Ulnar Nerve: High-Yield Clinical Facts

Overview

The Ulnar Nerve originates primarily from the C8–T1 nerve roots of the brachial plexus. It is responsible for sensation in the medial hand and motor supply to many intrinsic hand muscles.

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Common Sites and Causes of Ulnar Nerve Compression

Most Common Site

The most common site of compression is the cubital tunnel, especially between the two heads of the flexor carpi ulnaris (FCU).

Other Causes

Additional causes of ulnar nerve compression include:

• Anconeus epitrochlearis muscle
• Cubital tunnel narrowing
• Osteophytes or deformity around the elbow
• Ganglion cysts
• Arcade of Struthers
• Trauma or valgus instability
• Repetitive elbow flexion

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Root Value and Clinical Importance

Root Value

The ulnar nerve arises mainly from:

• C8–T1

Relation to Horner Syndrome

These same roots may also be involved in lesions producing Horner Syndrome.

Presence of Horner syndrome in a brachial plexus injury suggests a poor prognosis, usually indicating lower trunk or root avulsion involvement.

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First Dorsal Interosseous Muscle

Clinical Importance

The first dorsal interosseous muscle is supplied by the ulnar nerve.

Findings

• Wasting of this muscle indicates ulnar nerve palsy
• Visible wasting in the first web space suggests chronic denervation and poorer prognosis

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Thenar vs Interosseous Wasting

Thenar Wasting

Associated mainly with median nerve compression, especially Carpal Tunnel Syndrome.

Interosseous Wasting

Associated with ulnar nerve lesions.

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Martin–Gruber Anastomosis

Definition

A communicating branch between the median nerve and ulnar nerve in the forearm.

Importance

It can:

• Alter clinical findings
• Confuse nerve conduction studies and EMG interpretation
• Produce unexpected muscle preservation patterns

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Cubital Tunnel and Elbow Flexion

Key Fact

Elbow flexion significantly decreases cubital tunnel volume and increases pressure on the ulnar nerve.

Clinical Relevance

• Prolonged elbow flexion may worsen symptoms
• Excessive mobile phone use may aggravate cubital tunnel syndrome

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Dorsal Cutaneous Branch of the Ulnar Nerve

Important Sensory Branch

The dorsal cutaneous branch arises in the forearm proximal to the wrist.

High Ulnar Nerve Lesion (At/Above Elbow)

Causes:

• Sensory loss over dorsal medial hand
• Sensory loss over medial 1½ fingers dorsally

Low Ulnar Nerve Lesion (At Wrist)

Usually spares dorsal sensation because the dorsal cutaneous branch has already originated proximally.

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Sensory Distribution

Palmar Cutaneous Branch

Supplies sensation to the:

• Hypothenar eminence

Superficial Terminal Branch

Supplies sensation to:

• Little finger
• Medial half of ring finger

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Ulnar Claw Hand

Low Ulnar Nerve Lesion

Lesions at the wrist or distal forearm produce:

• Clawing of the ring and little fingers
• Hyperextension at MCP joints
• Flexion at IP joints

This occurs because the lumbricals and interossei are paralyzed while FDP to ring and little fingers remains intact.

High Ulnar Nerve Lesion

May produce:

• Less clawing (“ulnar paradox”)
• More sensory loss

Because FDP to the medial fingers is also weak.

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Claw Hand vs Benediction Sign

Ulnar Claw

Seen at rest involving:

• Ring finger
• Little finger

Benediction Sign

Occurs during attempted fist-making in median nerve injury.

The patient cannot flex:

• Index finger
• Middle finger

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Role of Flexor Digitorum Profundus (FDP)

Innervation

• Medial half (ring and little finger): ulnar nerve
• Lateral half (index and middle finger): median nerve

This explains the difference between high and low ulnar nerve lesions.

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Wartenberg Sign

Definition

Persistent abduction of the little finger due to weakness of:

• Palmar interossei (adductors)

Mechanism

The extensor digiti minimi remains unopposed, causing abduction.

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Wartenberg Syndrome

Different Condition

Wartenberg Syndrome is unrelated to ulnar nerve palsy.

It is caused by compression of the superficial radial nerve between:

• Brachioradialis
• Extensor carpi radialis longus

Commonly aggravated by:

• Tight watches
• Bracelets
• Handcuffs

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Arcade of Struthers vs Ligament of Struthers

Arcade of Struthers

Potential site of ulnar nerve compression proximal to the elbow.

Ligament of Struthers

A fibrous band extending from a supracondylar spur that may compress the median nerve.

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Froment Sign

Test

The patient attempts to hold a paper between thumb and index finger.

Positive Froment Sign

Because the adductor pollicis (ulnar nerve) is weak, the patient compensates by flexing the thumb IP joint using:

• Flexor pollicis longus (median nerve)

This produces thumb flexion during pinch.

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Froment Sign vs Anterior Interosseous Nerve (AIN) Palsy

Anterior Interosseous Nerve Palsy

The patient cannot make a normal “OK” sign because of weakness of:

• Flexor pollicis longus
• FDP to index finger

The pinch becomes flattened instead of circular.

Froment Sign

The patient can still flex the thumb, but does so abnormally to compensate for ulnar nerve weakness.

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Tinel Sign at the Cubital Tunnel

Technique

Tapping over the cubital tunnel reproduces tingling along the ulnar nerve distribution.

Clinical Use

Supports diagnosis of:

• Cubital tunnel syndrome
• Ulnar neuritis

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Differential Diagnosis of Ulnar Symptoms

Conditions that may mimic cubital tunnel syndrome include:

• Cervical radiculopathy (especially C8–T1)
• Thoracic outlet syndrome
• Double crush syndrome
• Lower brachial plexopathy

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Sensory Testing

Quick Bedside Test

Check sensation over:

• Little finger
• Medial half of ring finger

Useful in postoperative or trauma settings.

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Supracondylar Fracture and Nerve Injury

Extension-Type Supracondylar Fracture

Most commonly associated with:

• Median nerve injury
• Anterior interosseous nerve injury

Flexion-Type Supracondylar Fracture

More commonly associated with:

• Ulnar nerve injury

Flexion-type injuries are less common and may require open reduction more frequently.