Memory Effect of the Median Nerve: Can Ultrasound Reliably Depict Carpal Tunnel Release Success?

Abstract

© Georg Thieme Verlag KGStuttgart · New York. Purpose To evaluate whether ultrasound findings of the median nerve crosssection area (CSA) and wrist-to-forearm ratio (WFR) correlate with neurophysiological and patient-reported outcome after surgery for carpal tunnel syndrome (CTS). Materials and Methods Subjective pain, CSA,WFR, postoperative scar tissue formation and nerve conduction velocity in 21 hands of 20 patients with clinically confirmed CTS were examined before and after carpal tunnel release surgery. Group differences were compared via a 2-sided ratio paired t-test or one-way ANOVA, and correlations were calculated using a linear regression model. Results There were no significant pre- and postoperative changes of the CSA of the median nerve (p = 0.293, 95 % CI 0.826 – 1.063) orWFR (p = 0.230, 95 % CI 0.757 – 1.074). The nerve conduction velocity (p < 0.0001, 95 % CI 0.753 –0.886) and subjective pain during rest (p = 0.001, 95 % CI 1.615 – 5.797) and exercise (p = 0.008, 95 % CI 0.760 – 4.888) improved significantly, though. There was no correlation between changes in the median nerve CSA and nerve conduction velocity (p = 0.357, r = 0.217, R2 = 0.047) or reported pain intensity (p = 0.441, r = 0.200, R2 = 0.040). Conclusion Contrary to common assumptions, there is no significant reduction of the CSA of the median nerve after successful carpal tunnel release. Morphologic median nerve changes may persist for a longer period regardless of successful surgery and clinical improvement. Accordingly, ultrasound appears unsuitable as the primary means of assessing surgical success due to this "memory effect".