Ulnar Nerve: Anatomy, Course, Branches, Function, and Imaging

Topic

Ulnar nerve

The ulnar nerve is a major mixed peripheral nerve of the upper limb, providing motor and sensory innervation to the forearm and hand. It is one of the terminal branches of the medial cord of the brachial plexus (root values C8–T1). The nerve is critical for intrinsic hand muscle control and sensation of the ulnar one and a half digits (little finger and ulnar half of ring finger).

The ulnar nerve’s superficial position around the medial epicondyle of the humerus makes it particularly vulnerable to trauma, compression, and entrapment—most notably in cubital tunnel syndrome and Guyon’s canal syndrome at the wrist.

Synonyms

Nervus ulnaris
Medial nerve of the arm
Cubital nerve

Origin

Arises from the medial cord of the brachial plexus, carrying fibers from C8 and T1, occasionally with a contribution from C7.

Course and Relations

In the arm:
- Descends medial to the brachial artery, piercing the medial intermuscular septum near mid-arm to enter the posterior compartment.
- Lies posterior to the medial epicondyle in the ulnar groove (superficial and palpable).
In the forearm:
- Passes between the two heads of the flexor carpi ulnaris (FCU).
- Travels deep to FCU, alongside the ulnar side of the flexor digitorum profundus (FDP).
- Emerges superficially at the wrist lateral to the pisiform bone.
In the hand:
- Enters through the Guyon’s canal (ulnar tunnel), lying superficial to the flexor retinaculum and lateral to the pisiform.
- Divides into superficial (sensory) and deep (motor) branches.

Branches

In the forearm:
- Muscular branches: To flexor carpi ulnaris and medial half of flexor digitorum profundus.
- Palmar cutaneous branch: Supplies skin over hypothenar eminence.
- Dorsal cutaneous branch: Supplies dorsum of hand and dorsal aspect of ulnar 1½ digits.
In the hand:
- Superficial branch: Sensory to palmar surface of little finger and ulnar half of ring finger.
- Deep branch: Motor to hypothenar muscles, medial lumbricals (3rd & 4th), interossei, adductor pollicis, and deep head of flexor pollicis brevis.

Relations

Arm: Medial intermuscular septum, triceps, and medial epicondyle.
Elbow: Posterior to medial epicondyle in the cubital tunnel (common entrapment site).
Forearm: Between FCU heads, deep to FCU and superficial to FDP.
Wrist: Enters Guyon’s canal between pisiform and hook of hamate.
Hand: Lies deep to palmaris brevis and superficial palmar fascia.

Nerve Supply (Functional Distribution)

Motor:
- Forearm: FCU, medial half of FDP.
- Hand: Hypothenar muscles, medial lumbricals, interossei, adductor pollicis, deep head of FPB.
Sensory:
- Palmar surface of little finger and ulnar half of ring finger.
- Dorsal surface of ulnar 1½ digits.
- Skin of hypothenar region (via palmar branch).

Function

Motor:
- Flexes and adducts wrist (FCU).
- Flexes ring and little fingers (FDP).
- Enables fine movements of fingers (interossei and lumbricals).
- Adducts thumb (adductor pollicis).
Sensory:
- Provides tactile and pain sensation to ulnar digits and medial hand.
Clinical importance:
- Essential for grip strength, precision, and coordination of hand movements.

Clinical Significance

Cubital tunnel syndrome: Compression behind medial epicondyle causes paresthesia in ulnar digits and weakness in grip.
Guyon’s canal syndrome: Entrapment at wrist, often from ganglion cysts or handlebar pressure (cyclists’ palsy).
Ulnar claw hand: Loss of interossei and lumbrical function leads to hyperextension of MCP joints and flexion of IP joints (digits 4–5).
Froment’s sign: Thumb flexion during pinch grip indicates adductor pollicis weakness.
Surgical relevance: Vulnerable in medial elbow surgeries, fractures, and carpal tunnel decompressions.

MRI Appearance

T1-weighted images:
- Normal nerve: Intermediate signal intensity, slightly darker than muscle.
- Perineural fat: Bright, providing contrast against the nerve.
- Entrapment or neuritis: Focal or diffuse increased T1 signal or swelling with loss of fascicular definition.
- Adjacent muscle atrophy: Fatty replacement with bright T1 signal in chronic denervation.
T2-weighted images:
- Normal nerve: Intermediate-to-low signal, slightly darker than on T1.
- Pathology: Entrapment or edema causes increased hyperintense signal and thickening.
- Nerve continuity: Preserved in mild neuritis; interrupted in transection injuries.
- Cubital tunnel entrapment: Focal bright signal with adjacent FCU edema.
STIR:
- Normal nerve: Intermediate-to-dark signal.
- Inflamed or edematous nerve: Bright hyperintense signal, helpful in detecting subtle neuritis.
- Chronic entrapment: Mild thickening with less intense signal due to fibrosis.
Proton Density Fat-Saturated (PD FS):
- Normal nerve: Intermediate-to-dark with distinct fascicular pattern.
- Pathology: Bright hyperintensity and swelling in neuritis, trauma, or compression.
- Excellent for visualizing: Nerve continuity, edema, and perineural inflammation.
T1 Fat-Sat Post-Contrast:
- Normal nerve: Minimal enhancement.
- Inflamed nerve or perineural scar: Shows focal or diffuse enhancement.
- Tumors (schwannoma, neurofibroma): Show heterogeneous enhancement.
- Postoperative fibrosis: Enhancing scar tissue with non-enhancing residual nerve.

CT Appearance

Non-Contrast CT:

Nerve: Not well visualized; appears as a soft-tissue density along its anatomical course.
Indirect signs: Soft-tissue swelling, cubital tunnel narrowing, or associated bony deformity.
Fracture-related entrapment: May show proximity of fracture fragments to nerve course.

Post-Contrast CT (standard):

Enhancing soft tissue: Suggests inflammation, tumor, or scar around nerve path.
Useful for:
- Fracture-related compression, bony spurs, or osseous tunnel narrowing.
- Correlating bone and soft-tissue causes of ulnar neuropathy.