A 28-year-old woman with a 6-month history of progressive hand weakness and numbness in the thumb and index finger is evaluated. Electromyography shows denervation in the abductor pollicis brevis and flexor pollicis longus. What is the most common underlying cause of median nerve dysfunction in this patient?

## Most Common Cause of Median Nerve Dysfunction **Key Point:** Repetitive wrist flexion and occupational trauma is the most common *cause* of median nerve dysfunction, primarily through carpal tunnel syndrome (CTS). ### Clinical Correlation The patient's presentation indicates: - **Progressive course:** 6 months (chronic compression pattern) - **Motor involvement:** Abductor pollicis brevis (recurrent motor branch) and flexor pollicis longus (AIN branch) denervation - **Sensory involvement:** Thumb and index finger (median nerve distribution) - **EMG findings:** Denervation pattern consistent with distal median nerve lesion ### Epidemiology of Median Nerve Lesions | Cause | Frequency | Population at Risk | Presentation | |-------|-----------|-------------------|---------------| | **Repetitive trauma / CTS** | **>60%** | **Office workers, assembly line workers, typists** | **Insidious, progressive, bilateral often** | | Supracondylar fracture | ~15% | Children (fall on outstretched hand) | Acute, traumatic onset, proximal weakness | | Brachial plexus injury | ~10% | Birth trauma, motorcycle accidents | Neonatal or acute post-trauma | | Cervical spondylosis | ~5% | Elderly with degenerative disc disease | Myelopathic features, upper motor signs | | Laceration / penetrating injury | ~5% | Trauma, occupational accidents | Acute, obvious wound | **High-Yield:** CTS accounts for >50% of all entrapment neuropathies and >60% of all median nerve dysfunction. Repetitive wrist flexion is the leading occupational risk factor. ### Why Repetitive Trauma is the Answer 1. **Prevalence:** CTS affects 3–5% of the general population; occupational groups (assembly workers, typists) have incidence up to 10–15% 2. **Mechanism:** Repetitive flexion increases intra-carpal tunnel pressure, causing ischemic compression of the median nerve 3. **Clinical course:** Insidious onset over weeks to months (matches this patient's 6-month history) 4. **Demographics:** More common in women (3:1 ratio) and increases with age — this 28-year-old woman fits the typical CTS profile **Clinical Pearl:** Risk factors for CTS include: - Occupational: typing, assembly line work, vibrating tools - Medical: pregnancy, hypothyroidism, rheumatoid arthritis, diabetes - Anatomical: smaller carpal tunnel, wrist geometry ### Why Other Causes Are Less Likely **Supracondylar fracture** (15% of median nerve injuries): - Typically presents acutely after trauma, not progressively over 6 months - More common in children; this patient is 28 years old - Would show proximal weakness (pronator teres, flexor carpi radialis) before distal **Brachial plexus injury** (10% of median nerve injuries): - Birth trauma is the classic scenario; this patient is 28 years old - Presents acutely or in neonatal period, not as progressive 6-month decline - Would involve multiple nerve roots with broader motor/sensory loss **Cervical spondylosis** (5% of median nerve dysfunction): - Causes myelopathy with upper motor neuron signs (hyperreflexia, spasticity) - Would present with bilateral symptoms and gait disturbance - EMG would show denervation in multiple myotomes, not isolated median nerve **Mnemonic — Causes of Median Nerve Lesion (by frequency):** **CREST** - **C**arpal tunnel (repetitive trauma) — most common - **R**ecurrent compression (occupational) - **E**ntrapment syndromes (pronator teres, AIN) - **S**upracondylar fracture - **T**rauma (laceration, avulsion) [cite:Clinically Oriented Anatomy 8e Ch 6; Harrison 21e Ch 379]