Carpal tunnel syndrome: the role of median nerve ultrasound

How measuring the median nerve's cross-sectional area helps diagnose carpal tunnel syndrome, and where ultrasound fits alongside nerve conduction studies.

Carpal tunnel syndrome is the most common entrapment neuropathy, and ultrasound has become an accessible complementary tool to nerve conduction studies for evaluating the median nerve at the wrist.

The measurement that matters most

The cross-sectional area of the median nerve at the entrance of the carpal tunnel, at the level of the pisiform, is the most widely used parameter. The key finding is an increase in this area relative to more proximal segments of the same nerve — an indirect sign of compression and edema.

Other associated signs

Flattening of the nerve within the tunnel itself, bowing of the flexor retinaculum, and increased vascularity on Doppler in more inflammatory cases reinforce the suspicion when present alongside the increased area.

Complementary to nerve conduction studies, not a replacement

Ultrasound adds anatomical information — nerve variants, associated masses, flexor tendon synovitis — but does not measure nerve conduction velocity. The two exams answer different questions and complement rather than compete with each other.

Why the structural cause matters

Identifying an associated cause, such as a synovial cyst or an anatomical variant of the median nerve, can change the management of a case that would otherwise simply be treated as idiopathic carpal tunnel syndrome.