Ultrasonographic median nerve cross-section areas measured by 8-point "inching test" for idiopathic carpal tunnel syndrome: a correlation of nerve conduction study severity and duration of clinical symptoms

BACKGROUND Incremental palmar stimulation of the median nerve sensory conduction at the wrist, the "inching test", provides an assessment with reference to segments proximal and distal to the entrapment. This study used high-resolution ultrasonography (US) to measure the median nerve's cross-section areas (CSAs) like the "inching test" and to correlate with the nerve conduction study (NCS) severity and duration of carpal tunnel syndrome (CTS). METHODS Two hundred and twelve (212) "CTS-hands" from 135 CTS patients and 50 asymptomatic hands ("A-hands") from 25 control individuals were enrolled. The median nerve CSAs were measured at the 8-point marked as i4, i3, i2, i1, w, o1, o2, and 03 in inching test. The NCS severities were classified into six groups based on motor and sensory responses (i.e., negative, minimal, mild, moderate, severe, and extreme). Results of US studies were compared in terms of NCS severity and duration of clinical CTS symptoms. RESULTS There was significantly larger CSA of the NCS negative group of "CTS-hands" than of "A-hands". The cut-off values of the CSAs of the NCS negative CTS group were 12.5 mm2, 11.5 mm2 and 10.1 mm2 at the inlet, wrist crease, and outlet, respectively. Of the 212 "CTS-hands", 32 were NCS negative while 40 had minimal, 43 mild, 85 moderate, 10 severe, and two extreme NCS severities. The CSAs of "CTS-hands" positively correlated with different NCS severities and with the duration of CTS symptoms. By duration of clinical symptoms, 12 of the 212 "CTS-hands" were in the 1 month group; 82 in >1 month and ≤ 12 months group, and 118 in >12 months group. In "inching test", segments i4-i3 and i3-i2 were the most common "positive-site". The corresponding CSAs measured at i4 and i3, but not at i2, were significantly larger than those measured at points that were not "positive-site". CONCLUSIONS Using the 8-point measurement of the median nerve CSA from inlet to outlet similar to the "inching test" has positive correlations with NCS severity and duration of CTS clinical symptoms, and can provide more information on anatomic changes. Combined NCS and US studies using the 8-point measurement may have a higher positive rate than NCS alone for diagnosing CTS.