Image Interpretation: The median nerve is hypoechoic and enlarged in appearance, measuring 17mm2 at the wrist crease. The flexor tendons are normal in appearance without tear or evidence of tenosynovitis. Unremarkable Guyon's canal. Ultrasound findings are compatible with a clinical diagnosis of carpal tunnel syndrome.
Teaching Pearl: Ultrasound of the median nerve is an evidenced-based diagnostic tool for carpal tunnel syndrome (CTS). Studies show similar sensitivity and specificity of US as compared to the clinical CTS-6 score and nerve conduction studies in the diagnosis of CTS (Fowler JR, Cipolli W, Hanson T. A Comparison of Three Diagnostic Tests for Carpal Tunnel Syndrome Using Latent Class Analysis. J Bone Joint Surg Am. 2015 Dec 2;97(23):1958-61. doi: 10.2106/JBJS.O.00476. PMID: 26631997.). The main measurement of interest is maximal cross sectional area (CSA) at the carpal tunnel inlet, with the following thresholds: <10mm2 normal, 10-12mm2 borderline, >12mm2 abnormal (Smith, J. Carpal Tunnel with Dr. Jay Smith: AMSSM Sports Ultrasound Case Presentation. YouTube. December 18, 2020. https://www.youtube.com/watch?v=3rjdXSts8dc). If borderline, the Wrist-Forearm ratio (WFR) can also be used, with > or = 1.4 being abnormal (Hobson-Webb LD, Massey JM, Juel VC, Sanders DB. The ultrasonographic wrist-to-forearm median nerve area ratio in carpal tunnel syndrome. Clin Neurophysiol. 2008 Jun;119(6):1353-7. doi: 10.1016/j.clinph.2008.01.101. Epub 2008 Apr 1. PMID: 18387336.).