Ulnar Nerve Entrapment

Ulnar neuropathy at the elbow (cubital tunnel syndrome) is the second most common upper limb compressive neuropathy after carpal tunnel syndrome. Incidence is roughly 20 to 25 per 100,000 per year, with a male predominance. Compression at the wrist (Guyon canal) is much less common.

Anatomy of the nerve (C8 to T1, medial cord)

• It runs in the medial arm, passes behind the medial epicondyle in the ulnar groove, and enters the cubital tunnel between the two heads of flexor carpi ulnaris, beneath the Osborne ligament.
• It supplies flexor carpi ulnaris and the ulnar half of flexor digitorum profundus, then the hypothenar muscles, interossei, ulnar two lumbricals, adductor pollicis and deep flexor pollicis brevis.
• The dorsal ulnar cutaneous branch leaves about 5 to 8 cm proximal to the wrist, and the palmar cutaneous branch leaves in the forearm. Both are spared in Guyon canal lesions.

Sites of compression at the elbow

• Arcade of Struthers (about 8 cm proximal to the medial epicondyle) and medial intermuscular septum.
• The retrocondylar groove.
• The cubital tunnel and Osborne ligament.
• The deep flexor pronator aponeurosis.

Contributing factors

• Prolonged elbow flexion (phone use, sleeping with elbows flexed), leaning on the elbow, and repetitive flexion.
• Nerve subluxation or snapping over the epicondyle (10 to 16% of the population have a hypermobile nerve).
• Osteophytes and osteoarthritis, previous elbow fractures and valgus deformity (tardy ulnar palsy), anconeus epitrochlearis, ganglion, lipoma, and synovitis.
• Diabetes, alcohol, hypothyroidism, haemodialysis, and body habitus.

Guyon canal causes include cycling (handlebar palsy), hamate hook fracture, ganglion, ulnar artery thrombosis or aneurysm (hypothenar hammer syndrome), pisiform fracture, and repetitive hand use.

Pathophysiology: elbow flexion stretches the nerve by about 5 mm and narrows the tunnel, raising intraneural pressure by several fold, which impairs microcirculation. The McGowan grading is mild (I: sensory only), moderate (II: weakness) and severe (III: paralysis and atrophy).

Symptoms at the elbow

• Intermittent numbness and tingling of the little finger and the ulnar half of the ring finger, often waking the patient at night or after prolonged elbow flexion.
• Medial elbow pain, and sensory symptoms in the dorsal ulnar hand (spared in Guyon canal).
• Clumsiness, loss of grip strength, dropping objects, and difficulty with fine hand tasks.

Signs of progression

• Weak finger abduction and adduction, loss of intrinsic strength, and wasting of the first dorsal interosseous and hypothenar eminence.
• Froment sign (thumb interphalangeal flexion when gripping paper, substituting for weak adductor pollicis).
• Wartenberg sign (persistent abduction of the little finger), and Jeanne sign (thumb metacarpophalangeal hyperextension).
• Ulnar claw hand (ring and little finger hyperextension at MCP with flexion at the interphalangeal joints). In proximal lesions the claw can be less marked because the flexor digitorum profundus is also weak (ulnar paradox).
• Crossed finger sign is impaired.

Provocative tests

• Tinel sign over the cubital tunnel.
• Elbow flexion test (full elbow flexion with wrist extension held for 60 seconds reproduces paraesthesia).
• Direct pressure test over the tunnel for 60 seconds.
• Scratch collapse test.
• Palpation for nerve subluxation on elbow flexion.

Guyon canal

• Zone 1 (proximal to bifurcation): combined motor and sensory loss.
• Zone 2 (deep motor branch): pure motor loss with intrinsic atrophy and sensory sparing.
• Zone 3 (superficial branch): pure sensory loss over the palmar little finger and ring finger.

Differential diagnosis

• C8 to T1 radiculopathy, lower trunk plexopathy, thoracic outlet syndrome (medial antebrachial cutaneous and median involvement).
• Pancoast tumour, motor neuron disease, medial epicondylitis, and focal hand dystonia.

The diagnosis is clinical, and electrodiagnostic testing confirms location and severity.

Nerve conduction studies and EMG

• Motor conduction velocity across the elbow below 50 m/s, or a drop of more than 10 m/s compared with the forearm segment.
• Conduction block (more than 20% CMAP amplitude drop) across the elbow, assessed with inching techniques.
• Reduced dorsal ulnar cutaneous SNAP localises the lesion proximal to the wrist.
• EMG of flexor carpi ulnaris, first dorsal interosseous, and abductor digiti minimi quantifies denervation. Normal studies do not exclude mild entrapment.

Imaging

• Ultrasound shows nerve enlargement at the elbow (cross-sectional area above about 9 to 10 mm2 at the medial epicondyle), and dynamic subluxation.
• MRI shows nerve signal change, masses and denervation oedema.
• Radiographs of the elbow for osteophytes, previous fracture, and cubitus valgus. Cervical spine imaging when radiculopathy is suspected.
• CT of the wrist for hamate hook fracture, and Doppler or angiography for ulnar artery thrombosis.

Laboratory tests: glucose or HbA1c, vitamin B12, TSH, and renal function.

Conservative management for mild or intermittent symptoms (McGowan I)

• Avoid prolonged elbow flexion beyond 90 degrees and avoid leaning on the elbow.
• Night splint or a folded towel around the elbow keeping it at about 45 degrees of flexion, for 3 months.
• Elbow pads, nerve gliding exercises and ergonomic changes.
• NSAIDs (ibuprofen 400 mg three times daily) for short courses, and neuropathic pain agents (gabapentin 300 mg at night titrated) if needed.
• Steroid injection into the cubital tunnel is not recommended, because of limited benefit and nerve risk.

Indications for surgery

• Symptoms persisting beyond 3 to 6 months of conservative treatment.
• Motor weakness, muscle wasting, or severe sensory loss.
• Marked abnormalities on EMG, or nerve subluxation with symptoms.

Surgical options

• Simple in situ decompression (open or endoscopic) is preferred for mild to moderate disease without subluxation, with similar results to transposition and fewer complications.
• Anterior subcutaneous transposition for nerve subluxation or after failed decompression.
• Submuscular transposition for severe or recurrent disease.
• Medial epicondylectomy as an alternative.
• Cefazolin 2 g IV at induction. Complications include medial antebrachial cutaneous neuroma, hematoma, and incomplete decompression.
• Outcomes: about 85 to 90% improve, with better results in mild disease. Severe, long-standing disease recovers incompletely, and motor recovery takes 6 to 12 months.

Guyon canal

• Remove the cause: padded gloves and handlebar modification for cyclists, wrist rest, and ganglion excision.
• Hamate hook fracture: excision of the hook fragment.
• Ulnar artery thrombosis: antiplatelet therapy with aspirin 100 mg daily, cessation of smoking, and vascular surgery for aneurysm or persistent ischaemia.
• Surgical decompression of Guyon canal for persistent compression or motor deficits.

Late reconstruction

• Tendon transfers (for example, extensor carpi radialis brevis based procedures, and flexor digitorum superficialis lasso procedure for claw correction) and Brand-type reconstruction of pinch for established paralysis.
• Hand therapy for strength and function.