Carpal Tunnel Syndrome
Overview
Carpal Tunnel Syndrome (CTS) is the most common peripheral nerve entrapment, caused by compression of the median nerve within the carpal tunnel, bounded anteriorly by the flexor retinaculum and posteriorly by the concave carpal row.
Carpal Tunnel Contents
- Median nerve (most superficial, just deep to the retinaculum).
- Flexor Digitorum Superficialis x4, Flexor Digitorum Profundus x4, Flexor Pollicis Longus x1.
- Note: The ulnar nerve exits via Guyon's canal (medial to the carpal tunnel) and is NOT compressed here.
Clinical Presentation
- Paraesthesia and numbness in the lateral 3.5 digits (thumb, index, middle, lateral ring finger).
- Worsens at night and with wrist flexion (driving, phone use).
- Thenar eminence wasting in advanced cases.
WarningThenar Sensation is Spared
The palmar cutaneous branch of the median nerve departs PROXIMAL to the carpal tunnel, passing superficially over the flexor retinaculum. Therefore, sensation over the thenar eminence is specifically PRESERVED in CTS — this identifies the lesion as distal (at the wrist), not more proximal.
Diagnostic Tests
- Phalen's Test: Forced wrist flexion for 60 seconds reproduces paraesthesia.
- Tinel's Sign: Percussion over the flexor retinaculum elicits tingling.
- Ultrasound: Cross-sectional area of the median nerve at the level of the pisiform >9-10 mm2 is diagnostic.
High Yield Facts
LightbulbFRCR / MD Prep Pearl
Common secondary causes: pregnancy, hypothyroidism, rheumatoid arthritis, acromegaly, amyloidosis, gout. On ultrasound: proximal nerve swelling with loss of normal echogenicity at the tunnel. MRI: T2 hyperintensity within the nerve at the tunnel level.