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

Abstract

Purpose To evaluate whether ultrasound findings of the median nerve cross-section 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) or WFR (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”.

Key Points

• CSA of the median nerve does not change significantly after successful carpal tunnel release.

• Morphologic median nerve alterations may persist regardless of functional outcome (“memory effect”).

• Therefore, ultrasound imaging is not ideally suited to assess the outcome after carpal tunnel release.

Citation Format

• Steinkohl Fabian, Gruber Leonhard, Gruber Hannes et al. Memory Effect of the Median Nerve: Can Ultrasound Reliably Depict Carpal Tunnel Release Success?. Fortschr Röntgenstr 2017; 189: 57 – 62

Zusammenfassung

Ziel Evaluierung der postoperativen Ultraschalluntersuchung der Querschnittsfläche und Wrist-to-Forearm-Ratio des Nervus medianus zur Prognoseabschätzung nach Karpaltunnelspaltung bei Karpaltunnelsyndrom.

Material und Methoden 21 Hände von 20 Patienten mit klinisch diagnostiziertem Karpaltunnelsyndrom wurden in Bezug auf subjektive Schmerzen, Querschnittsfläche und Wrist-Forearm-Ratio des N. medianus, postoperatives Narbengewebe kurz vor der Karpaltunnelspaltung und 6 Monate danach untersucht. Gruppenunterschiede wurden mittels eines zweiseitigen ratio-paired t-Tests und one-way ANOVA, Korrelationen mittels eines linearen Regressionsmodells untersucht.

Ergebnisse Es kam zu keiner signifikanten Änderung zwischen präoperativer und postoperativer Querschnittsfläche des Nervus medianus (p = 0,293, 95 % CI 0,826 – 1,063) oder WFR (p = 0,230, 95 % CI 0,757 – 1,074). Die Nervenleitgeschwindigkeit besserte sich nach der Karpaltunnelspaltung signifikant (p < 0,0001, 95 % CI 0,753 – 0,886), ebenso das subjektive Schmerzempfinden in Ruhe (p = 0,001, 95 % CI 1,615 – 5,797) und bei Bewegung (p = 0,008, 95 % CI 0,760 – 4,888). Es bestand keine statistische Korrelation zwischen Änderung der Medianus-Querschnittsfläche und Nervenleitgeschwindigkeit (p = 0,357, r = 0,217, R2 = 0,047) oder karpaltunneltypischer Beschwerden (p = 0,441, r = 0,200, R2 = 0,040).

Schlussfolgerungen Anders als oft angegeben, kommt es nach einer erfolgreichen Karpaltunnelspaltung nicht zu einer signifikanten Reduktion der Querschnittsfläche der Nervus medianus. Ein einmal geschädigter Nervus medianus bleibt auch längere Zeit nach Behebung der Ursache der Kompression verdickt. Aufgrund dieses „Memory Effects“ ist die Annahme, dass ein postoperativ verdickter Nervus medianus mit einer erfolglosen Karpaltunnelspaltung einhergeht, nicht korrekt.

Kernaussagen

• Die CSA des Nervus medianus ändert sich nicht signifikant nach erfolgreicher Karpaltunnelchirurgie.

• Morphologische Veränderungen des N. medianus können ungeachtet des funktionellen Outcomes persistieren ("Memory Effect").

• Aus diesem Grund eignet sich die Ultraschalluntersuchung nicht, um ein erfolgreiches funktionelles Outcome nach operativer Entlastung des Nervus medianus zu beurteilen.

Key words

ultrasound - carpal tunnel syndrome - median nerve - carpal tunnel release - wrist-to-forearm ratio

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