carpal tunnel
Summary
Carpal tunnel syndrome is compression neuropathy of the median nerve as it passes through the carpal tunnel at the wrist, causing pain, paresthesias, and weakness in the median nerve distribution (palmar thumb, index, middle, and half of ring finger). It is the most common entrapment neuropathy, often associated with repetitive wrist motion, pregnancy, hypothyroidism, diabetes, rheumatoid arthritis, and acromegaly.
Detail
The carpal tunnel is a narrow anatomical passage on the palmar side of the wrist bounded by the carpal bones dorsally and the flexor retinaculum (transverse carpal ligament) volarly. It contains the median nerve and 9 flexor tendons (4 flexor digitorum superficialis, 4 flexor digitorum profundus, 1 flexor pollicis longus). Increased pressure within this confined space compresses the median nerve, leading to ischemia and demyelination.
Pathophysiology: Any condition causing swelling of the tendons/synovium or narrowing of the tunnel increases pressure on the median nerve. Common causes include repetitive motion (typing, assembly work), pregnancy (fluid retention), hypothyroidism (myxedema), diabetes mellitus (neuropathy + tenosynovial thickening), rheumatoid arthritis (synovitis), acromegaly (soft tissue overgrowth), amyloidosis, and obesity.
Clinical presentation: Numbness, tingling, and burning pain in the median nerve distribution (thumb, index, middle finger, and radial half of ring finger), often worse at night, sometimes relieved by shaking the hand ("flick sign"). With chronic/severe compression, thenar muscle weakness and atrophy occur (weak thumb abduction/opposition due to involvement of the recurrent motor branch innervating the LOAF muscles: Lumbricals 1&2, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis).
Diagnosis: Clinical exam plus provocative tests—Tinel sign (tapping over median nerve reproduces symptoms) and Phalen sign (wrist flexion for 60 seconds reproduces symptoms). Nerve conduction studies/EMG confirm diagnosis by showing prolonged distal latency.
Treatment: Conservative measures include wrist splinting (especially at night), NSAIDs, and corticosteroid injections. Treat underlying causes (e.g., thyroid hormone replacement for hypothyroidism). Surgical carpal tunnel release (division of the transverse carpal ligament) is indicated for refractory cases or significant motor involvement.
High-yield board points: Remember the sensory sparing of the palm (palmar cutaneous branch of median nerve arises proximal to the carpal tunnel and is NOT compressed), differentiating carpal tunnel from more proximal median nerve lesions. Also associate carpal tunnel syndrome with pregnancy, hypothyroidism, RA, diabetes, and amyloidosis as classic test-taking associations.
Sources
- First Aid for the USMLE Step 1
- Kaplan USMLE Step 1 Lecture Notes: Anatomy
- UpToDate: Carpal Tunnel Syndrome
- Netter's Atlas of Human Anatomy
Reviewed by AnkiBoss editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.