Carpal Tunnel Syndrome (CTS) is the most common entrapment neuropathy of the upper extremity. It results from the compression of the median nerve as it passes through the fibro-osseous carpal tunnel at the wrist. For the board-certified physician, diagnosing CTS requires a precise understanding of the tunnel’s boundaries and the ability to differentiate median nerve entrapment from more proximal issues like cervical radiculopathy or Pronator Syndrome.

1. Anatomy of the Carpal Tunnel

The carpal tunnel is a narrow passageway on the palmar side of the wrist. Because its boundaries are rigid, any increase in the volume of the contents leads to increased pressure on the median nerve.

• Boundaries: * Posterior/Lateral/Medial: The carpal bones (forming a concave arch).
  • Anterior: The Flexor Retinaculum (Transverse Carpal Ligament), which attaches to the scaphoid and trapezium laterally, and the hamate and pisiform medially.
• Contents: The carpal tunnel contains ten structures:
  • The Median Nerve.
  • Four tendons of the Flexor Digitorum Superficialis (FDS).
  • Four tendons of the Flexor Digitorum Profundus (FDP).
  • The one tendon of the Flexor Pollicis Longus (FPL).

2. Clinical Presentation and Physical Exam

Patients typically present with numbness and tingling in the median nerve distribution (thumb, index, middle, and the radial half of the ring finger). A hallmark of CTS is nocturnal symptoms, where patients wake up needing to “shake out” their hands (the Flick Sign).

High-Yield Physical Exam Maneuvers

• Phalen’s Test: The patient holds their wrists in full flexion for 60 seconds. A positive test is the reproduction of paresthesia in the median distribution.
• Reverse Phalen’s (Prayer) Test: Full wrist extension for 60 seconds.
• Tinel’s Sign: Percussion over the median nerve at the carpal tunnel.
• Durkan’s Compression Test: The clinician applies direct pressure over the carpal tunnel for 30 seconds. Research suggests this is the most sensitive clinical test for CTS.
• Semmes-Weinstein Monofilament Testing: Used to assess for early sensory loss, which often precedes motor weakness.

3. Electrodiagnostic (EMG/NCS) Correlation

While the diagnosis is clinical, EMG/NCS is the gold standard for confirming severity and ruling out other pathologies.

• Sensory Nerve Conduction: Usually the first to become abnormal. A delay in the median sensory latency across the wrist compared to the ulnar nerve is a classic finding.
• Motor Nerve Conduction: Prolonged Distal Motor Latency (DML) to the Abductor Pollicis Brevis (APB) indicates more advanced compression.
• Electromyography (EMG): The presence of fibrillations or motor unit changes in the APB muscle suggests axonal loss and is an indication for surgical referral.
• Proximal Sparing: In CTS, sensation over the thenar eminence is preserved because the palmar cutaneous branch of the median nerve arises proximal to the carpal tunnel and passes over the retinaculum.

4. Differential Diagnosis: The “Double Crush”

A high-yield board concept is the Double Crush Syndrome, where a proximal compression (e.g., C6 nerve root) makes the distal nerve more susceptible to compression at the carpal tunnel.

• Pronator Syndrome: Compression of the median nerve in the forearm by the pronator teres. Unlike CTS, this causes pain in the forearm and sensory loss over the thenar eminence.
• C6/C7 Radiculopathy: Pain and paresthesia will follow a dermatomal pattern and may be accompanied by neck pain and diminished reflexes (brachioradialis or triceps).

5. Management Strategies

• Conservative: Neutral-angle wrist splinting at night is the first-line treatment to prevent sustained wrist flexion during sleep.
• Interventional: Corticosteroid injections into the carpal tunnel provide significant temporary relief and can serve as a diagnostic tool. If an injection provides no relief, the diagnosis of CTS should be questioned.
• Surgical: Transverse Carpal Ligament Release (open or endoscopic) is indicated for patients with failed conservative care, constant numbness, or thenar atrophy.

6. Ergonomics and Prevention

CTS is an overuse injury.

• Ergonomics: Adjusting keyboard height to maintain a neutral wrist position and using “vertical” mice can reduce pressure within the tunnel.
• Tendon Gliding Exercises: Specific exercises designed to ensure the tendons and nerve move smoothly against each other, preventing adhesions.
• Occupation: Workers in high-vibration industries or those performing repetitive forceful gripping are at highest risk.

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High-Yield Board “Fast Facts”

• Abductor Pollicis Brevis (APB): The primary motor muscle tested in CTS; innervated by the recurrent branch of the median nerve.
• Palmar Cutaneous Branch: Sensation over the palm/thenar eminence is spared in CTS.
• Nocturnal Paresthesia: The most specific historical finding for CTS.
• Vitamin B6: Historically used as a supplement, though evidence for its efficacy is weak.
• Ape Hand Deformity: Occurs in late-stage CTS due to thenar atrophy and loss of thumb opposition.