Deep Branch of Ulnar Nerve: Anatomy, Course, Function, Imaging

Topic

Deep branch of ulnar nerve

The deep branch of the ulnar nerve is the primary motor branch of the ulnar nerve in the hand. It arises within the ulnar (Guyon’s) canal, passes deeply through the hypothenar region, and supplies most of the intrinsic muscles of the hand. It is crucial for fine motor control, grip strength, and coordinated finger movements.

The branch travels with the deep branch of the ulnar artery, curving laterally across the palm within the deep plane, under the flexor tendons and palmar aponeurosis. Damage to this nerve results in significant motor deficits such as clawing of the hand and loss of fine finger motion.

Synonyms

Deep terminal branch of the ulnar nerve
Deep motor branch of the ulnar nerve
Palmar deep branch of the ulnar nerve

Origin, Course, and Termination

Origin: Arises from the ulnar nerve in the distal portion of the ulnar canal (Guyon’s canal), opposite the pisiform bone.
Course:
- Passes deep to the abductor digiti minimi muscle in the hypothenar region.
- Curves laterally across the palm, lying deep to the flexor tendons, close to the deep palmar arch.
- Travels across the bases of the metacarpals toward the thumb.
Termination: Ends by innervating the adductor pollicis and the first dorsal interosseous muscle, often continuing as a small branch to the second interosseous space.

Relations

Anteriorly: Flexor tendons of the fingers and the palmar aponeurosis
Posteriorly: Interosseous muscles and metacarpal bones
Medially: Hypothenar muscles and pisohamate ligament
Laterally: Deep palmar arch and adductor pollicis
Superiorly: Fibrous arch of the flexor tendons and lumbricals

Branches and Distribution

Muscular branches:
- To the three hypothenar muscles: abductor digiti minimi, flexor digiti minimi brevis, and opponens digiti minimi
- To the medial two lumbricals (3rd and 4th)
- To the dorsal and palmar interossei (7 total)
- To the adductor pollicis
- Occasionally to the deep head of the flexor pollicis brevis
Articular branches: Small twigs to metacarpophalangeal and intercarpal joints

Function

Motor control:
- Innervates almost all intrinsic hand muscles except the thenar muscles (mostly median-innervated).
- Enables finger abduction/adduction via interossei.
- Aids in thumb adduction and deep flexion movements.
Grip strength: Provides fine control of hand power and precision grip.
Coordination: Ensures synchronized finger movement, essential for writing and tool use.

Clinical Significance

Ulnar neuropathy in Guyon’s canal: Compression at the wrist may selectively affect the deep branch, causing motor weakness without sensory loss.
Claw hand deformity: Results from paralysis of lumbricals and interossei (MCP hyperextension and IP flexion).
Froment’s sign: Indicates adductor pollicis weakness when patient uses thumb flexion (FPL compensation) to grasp paper.
Injury causes: Trauma, ganglion cysts, hook of hamate fractures, repetitive pressure (cyclist’s palsy).
Surgical relevance: Must be preserved during carpal tunnel or ulnar decompression procedures.
Imaging role: MRI and ultrasound critical for detecting entrapment, injury, or denervation-related muscle atrophy.

MRI Appearance

T1-weighted images:

Nerve appears as a thin, low-to-intermediate signal structure coursing along the deep palmar plane.
Surrounded by bright fat, aiding contrast with adjacent tendons and vessels.
Normal muscles: Show intermediate signal intensity.
Denervation: Chronic denervated muscles show fatty replacement, appearing bright on T1.

T2-weighted images:

Nerve: Intermediate-to-mildly hyperintense compared to muscle.
Acute neuropathy or entrapment: Increased T2 signal within the nerve due to edema.
Muscle denervation (acute): Hyperintense signal within affected muscles due to edema.
Chronic neuropathy: Loss of edema with progressive fatty atrophy (dark on T2).

STIR:

Nerve: Low-to-intermediate signal, with edema or inflammation appearing bright.
Denervated muscles: Bright hyperintense regions indicating active denervation or inflammation.

Proton Density Fat-Saturated (PD FS):

Normal nerve: Low-to-intermediate signal.
Pathology: Focal or diffuse bright signal in compressed or inflamed segments.
Denervated muscle shows bright signal intensity during acute phase.

T1 Fat-Sat Post-Contrast:

Normal: Minimal or no enhancement.
Neuritis or entrapment: Diffuse or nodular enhancement along nerve course.
Neuroma or fibrosis: Heterogeneous or irregular enhancement at site of injury.
Postoperative scar: Enhances peripherally; recurrent neuroma shows central enhancement.

CT Appearance

Non-Contrast CT:

Nerve not directly visualized due to low contrast with adjacent soft tissue.
Indirect evidence of entrapment may include bony irregularity of the pisiform or hamate or mass lesions in Guyon’s canal.

MRI images

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