Reference Values for Cross-Sectional Area of Peripheral Nerves on Ultrasound
Median Nerve (Carpal Tunnel Syndrome)
According to Jacobson (Fundamentals of Musculoskeletal Ultrasound, 4th ed., 2026), the cross-sectional area (CSA) of the median nerve less than 6 mm² typically excludes carpal tunnel syndrome, while greater than 12 mm² indicates it.
More accurate assessment is provided by analyzing the change in median nerve CSA: an increase in area greater than 2 mm² when comparing the proximal level (at the proximal border of the pronator quadratus) with the distal level (maximum nerve enlargement at the level of the wrist crease) allows diagnosis of carpal tunnel syndrome with 99% accuracy.
Additionally, a change in median nerve CSA from the proximal to distal level of 6 mm² indicates moderate, and 9 mm² indicates severe carpal tunnel syndrome.
Ulnar Nerve (Cubital Tunnel Syndrome)
According to Griffith (Diagnostic Ultrasound: Musculoskeletal, 3rd ed., 2025), the normal mean CSA of the ulnar nerve in the cubital tunnel is 9 mm². Most studies use 10 mm² as the threshold for abnormality.
Due to wide variability in ulnar nerve size, cubital tunnel syndrome can be confidently diagnosed when CSA > 15 mm². On MRI, ulnar nerve swelling >15 mm² is also described. Localization is between the medial epicondyle of the humerus and the olecranon; on longitudinal scanning, a spindle-shaped appearance is characteristic due to localized thickening.
Reference Values for CSA
| Nerve | Parameter | Value (Source) |
|---|---|---|
| Median | Excludes CTS | < 6 mm² (Jacobson, 2026) |
| Median | Indicates CTS | > 12 mm² (Jacobson, 2026) |
| Median | Proximal→distal increase | > 2 mm² (99% accuracy) |
| Median | Moderate / Severe CTS | +6 mm² / +9 mm² |
| Ulnar (cubital tunnel) | Normal (mean) | 9 mm² (Griffith, 2025) |
| Ulnar | Threshold for abnormality | 10 mm² |
| Ulnar | Confident CTS diagnosis | > 15 mm² |
Measurement Technique
The circumferential trace method is preferred, accounting for variations in median nerve shape; measurement is performed along the inner border of the echogenic epineurium.
For systemic reference values of CSA for peripheral nerves and brachial plexus, published data exist (Kerasnoudis et al., Clin Neurophysiol 2013); however, specific threshold values are not provided in the supplied fragments [to be clarified].
Frequently asked questions
What median nerve CSA excludes carpal tunnel syndrome?
An area less than 6 mm² typically excludes carpal tunnel syndrome, while greater than 12 mm² indicates it (Jacobson, 2026).
How can the accuracy of CTS ultrasound diagnosis be improved?
Compare CSA proximally (at the proximal border of the pronator quadratus) and distally (at the level of the wrist crease): an increase greater than 2 mm² diagnoses CTS with 99% accuracy.
How to assess CTS severity by CSA?
A change in median nerve CSA from the proximal to distal level of 6 mm² corresponds to moderate severity, and 9 mm² to severe.
What is the normal CSA of the ulnar nerve in the cubital tunnel?
Normal mean CSA is ~9 mm², abnormality threshold is 10 mm². Due to wide variability, cubital tunnel syndrome is confidently diagnosed at CSA > 15 mm² (Griffith, 2025).
What is the preferred method for measuring nerve area?
The circumferential trace method inside the echogenic epineurium is preferred—it accounts for variations in nerve shape.