Ultrasound (US) of the carpal tunnel adds value to the usual electrophysiology diagnosis by confirming doubtful results or evaluating patients who do not improve after surgery. US sometimes provides the surgeon or the referring physician with important information about normal variants or common or even rare causes of the nerve compression. The size of the median nerve must be calculated in the short axis by measuring the nerve cross-sectional area where the nerve is the thickest, in most cases at the proximal entrance of the carpal tunnel. Because the threshold of the nerve size varies in the literature, it is best to consider the cross-sectional median nerve area <8 mm(2) to rule out and ≥12 mm(2) to rule in the diagnosis of carpal tunnel syndrome.
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