter than muscle)

    • Course visible as a fine tubular or linear structure within subcutaneous fat (bright background)

    • Perineural fat provides good contrast; no enhancement in normal state

    • Pathology: compression or neuroma shows focal thickening and intermediate-to-bright signal

  • T2-weighted images:

    • Normal nerve: intermediate signal, darker than surrounding fat

    • Pathologic: nerve injury or neuritis appears as focal or diffuse bright hyperintense signal

    • Perineural edema or fibrosis produces surrounding bright halo

  • STIR:

    • Normal nerve: intermediate-to-dark

    • Pathology: bright hyperintensity due to edema or inflammation; helps identify neuritis and entrapment

  • Proton Density Fat-Saturated (PD FS):

    • Normal nerve: intermediate-to-dark linear signal

    • Pathology: bright signal increase within the nerve or perineural tissue in injury, traction, or compression neuropathy

  • T1 Fat-Sat Post-Contrast:

    • Normal: minimal or no enhancement

    • Inflamed or injured nerve: shows linear or nodular enhancement

    • Neuroma or scar tissue: shows focal enhancing mass at injury site

CT Appearance

Non-Contrast CT:

  • Nerve not directly visualized due to small caliber

  • Course inferred by location relative to basilic vein, medial epicondyle, and ulnar border of forearm

  • Chronic injury may show fibrotic tissue density or scarring in soft tissues

Post-Contrast CT (standard):

  • May show subtle linear or focal enhancement in perineural soft tissue with inflammation

  • Useful for identifying postoperative fibrosis, mass effect, or traumatic scarring in the medial forearm region

MRI image

Medial cutaneous nerve of forearm (anterior branch) axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000

MRI image

Medial cutaneous nerve of forearm (anterior branch) axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000_00001

MRI image

Medial cutaneous nerve of forearm (anterior branch) axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000_00005