[The usefulness of ultrasonography in the diagnosis of carpal tunnel syndrome--a review].

Abstract

Ultrasonography has had relatively long history (approximately 20 years) in the diagnosing carpal tunnel syndrome, but as late as in last decade has gained greater popularity and has been applied in the clinic. Numerous studies revealed that the most accurate parameter indicating the compression of the median nerve in the carpal tunnel is the cross sectional area of the nerve at the inlet level. However, contrary to the nerve conduction studies, sonographic measurements are characterised by wide range of normal, physiological (a mean of 4.8 to 9.7 mm2), pathological, indicating compression of the nerve values (a mean of 10.7 to 16.8 mm2) and cut-off coefficients between normal state and pathology (a mean of 6.5 to 14 mm2). Sensitivity and specificity of the method, calculated for different cut-off values is estimated of 60-90%, hence, is around 10% lower than the same parameters of electrophysiological tests. Difficulties in standardisation of cross-sectional nerve area (considered a classical parameter) motivated investigators to searching other indicators of the nerve compression, e.g. "wrist-forearm" ratio, which is a quotient of the cross sectional area of the median nerve at the carpal tunnel inlet and 12-15 proximally at the forearm level. Some studies showed greater accuracy of this ratio, allowing to obtain the sensitivity and specificity of more than 95%. After review of the studies, authors critically conclude that actual state of art does not justify considering ultrasonography a valuable additional test in diagnosing carpal tunnel syndrome and for routine use this technique in typical cases. Ultrasonography may be useful in patients with doubtful clinical picture, as a screening test, as well as in suspicion of intra-tunnel pathology. However, in atypical clinical situation, nerve conduction studies provide significantly more information on the function of the median nerve, presence of more than one compression sites or other pathology.