Ultrasound of the Median Nerve in Carpal Tunnel Syndrome: Technique and Threshold Values for Cross-Sectional Area

Research Methodology

The hand is placed on a flat surface with relaxed fingers. A high-frequency probe of 12–17 MHz is used. The probe is held strictly perpendicular to the nerve, as tilting increases the measured caliber; slight angulation can use anisotropy for better delineation of the nerve boundaries from tendons. Excessive pressure with the probe should be avoided, as it reduces the nerve caliber and enhances its flattening (Griffith, 2025).

The cross-sectional area (CSA) is measured using the continuous tracing method around the perimeter of the nerve surrounding the hypoechoic nerve bundles. The tracing method is preferred due to variations in the shape of the median nerve; measurements are conducted within the echogenic epineurium (Jacobson, 2026). However, the echogenic epineurium is poorly delineated from perineural fat on ultrasound (Griffith, 2025). If the median nerve is bifid, the CSA of both components is measured and summed.

CSA Measurement Points

According to the protocol (Griffith, 2025), CSA is measured in the distal forearm, proximal to the canal entrance, at the canal entrance, at the canal exit, and distal to the exit. The optimal cutoff point: CSA >14 mm² at any location is a strong positive predictor of CTS (sensitivity/specificity/accuracy >88%). The nerve is usually most swollen immediately proximal or distal to the canal due to compression at the entrance or exit.

Normal Values and Thresholds

ParameterValueSourceNormal CSA7–12 mm²Griffith, 2025Nerve swelling suggested atCSA >10 mm²Griffith, 2025Usually excludes CTSCSA <6 mm²Jacobson, 2026Indicates CTSCSA >12 mm²Jacobson, 2026Strong predictor of CTSCSA >14 mm² at any pointGriffith, 2025Increase at the proximal canal levelCSA >15 mm²Hagen-Ansert, 2023

The normal CSA of the median nerve measured by ultrasound is 15–20% less than on MRI or anatomically, as ultrasound measurements do not include the epineurium (Griffith, 2025).

Dynamic Assessment (Delta CSA)

A more accurate assessment is based on the change in CSA of the median nerve. An increase of more than 2 mm² when comparing the proximal level (at the proximal edge of the pronator quadratus) with the distal (maximum nerve enlargement at the wrist crease) allows diagnosing CTS with 99% accuracy (Jacobson, 2026). A CSA change from proximal to distal of 6 mm² indicates moderate, and 9 mm² indicates severe CTS.

Additional Signs

A characteristic "hourglass" deformation of the nerve due to compression in the canal with swelling proximally and distally is noted. An increase in nerve caliber is the most reliable ultrasound criterion for CTS. It should be remembered that rigid diagnostic criteria are of little use in clinical practice, as median nerve compression is a continuum, not an "all or nothing" phenomenon (Griffith, 2025). Vascularization of the nerve is also assessed (neural hyperemia is rare); in cases of ambiguous CSA values, palmar bulging of the flexor retinaculum at the canal exit is measured.