Abstract

Myotonia, i.e., delayed muscle relaxation in certain hereditary muscle disorders, can be assessed quantitatively using different techniques ranging from force measurements to electrodiagnostics. Ultrasound shear wave elastography (SWE) has been proposed as a novel tool in biomechanics and neuromuscular medicine for the non-invasive estimation of muscle elasticity and, indirectly, muscle force. The aim of this study is to provide ‘proof-of-principle’ that SWE allows a quantitative measurement of the duration of delayed muscle relaxation in myotonia in a simple clinical setting. In six myotonic muscle disorder patients and six healthy volunteers, shear wave velocities (SWV) parallel to the fiber orientation in the flexor digitorum superficialis muscle in the forearm were recorded with a temporal resolution of one per second during fist-clenching and subsequent relaxation; the relaxation time to 10% of normalized shear wave velocity (RT0.1) was calculated. Forty-six SWE imaging sequences were acquired, yielding a mean RT0.1 of 7.38 s in myotonic muscle disorder patients, significantly higher than in healthy volunteers (1.36 s), which is comparable to data obtained by mechanical dynamometry. SWV measurements during the baseline relaxation and voluntary contraction phases did not differ significantly between groups. We conclude that SWE is a promising, non-invasive, widely available tool for the quantitative assessment of myotonia to aid in diagnosis and therapeutic monitoring.

Highlights

  • Myotonia describes delayed muscle relaxation after activation

  • RT0.1, rSWVmean baseline phase and rSWVmean fist clenching phase were compared between groups

  • For the differentiation between groups according to RT0.1, a receiver operating characteristic (ROC) curve analysis was performed

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Summary

Introduction

Myotonia describes delayed muscle relaxation after activation. It is the eponymous symptom and clinical sign of myotonic muscle disorders, causing disability and reduction in quality of life [1,2]. Previous clinical studies used the following options: (1) patientreported outcomes, such as the Myotonia Behaviour Scale [7]; (2) clinical observation and timing of myotonia after closing eyes or clenching a fist, as well as the presence of percussion myotonia [3,5]; (3) quantitative measurement of the increased relaxation time of finger flexors with a dynamometer [8,9,10,11]; (4) functional tests of the lower extremities such as the Timed Up And Go or 14 Step Stair Test [6]; and (5) electrophysiological tests such as electromyographical relaxation time [3] or the maximal post-exercise decrement in compound muscle action potential after short and long exercise [5]

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