Sonographic measurement of cross-sectional area of the median nerve in the diagnosis of carpal tunnel syndrome: correlation with nerve conduction studies.

PURPOSE

To assess the usefulness of sonographic measurement of the median nerve cross-sectional area (CSA) in the diagnosis of carpal tunnel syndrome (CTS) and grading of its severity using nerve conduction (NC) studies as the standard.

METHOD

The CSA of the median nerve was measured at the tunnel inlet and outlet using the ellipse formula and automatic tracing in 72 hands with suspicion of CTS.

RESULT

The lack of inter-reader reliability led to excluding CSA measurements obtained at the tunnel outlet. Based on the receiver operating characteristic curves, the following cut-off points for the CSA of the median nerve at the tunnel inlet was selected: 9.8 mm and 12.3 mm(2) for the ellipse formula and 11 and 13 mm(2) for automatic tracing. For the ellipse formula, a CSA less than or equal to 9.8 mm(2) excluded CTS whereas a CSA greater than or equal to 12.3 mm(2) was diagnostic of CTS with measurements between 9.8 and 12.3 mm(2) being indeterminate and requiring NC studies. For automatic tracing, the cutoff value of 11 mm(2) was excluded because of the high percentage of false negatives, whereas CSAs greater than or equal to 13 mm(2) were diagnostic of CTS. There were no statistically significant differences in CSA measurements between the various degrees of CTS severity determined by NC studies.

CONCLUSION

Sonographic measurement of median nerve CSA at the tunnel inlet is a good alternative to NC studies as the initial diagnostic test for CTS, but it cannot grade the severity of CTS as well as NC studies.