A 32-year-old carpenter from Delhi presents with weakness of thumb opposition and abduction, along with sensory loss over the lateral three and a half fingers. Nerve conduction studies show slowing across the wrist. What is the most common site of median nerve compression in this patient?

Explanation

## Most Common Site of Median Nerve Compression **Key Point:** Carpal tunnel syndrome (CTS) is the most common entrapment neuropathy of the median nerve, accounting for >50% of all nerve compression syndromes. ### Clinical Presentation The patient's presentation is classic for CTS: - **Motor deficit:** Weakness of thumb opposition and abduction (thenar eminence atrophy) — supplied by recurrent motor branch of median nerve distal to carpal tunnel - **Sensory loss:** Lateral three and a half fingers (radial 3.5 digits) - **NCS finding:** Slowing across the wrist localizes the lesion to the carpal tunnel ### Anatomical Basis The carpal tunnel is a fibro-osseous canal bounded by: - Anteriorly: flexor retinaculum (transverse carpal ligament) - Posteriorly: carpal bones - Contents: median nerve + 8 flexor tendons (FDS, FDP, FPL) **High-Yield:** The median nerve is the most superficial structure within the carpal tunnel and most vulnerable to compression. ### Comparison of Median Nerve Compression Sites | Site | Frequency | Clinical Features | NCS Findings | |------|-----------|-------------------|---------------| | **Carpal tunnel (wrist)** | **Most common (>50%)** | **Thenar weakness, sensory loss digits 1–3.5** | **Slowing across wrist** | | Pronator teres (forearm) | Rare | Forearm pain, weakness of pronation | Slowing in proximal forearm | | Ligament of Struthers | Very rare | Supracondylar pain, proximal weakness | Slowing above elbow | | AIN (anterior interosseous) | Uncommon | Weakness of FPL + EIP only, no sensory loss | Normal routine NCS | **Clinical Pearl:** CTS is 3–5 times more common in women and increases with age, repetitive wrist flexion (occupational), pregnancy, hypothyroidism, and rheumatoid arthritis. ### Why Carpal Tunnel is the Answer 1. Anatomical narrowing: The carpal tunnel has the smallest cross-sectional area along the median nerve course 2. Mechanical vulnerability: Flexor tendons swell with repetitive use, compressing the nerve 3. Epidemiological dominance: CTS prevalence in general population is ~3–5%; other sites are <1% **Mnemonic — Sites of Median Nerve Compression (proximal to distal):** **LAPS-C** - **L**igament of Struthers (supracondylar) - **A**ntebrachial fascia (rare) - **P**ronator teres - **S**uperficial flexor origin - **C**arpal tunnel ← Most common [cite:Clinically Oriented Anatomy 8e Ch 6]