# Carpal Tunnel Syndrome and Common Entrapment Neuropathies

## Introduction

Entrapment neuropathies result from chronic compression of peripheral nerves at anatomically vulnerable sites. Carpal tunnel syndrome is the most common entrapment neuropathy, affecting approximately 3 to 6 percent of the general population. Neurosurgeons manage these conditions when conservative treatment fails, performing decompression procedures that provide reliable symptom relief. Understanding the anatomy, electrodiagnostic findings, and surgical techniques for common entrapment neuropathies is essential for clinical practice.

## Carpal Tunnel Syndrome

### Anatomy

The carpal tunnel is bounded by the carpal bones forming the floor and sides, with the transverse carpal ligament, also called the flexor retinaculum, serving as the roof. The tunnel contains the median nerve and nine flexor tendons, specifically four flexor digitorum superficialis tendons, four flexor digitorum profundus tendons, and the flexor pollicis longus. The median nerve is the most superficial and radial structure in the tunnel. The palmar cutaneous branch exits proximal to the tunnel and is not affected in carpal tunnel syndrome. The recurrent motor branch exits distal to the tunnel to innervate the thenar muscles.

### Clinical Presentation

Patients present with paresthesias and numbness in the median nerve distribution, affecting the thumb, index finger, middle finger, and radial half of the ring finger. Symptoms are characteristically worse at night, and patients often shake their hands to relieve symptoms, a finding known as the flick sign. In advanced cases, weakness and atrophy of the thenar muscles, specifically the abductor pollicis brevis and opponens pollicis, develop. The Phalen test reproduces symptoms with wrist flexion maintained for 60 seconds and has a sensitivity of approximately 70 percent. The Tinel sign, elicited by percussion over the carpal tunnel producing tingling in the median distribution, has a sensitivity of approximately 50 percent. The Durkan compression test involves direct pressure over the carpal tunnel for 30 seconds.

### Risk Factors

Risk factors include female sex with a 3:1 ratio, pregnancy, diabetes, hypothyroidism, rheumatoid arthritis, and obesity. Repetitive wrist motion from occupational activities is a contributing factor but not the sole cause. The condition is bilateral in up to 50 percent of cases.

### Electrodiagnosis

Sensory nerve conduction studies reveal prolonged median sensory distal latency greater than 3.5 milliseconds, with reduced SNAP amplitude in severe cases. Motor nerve conduction studies show prolonged median motor distal latency greater than 4.2 milliseconds, and reduced CMAP amplitude indicates axonal loss. Comparison studies measuring the median-ulnar latency difference across the wrist represent the most sensitive test. EMG of the abductor pollicis brevis showing fibrillation potentials and neurogenic MUAPs indicates severe denervation.

### Management

Conservative management includes neutral wrist splinting especially at night, activity modification, and corticosteroid injection which provides temporary relief in 70 to 80 percent of patients. Surgical decompression is indicated for failed conservative therapy, progressive weakness, or severe electrodiagnostic findings. Open carpal tunnel release is the standard approach using a longitudinal incision over the thenar crease with division of the transverse carpal ligament. Endoscopic carpal tunnel release uses a smaller incision with faster return to work and comparable outcomes. The success rate exceeds 90 percent with high patient satisfaction. Complications include pillar pain, scar tenderness, recurrence in 1 to 3 percent, and rare injury to the palmar cutaneous or recurrent motor branch.

## Ulnar Neuropathy at the Elbow (Cubital Tunnel Syndrome)

### Anatomy and Pathophysiology

Cubital tunnel syndrome is the second most common entrapment neuropathy. The ulnar nerve passes through the cubital tunnel behind the medial epicondyle, beneath the Osborne ligament, also called the arcuate ligament. Compression sites include the retroepicondylar groove, the Osborne ligament, and the space between the two heads of the flexor carpi ulnaris.

### Clinical Presentation

Patients experience numbness and tingling in the small finger and ulnar half of the ring finger. Weakness affects the intrinsic hand muscles including the interossei for finger abduction and adduction, the hypothenar muscles, and the adductor pollicis. The Froment sign demonstrates flexion of the thumb interphalangeal joint during key pinch, compensating for a weak adductor pollicis. The Wartenberg sign is the inability to adduct the small finger, resulting in an abducted posture. Claw hand deformity consists of hyperextension at the metacarpophalangeal joints and flexion at the proximal and distal interphalangeal joints of the ring and small fingers. The ulnar paradox describes how claw deformity is worse with more distal lesions.

### Electrodiagnosis

Electrodiagnostic findings include slowed ulnar motor conduction velocity across the elbow below 50 meters per second and reduced ulnar SNAP amplitude. EMG demonstrates denervation in ulnar-innervated hand muscles including the first dorsal interosseous and abductor digiti minimi.

### Surgical Management

In situ decompression involves release of the Osborne ligament and fascial bands while preserving the nerve blood supply. Anterior transposition, which may be subcutaneous, intramuscular, or submuscular, is indicated for nerve subluxation or failed in situ decompression. Medial epicondylectomy is a less common option that removes the bony prominence. Outcomes for cubital tunnel surgery are less predictable than for carpal tunnel syndrome, and earlier intervention yields better results.

## Other Common Entrapment Neuropathies

### Radial Nerve Entrapment

Posterior interosseous nerve syndrome results from compression at the arcade of Frohse and produces a pure motor deficit with finger and wrist drop. The absence of wrist extension weakness distinguishes it from radial nerve palsy proper, and there is no sensory loss. Radial tunnel syndrome presents with pain in the lateral forearm without motor deficit and is a controversial entity that remains a diagnosis of exclusion.

### Meralgia Paresthetica

Meralgia paresthetica results from entrapment of the lateral femoral cutaneous nerve under the inguinal ligament. It is a pure sensory neuropathy producing burning pain and numbness over the anterolateral thigh. Associated conditions include obesity, pregnancy, tight clothing, and diabetes. Management begins with weight loss and avoiding compression, progresses to local anesthetic and steroid injection, and may require surgical decompression for refractory cases.

### Peroneal Neuropathy

Common peroneal nerve compression at the fibular head presents with foot drop, manifesting as weakness of ankle dorsiflexion and eversion, along with numbness over the dorsum of the foot. Causes include habitual leg crossing, prolonged recumbency, casts, weight loss, and ganglion cysts. EMG and NCS distinguish this condition from L5 radiculopathy because peroneal neuropathy spares the tibialis posterior and gluteus medius. Management includes an ankle-foot orthosis, avoidance of compression, and surgical decompression if conservative measures fail.

| Entrapment | Nerve | Site | Motor Deficit | Sensory Deficit |
|-----------|-------|------|--------------|-----------------|
| Carpal tunnel | Median | Transverse carpal ligament | Thenar weakness (APB, opponens) | Thumb, index, middle, radial ring finger |
| Cubital tunnel | Ulnar | Medial epicondyle / Osborne ligament | Interossei, hypothenar, adductor pollicis | Small finger, ulnar ring finger |
| PIN syndrome | Posterior interosseous (radial) | Arcade of Frohse | Finger/wrist extension (no wrist drop) | None (pure motor) |
| Meralgia paresthetica | Lateral femoral cutaneous | Inguinal ligament | None (pure sensory) | Anterolateral thigh |
| Peroneal neuropathy | Common peroneal | Fibular head | Ankle dorsiflexion, eversion | Dorsum of foot |
| Tarsal tunnel | Posterior tibial | Flexor retinaculum (medial ankle) | Intrinsic foot muscles | Plantar foot |

### Tarsal Tunnel Syndrome

Tarsal tunnel syndrome results from compression of the posterior tibial nerve beneath the flexor retinaculum at the medial ankle. Patients present with burning pain and numbness in the plantar aspect of the foot. A positive Tinel sign at the tarsal tunnel supports the diagnosis. Electrodiagnostic confirmation is followed by surgical release for refractory cases.

## Clinical Pearls

Carpal tunnel syndrome is the most common entrapment neuropathy, and nocturnal symptoms combined with a positive flick sign are highly suggestive of the diagnosis. Thenar atrophy in carpal tunnel syndrome indicates advanced disease with axonal loss, and surgical decompression should not be delayed. In cubital tunnel syndrome, the Froment sign and Wartenberg sign are specific clinical findings for ulnar nerve dysfunction. Electrodiagnostic studies are essential for confirming the diagnosis, grading severity, and excluding proximal pathology before surgery. Peroneal neuropathy at the fibular head must be distinguished from L5 radiculopathy, with paraspinal EMG and tibialis posterior function serving as key differentiators.

## References
1. Padua L, Coraci D, Erra C, et al. Carpal tunnel syndrome: clinical features, diagnosis, and management. Lancet Neurol. 2016;15(12):1273-1284.
2. Caliandro P, La Torre G, Padua R, et al. Treatment for ulnar neuropathy at the elbow. Cochrane Database Syst Rev. 2016;11:CD006839.
3. Mackinnon SE. Pathophysiology of nerve compression. Hand Clin. 2002;18(2):231-241.
4. American Academy of Orthopaedic Surgeons. Management of carpal tunnel syndrome evidence-based clinical practice guideline. AAOS; 2016.