Sonographic diagnosis of carpal tunnel syndrome — diagnostic value of the triangular cross-section sign

Abstract Background and purpose Carpal tunnel syndrome (CTS) is a common neuropathy resulting from compression of the median nerve at the carpal tunnel. Sonographic diagnosis of this condition relies on enlargement, flattening and/or demonstration of impaired mobility of the median nerve, as well as bowing or thickening of the flexor retinaculum. In most patients the cross-section of the median nerve at the level of the carpal tunnel has an oval or elliptical shape. We have noticed, however, that in a number of cases the cross-section of the median nerve at the inlet of the carpal canal was deformed, assuming a triangular shape. The purpose of this study was to assess the diagnostic value of the triangular median nerve cross-section sign. Material and methods One hundred and thirty-nine sonographic examinations in 76 patients with clinical signs of CTS, and 25 examinations in 14 healthy volunteers were performed. Standard electrodiagnostic studies were performed in all symptomatic patients. Results Triangular shape of the median nerve cross-section in the carpal canal was observed in 13/118 (11.0%) median nerves with abnormal result of the electrodiagnostic study, and in 2/21 (9.5%) cases with a normal electrodiagnostic examination result. No such deformation was seen in the control group. Conclusions The triangular cross-section sign has high specificity (0.90) and positive predictive value (0.87) in diagnosis of CTS.