Carpal Tunnel Syndrome

Populations and Risk Factors

• Females — approximately 3 times more likely than males, due to smaller carpal tunnel dimensions and hormonal factors affecting connective tissue fluid dynamics
• Pregnancy — up to 41% incidence; fluid retention increases interstitial pressure within the tunnel; typically resolves postpartum but may persist if structural changes occur
• Ages 40–60 peak incidence for non-pregnancy-related CTS
• Occupational repetitive hand use — sustained wrist flexion or extension, repetitive gripping, vibrating tools (assembly line workers, computer users, hairdressers, manual laborers)
• Systemic contributors: diabetes mellitus (8% of diabetics develop CTS — neuropathy reduces nerve resilience to compression), rheumatoid arthritis (synovial proliferation directly narrows tunnel), hypothyroidism (myxedematous tissue deposition increases tunnel pressure), obesity (adipose tissue deposition increases tunnel volume), amyloidosis, acromegaly
• Prior wrist fracture (Colles' fracture) or dislocation — bony deformity narrows the tunnel
• Individuals with a "square wrist" (wrist depth-to-width ratio approaching 0.7) have anatomically smaller tunnel volume

Causes and Pathophysiology

Carpal Tunnel Anatomy

The carpal tunnel is a rigid, non-expandable osteofibrous canal at the volar wrist. Its boundaries are:

• Floor and walls: the carpal bones arranged in a concave arch — scaphoid and trapezium laterally, pisiform and hook of hamate medially
• Roof: the flexor retinaculum (transverse carpal ligament), a thick, inelastic band spanning between the carpal bone margins

The tunnel contains ten structures in a fixed space: the median nerve and nine flexor tendons (four tendons of flexor digitorum superficialis, four tendons of flexor digitorum profundus, and the tendon of flexor pollicis longus). Each tendon is enclosed in a synovial sheath. The median nerve is the only non-compressible structure — the tendons can deform slightly under pressure, but the nerve cannot. This makes the median nerve the first structure to suffer when tunnel pressure rises.

Compression Mechanism

• Tenosynovitis (most common cause): Repetitive flexor tendon gliding produces friction and microtrauma within the synovial sheaths. The inflamed sheaths swell, increasing volume within the fixed tunnel space. Because the carpal bones and retinaculum cannot expand, the increased volume is accommodated at the expense of the median nerve. Even modest synovial thickening (1–2 mm) can raise intraneural pressure above the threshold for ischemic damage.
• Intraneural ischemia sequence: Elevated tunnel pressure compresses the vasa nervorum (the nerve's own blood supply) before it compresses the nerve fibers themselves. This produces ischemic paresthesia — tingling and numbness — before any structural nerve damage occurs. Clinically, this explains why early CTS is intermittent and reversible: the nerve is ischemic, not damaged. Prolonged ischemia, however, leads to segmental demyelination and eventually to axonal degeneration — which is irreversible.
• Wrist position effect: Wrist flexion and extension both narrow the tunnel by up to 30% — flexion buckles the retinaculum inward; extension stretches it taut over the carpal arch. This is why symptoms are worst at night (unconscious wrist flexion during sleep) and why Phalen's test reproduces symptoms by sustained flexion.

Pregnancy Mechanism

Hormonal changes during pregnancy (elevated progesterone and relaxin) increase fluid retention systemically. In the carpal tunnel, interstitial edema raises hydrostatic pressure within the already-full tunnel, compressing the median nerve. This mechanism is usually self-limiting — fluid balance normalizes postpartum and symptoms resolve within weeks to months. However, if pregnancy-induced CTS is severe or prolonged, the nerve may sustain demyelination that does not fully reverse.

Double/Multiple Crush Phenomenon

The median nerve can be compressed at multiple points along its course: the cervical spine (C6–C7 nerve roots), the interscalene triangle (thoracic outlet), the pronator teres (proximal forearm), and the carpal tunnel. When compression exists at two or more sites, each individual site may be subclinical, but the cumulative effect exceeds the threshold for symptoms. Clinically, this explains why some patients have incomplete relief after carpal tunnel treatment alone — a proximal compression site (scalenes, pronator teres) may be contributing. Assessment must always include the proximal chain.

Signs and Symptoms

Early Presentation (Intermittent — Reversible Stage)

• Intermittent paresthesia: Tingling and numbness in the median nerve distribution (thumb, index, middle, and radial half of ring finger) — initially episodic, triggered by sustained wrist positions or repetitive hand activity
• Nocturnal symptoms (hallmark): Paresthesia wakes the patient at night — caused by unconscious wrist flexion during sleep increasing tunnel pressure; patient shakes or flicks the hand to restore sensation (flick sign, a behavioral hallmark of CTS)
• Aching at the wrist and forearm: Dull ache may radiate proximally into the forearm; occasionally misinterpreted as forearm strain
• Symptoms relieved by position change: Shaking the hand, extending the wrist, or running the hand under warm water restores circulation and relieves symptoms — indicates the nerve is ischemic, not structurally damaged
• Grip clumsiness: Occasional dropping of objects due to transient numbness, not true weakness at this stage

Advanced Presentation (Constant — Irreversible Stage)

• Constant numbness: Paresthesia becomes persistent rather than episodic — indicates segmental demyelination has occurred; partial sensory loss in the median distribution may become permanent
• Thenar atrophy: Visible wasting of the thenar eminence (abductor pollicis brevis, opponens pollicis) — indicates axonal degeneration of motor fibers; this is a late finding and signals poor prognosis for full recovery
• Weakness of opposition and pinch grip: Difficulty with fine motor tasks — buttoning, writing, turning keys — reflects motor fiber involvement
• Loss of two-point discrimination: Inability to distinguish two closely spaced touch points on the fingertips — quantifiable sensory deficit indicating advanced nerve damage

Assessment Profile

Subjective Presentation

• Chief complaint: "My hand goes numb and tingly at night — I have to shake it out to get feeling back"; "I keep dropping things"; often bilateral but one side dominant
• Pain quality: Tingling, burning, or "pins and needles" in thumb, index, and middle fingers; dull ache at the wrist that may radiate proximally into the forearm; pain is secondary to paresthesia in most patients
• Onset: Gradual and insidious; initially triggered by repetitive hand activity or sustained wrist positions; often bilateral with dominant hand worse; may begin or worsen during pregnancy or with new occupational demands
• Aggravating factors: Sustained wrist flexion (sleeping, driving, reading), repetitive gripping or pinching, vibrating tools, sustained keyboard/mouse use, carrying objects with flexed wrists
• Easing factors: Shaking or flicking the hand (flick sign), extending the wrist, running the hand under warm water — all reduce tunnel pressure or restore vasa nervorum blood flow; wrist splinting at night prevents flexion-induced compression
• Red flags: Acute hand weakness with no history of gradual progression → suspect acute median nerve injury or cervical cord pathology; severe thenar atrophy without pain history → refer for electrodiagnostic testing before treating

Observation

• Local inspection: Thenar eminence atrophy (advanced cases) — compare bilaterally; mild swelling at the volar wrist is occasionally visible; "square wrist sign" (wrist depth approximates wrist width) is a predisposing anatomical variant, not a pathological finding
• Posture: Sustained wrist flexion posture at rest; protective wrist guarding may be present; rounded shoulders and forward head posture may accompany proximal chain involvement (double crush)
• Gait: Not clinically relevant to CTS — omit from assessment

Palpation

• Tone: Forearm flexor group (FDS, FDP, FCR) — hypertonic, often bilaterally. Pronator teres — assess for tightness as a proximal median nerve compression site (double crush). Thenar muscles — may feel atrophied and soft in advanced cases rather than hypertonic.
• Tenderness: Carpal tunnel compression test (Durkan's) — direct sustained pressure over the flexor retinaculum at the volar wrist reproduces median nerve paresthesia; flexor retinaculum — taut and tender to direct palpation; proximal forearm flexor-pronator mass — tenderness and taut bands consistent with overuse; referred path tenderness: the median nerve may be tender to palpation through the proximal forearm (between the two heads of pronator teres) and into the thenar eminence — tenderness along this path suggests neural irritability beyond the tunnel and should prompt assessment for double crush
• Temperature: Usually normal; mild warmth over the volar wrist if active tenosynovitis is present
• Tissue quality: Forearm flexors — ropy, taut bands with reduced fascial mobility. Flexor retinaculum — thickened and inelastic. Thenar eminence — reduced muscle bulk with soft, inelastic tissue quality in advanced cases. Wrist flexor tendons may feel thickened at the tunnel entrance.

Motion Assessment

• AROM: Wrist extension may be limited by tight forearm flexors (non-capsular pattern); wrist flexion typically full but may reproduce paresthesia at end range; finger flexion/extension usually full; grip strength reduced (test with dynamometer — compare bilaterally)
• PROM / end-feel: Wrist extension end-feel is tissue stretch (tight flexors), not capsular; full PROM available unless concurrent wrist joint pathology exists; passive wrist flexion held at end range for 60 seconds functions as Phalen's test
• Resisted testing: Thumb opposition weakness (opponens pollicis — median nerve motor branch); weak pinch grip (flexor pollicis longus, lumbricals I–II); resisted wrist flexion and pronation are typically strong but may reproduce forearm pain if flexor overuse is present; normal resisted testing in early CTS is itself a finding — it indicates motor fibers are not yet involved

Special Test Cluster

Cluster interpretation: A positive Phalen's + positive Tinel's + positive Durkan's strongly confirms CTS. If ULNT median bias is also positive, assess the full proximal chain (cervical spine, scalenes, pronator teres) for double crush. If Finkelstein's is positive, de Quervain's may coexist — the two conditions are not mutually exclusive.

Differential Diagnoses