Carpal tunnel syndrome: Ultrasonographic evaluation of median nerve diameter

Abstract

Background: Carpal tunnel syndrome is the most frequently encountered peripheral compression neuropathy. Diagnosis of carpal tunnel syndrome involves physical examination, nerve conduction studies and electromyography. Ultrasonography (USG) examination of median nerve in carpal tunnel has been proposed as a useful alternative in diagnosing carpal tunnel syndrome. Materials and Methods: Patients were selected from those undergoing diagnostic workup for carpal tunnel syndrome in pre-treatment period. USG was performed using 11 MHz linear array transducer. Cross-sectional area at each level, major and minor axes were measured. Results: In our study, the mean median nerve cross sectional area at proximal part of carpal tunnel by direct method was 12.33 mm 2 in patients and 7.33 mm 2 in controls. By indirect method it was 12.01 mm 2 and 6.633 mm 2 in cases and controls respectively. In this study we found significant difference in flattening ratio between cases and controls. The mean flattening ratio in distal part of tunnel (at the level of hook of hamate) was 2.97 and 2.38 in cases and controls respectively. The sensitivity and specificity for cut-off value ≥2.5 was 76% and 63% respectively. Conclusion: We found that best discriminatory criterion for diagnosis of carpal tunnel syndrome are median nerve cross sectional area in the proximal part of carpal tunnel ≥9 mm 2 (Direct method) and ≥8.5 mm 2 (Indirect method). With our experience, we found it easier to evaluate the median nerve in the carpal tunnel in the disto proximal sequence by identifying the flexor pollicislongus (FPL) first with dynamic evaluation. As the percentage of space occupying lesions causing symptoms in unilateral (atypical) carpal tunnel syndrome is 35%, we highly recommend this pre-operative investigation in all carpal tunnel syndrome patients.