An enlarged median nerve cross-sectional area at the tunnel inlet (pisiform level) — commonly above 10–12 mm² (verify local threshold) — with flattening under the retinaculum, and a search for a space-occupying cause.
Orient first
- Nerve conduction studies remain the reference; ultrasound adds cause and anatomy.
- A bifid median nerve or persistent median artery changes surgery.
- Causes: tenosynovitis, ganglion, anomalous muscles, amyloid, fracture malunion.
Acquire the study
- High-frequency linear probe, wrist in neutral, transverse sweep from the distal forearm to the hook of hamate; colour Doppler.
The manoeuvre
- Transverse view at the pisiform: trace the inner margin of the epineurium — cross-sectional area in mm².
- Compare with the area at the pronator quadratus level (wrist-to-forearm ratio).
- Distal tunnel: flattening ratio of the nerve; bowing of the flexor retinaculum.
- Tendons: tenosynovitis; ganglion; anomalous muscle bellies.
- Bifid nerve and persistent median artery on colour Doppler.
What confirms it
- Enlarged median nerve area at the inlet with compatible nerve conduction studies.
What licenses you to exclude it
- A normal cross-sectional area lowers the likelihood; it does not exclude carpal tunnel syndrome.
The classic misread
- Including the epineural fat in the traced area.
- Measuring at the hamate level where the nerve is flattened rather than swollen.