Carpal Tunnel Syndrome: How a Pinched Nerve Numbs the Hand

The normal anatomy: a crowded tunnel with one nerve at risk

The carpal tunnel is a rigid passage on the palm side of the wrist. Its floor and walls are formed by the arch of the eight carpal bones, and its roof is a tough, inelastic band of fibrous tissue called the transverse carpal ligament (flexor retinaculum). Through this tunnel pass nine flexor tendons (four flexor digitorum superficialis, four profundus, and flexor pollicis longus) and one nerve — the median nerve.

Because the boundaries are bone and unyielding ligament, the tunnel cannot expand. It is a fixed-volume compartment. Anything that adds volume — swollen tendon sheaths (tenosynovitis), fluid, a mass, or simply a wrist bent into flexion or extension — must raise the pressure inside. And the median nerve, being the softest structure in that crowded space, is the one that pays the price.

The median nerve supplies sensation to the palmar surface of the thumb, index, middle, and the radial (thumb-side) half of the ring finger, and it powers the thenar muscles that let the thumb oppose. Crucially, the palmar cutaneous branch that supplies the base of the palm branches off before the tunnel — so it is spared. That anatomical detail is a fingerprint that distinguishes CTS from lesions higher up the arm.

The mechanism: how rising pressure strangles a nerve

The pathophysiology of CTS is a two-stage failure — first of blood supply, then of the nerve's structure. The causal chain runs like this:

1. Pressure rises inside the tunnel. Normal resting hydrostatic pressure is only about 2–10 mmHg. In CTS it commonly runs 20–30 mmHg at rest and can spike above 90–100 mmHg with wrist flexion or extension.
2. Venous return is obstructed first. The thin-walled epineurial venules collapse once tunnel pressure approaches 20–30 mmHg. Blood pools, the nerve becomes congested and edematous, and this swelling raises pressure further — a vicious cycle.
3. Arterial inflow and axonal transport fail. As pressure climbs toward 40–50 mmHg, capillary blood flow and the nerve's internal transport of proteins are choked. The nerve is now ischemic. Early symptoms at this stage are fully reversible — this is why shaking the hand (which restores flow) relieves tingling.
4. Chronic compression damages myelin. Sustained pressure mechanically distorts the myelin sheath, causing focal demyelination. Because myelin enables fast saltatory conduction, the nerve now conducts slowly — the measurable hallmark of CTS.
5. Axons die (advanced disease). If compression persists for months to years, axons themselves degenerate. This is the point of no full return: the thenar muscle wastes and sensory loss becomes fixed.

So the sequence is congestion → ischemia → demyelination → axon loss — each stage more permanent than the last.

Why it produces exactly these symptoms

Every classic feature of CTS maps directly onto the mechanism above.

Numbness and tingling in the median fingers reflect ischemia and demyelination of sensory fibers — the thumb, index, middle, and radial half of the ring finger, precisely the median territory. The little finger, supplied by the ulnar nerve, is spared; patients who report the little finger going numb usually do not have pure CTS.

Night-time symptoms are the single most characteristic feature. During sleep the wrist naturally curls into flexion, spiking tunnel pressure, and the recumbent posture increases interstitial fluid around the nerve. Patients wake with a burning, tingling hand and instinctively shake it out — the so-called flick sign — which milks venous congestion away and restores flow. Its presence is highly suggestive of CTS.

Sensory before motor: the large sensory fibers are more susceptible to compression ischemia than motor fibers, so numbness precedes weakness by months. Thenar weakness and wasting — difficulty with fine pinch, dropping objects, a flattened muscle bulge at the base of the thumb — signal that axon loss has begun and are a marker of advanced, less reversible disease.

Palm sensation is preserved because the palmar cutaneous branch bypasses the tunnel — a subtle sign that points the diagnosis firmly to the wrist rather than the neck.

Who gets it, and why: risk factors and epidemiology

CTS is common: population studies estimate a prevalence of roughly 3–6% of adults, with an annual incidence around 1–3 per 1,000 person-years. It is 2–3× more frequent in women and peaks between 45 and 60 years. Women's smaller wrist cross-sectional area is thought to leave less room for the nerve.

Anything that reduces tunnel volume or increases its contents raises risk:

• Repetitive / forceful wrist use and vibration — assembly work, sustained gripping (the ergonomic link is real but often overstated for typing alone).
• Obesity — one of the strongest modifiable risk factors.
• Pregnancy — fluid retention causes a transient CTS that often resolves after delivery.
• Diabetes mellitus — both by glycation-related nerve vulnerability and connective-tissue thickening.
• Hypothyroidism — myxedematous deposition of mucopolysaccharide in the tunnel.
• Rheumatoid arthritis and other inflammatory tenosynovitis.
• Amyloidosis — including dialysis-associated β₂-microglobulin amyloid; bilateral CTS can be an early clue to hereditary transthyretin (ATTR) amyloidosis.
• Acromegaly and prior wrist fracture (e.g., distal radius, Colles') distorting the tunnel.

Because so many systemic diseases funnel into the same tunnel, new bilateral CTS in an atypical patient warrants screening for thyroid disease, diabetes, and — increasingly recognized — amyloid.

Diagnosis: from bedside to nerve conduction studies

CTS is fundamentally a clinical diagnosis supported by testing. The history — nocturnal numbness in a median distribution relieved by shaking — carries the most weight.

Provocative bedside tests:

• Phalen's test — holding the wrists in full flexion for up to 60 s reproduces tingling by raising tunnel pressure (sensitivity ≈ 50–80%).
• Tinel's sign — tapping over the median nerve at the wrist provokes an electric tingle into the fingers (specific but less sensitive).
• Carpal compression (Durkan's) test — direct thumb pressure over the tunnel for 30 s; often the most sensitive of the three.

Nerve conduction studies (NCS) and electromyography (EMG) are the confirmatory gold standard. They demonstrate focal slowing of median conduction across the wrist: a distal motor latency > 4.0–4.5 ms or a sensory latency > 3.5 ms, and — most sensitively — median-vs-ulnar or median-vs-radial latency differences > 0.4–0.5 ms across the same distance. EMG detects axon loss in advanced cases. NCS also grades severity, which guides treatment.

Ultrasound and MRI can show an enlarged, flattened nerve (increased cross-sectional area, typically > 10–12 mm² at the tunnel inlet) and are useful when a mass or anatomical cause is suspected. Notably, a normal NCS occurs in a minority of clinically clear cases — so a negative study does not exclude CTS.

Treatment and the natural history — why each therapy works

Management follows the mechanism: relieve the pressure, restore flow, protect the axons.

• Neutral-wrist night splinting is first-line for mild–moderate CTS. Holding the wrist straight keeps tunnel pressure at its minimum overnight, breaking the nocturnal congestion cycle. It relieves symptoms in a majority within weeks.
• Corticosteroid injection into the tunnel reduces tenosynovial swelling, lowering pressure. It gives durable relief in many patients and is both therapeutic and, when it works, diagnostically confirmatory. Oral steroids give shorter benefit.
• Treating the underlying cause — controlling hypothyroidism, diabetes, or weight; awaiting postpartum fluid resolution — can reverse early disease.
• Surgical carpal tunnel release — dividing the transverse carpal ligament — is definitive. Cutting the roof lets the tunnel expand and pressure fall permanently. It is indicated for severe CTS (thenar atrophy, constant numbness, or axon loss on EMG) or failed conservative care, with symptom resolution in ~75–90%.

Natural history if untreated: intermittent tingling progresses to constant numbness, then to weakness and thenar wasting as axons die. The key clinical teaching point — and a common misconception — is that numbness is not benign to "wait out." Once fixed sensory loss or muscle wasting appears, even a technically perfect surgery may not fully restore function, because you cannot re-grow long-dead axons. The window for full recovery is the reversible ischemic/demyelinating phase — which is precisely why persistent numbness should prompt evaluation rather than watchful waiting.

Carpal tunnel syndrome vs. cervical (C6/C7) radiculopathy — the most common mimic

Feature	Carpal Tunnel Syndrome	C6/C7 Radiculopathy
Site of lesion	Median nerve at the wrist	Nerve root at the neck (foramen)
Sensory territory	Thumb, index, middle, radial half of ring finger; spares palm	Follows a dermatome up the forearm/arm; may spare fingers
Palm sensation	Spared (palmar cutaneous branch bypasses tunnel)	Often involved / variable
Neck movement effect	No change	Worsens with extension/rotation (Spurling +)
Night symptoms	Hallmark — wakes patient, relieved by shaking (flick sign)	Less positional; often constant
Reflexes	Normal	Biceps/triceps reflex may be reduced