Entrapment Neuropathies and Carpal Tunnel Syndrome
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Direct answer
Entrapment neuropathy is focal compression of a peripheral nerve at a fixed anatomical tunnel, producing pain, paraesthesia and motor loss in the territory that nerve supplies. Carpal tunnel syndrome, compression of the median nerve under the flexor retinaculum, is the commonest example and classically causes nocturnal tingling in the thumb, index and middle fingers. Nerve conduction studies confirm the diagnosis and grade its severity.
What you must remember
- Median nerve at the wrist: carpal tunnel syndrome gives thenar weakness and sensory loss over the thumb, index, middle and radial half of the ring finger, with the palm spared.
- Palm sparing: the palmar cutaneous branch leaves the median nerve before the carpal tunnel, so palmar sensation is preserved in carpal tunnel syndrome.
- Confirmation: nerve conduction studies show prolonged distal motor latency and slowed sensory conduction across the wrist, while clinical tests are sensitive but not specific.
- Provocative tests: Phalen's test (sustained wrist flexion), Tinel's sign at the wrist and the carpal compression test reproduce the symptoms.
- Ulnar nerve at the elbow: cubital tunnel syndrome causes milder clawing of the ring and little fingers than a wrist lesion (the ulnar paradox), Froment's sign from adductor pollicis weakness, and sensory loss over the little finger.
- Radial nerve at the spiral groove: wrist drop with the triceps spared, since its branches leave above the groove; classic after humeral shaft fracture or prolonged arm compression.
- Peroneal nerve at the fibular neck: foot drop with eversion weakness, a high-stepping gait and sensory loss over the dorsum of the foot.
A bedside pathway from numbness to the right nerve
Localising a nerve entrapment means matching the pattern of weakness and sensory loss to a single nerve and its compression site.
- Take the sensory map: Ask which digits are numb and whether the palm is involved, since median wrist compression spares the palm.
- Test the thenar and intrinsic muscles: Thenar wasting and weak thumb abduction point to median wrist compression, while clawing of the ring and little fingers points to an ulnar lesion.
- Check the spiral groove: A wrist drop with the triceps preserved places the lesion at the spiral groove, whereas a triceps-weak wrist drop suggests a more proximal, axillary lesion.
- Test the peroneal nerve: Foot drop with weak eversion, while ankle inversion from the tibial nerve is spared, points to the common peroneal nerve at the fibular neck.
- Confirm with electrodiagnosis: Nerve conduction studies and needle electromyography localise the lesion to a segment and show whether axons are lost, which guides the choice of treatment.
- Decide treatment by severity: Intermittent symptoms respond to night splints in neutral wrist position and a local steroid injection, whereas constant numbness with thenar wasting warrants carpal tunnel release.
Where the exam tries to catch you
Most traps here confuse a general symptom with a nerve-specific sign.
- Trap: Accepting a positive Phalen test as proof of carpal tunnel syndrome Correct: Phalen's test is sensitive but nonspecific, so nerve conduction studies are needed to confirm the diagnosis and grade it.
- Trap: Calling any hand numbness carpal tunnel syndrome Correct: C6 or C7 radiculopathy and diabetic neuropathy also cause hand symptoms, and a dermatomal or symmetrical pattern separates them.
- Trap: Confusing the ape hand of median palsy with the claw hand of ulnar palsy Correct: median lesions waste the thenar eminence, giving an ape hand, while ulnar lesions claw the ring and little fingers.
- Trap: Assuming every radial palsy spares the triceps Correct: the triceps is spared only in a spiral groove lesion, because its branches arise above the groove, while axillary lesions weaken it.
Frequently asked questions
Which investigation confirms carpal tunnel syndrome?
Nerve conduction studies, usually with needle electromyography, confirm carpal tunnel syndrome by showing prolonged distal motor latency and slowed sensory conduction across the wrist. They also grade severity, while clinical tests alone are not confirmatory.
Why does carpal tunnel syndrome spare the palm?
The palmar cutaneous branch of the median nerve leaves the nerve proximal to the carpal tunnel and runs superficial to the flexor retinaculum. Because it bypasses the compression site, palmar sensation remains intact in carpal tunnel syndrome. This palm-sparing is a useful bedside clue.
How does ulnar nerve palsy at the elbow differ from carpal tunnel syndrome?
Ulnar palsy at the cubital tunnel affects the ring and little fingers, causing clawing, intrinsic wasting and sensory loss over the little finger. Carpal tunnel syndrome affects the thumb side of the hand, causes thenar wasting and classically spares the palm.
What is the treatment of carpal tunnel syndrome in pregnancy?
Conservative management is usually preferred, with night splinting in neutral wrist position and, if needed, a local corticosteroid injection. Symptoms often settle after delivery, so surgery is reserved for persistent disease or established motor deficit.
Which nerve is injured in wrist drop after a humeral shaft fracture?
The radial nerve is injured at the spiral groove of the humerus, which is the classic site after a humeral shaft fracture. Wrist drop occurs with the triceps spared, because its branches arise above the groove.