P 73 Muscle relaxation in myotonia and longitudinal nerve mobility in carpal tunnel syndrome as possible clinical applications for “dynamic” ultrasound shear-wave elastography

Abstract

Shear wave elastography (SWE) is an emerging modality in clinical ultrasound. Using acoustic radiation force impulses to generate transversal shear waves and fast imaging to track their propagation, it enables the non-invasive estimation of two-dimensional maps of tissue elasticity. Various applications in neuromuscular medicine have been investigated, e.g., altered muscle elasticity has been reported in muscular dystrophy and inflammatory myopathies, and elasticity of the median nerve at the wrist has been shown to be increased in carpal tunnel syndrome. Currently available SWE systems have a temporal resolution of about 1/second, making “dynamic”, functional assessments of nerve or muscle possible; we propose two such possible applications: (1) Myotonia describes delayed muscle relaxation in rare hereditary muscle disorders. In six patients with different myotonic muscle disorders and six controls, we performed SWE of the flexor digitorum superficialis muscle in the forearm with a frequency of one measurement per second. From measurements before, during and after brief voluntary contraction, the relaxation time to 10% of normalized shear wave velocity (RT0.1) was determined. 46 imaging sequences were acquired, with a mean RT0.1 of 7.38 s in patients and 1.36 s in control subjects, which is comparable to data obtained by mechanical dynamometry. (2) In carpal tunnel syndrome, reduced nerve mobility mostly in the lateral plane, but also longitudinally, has been described. Wrist and elbow movements, in healthy subjects, are known to alter median nerve tension and, thus, elasticity as detectable by SWE. We present a protocol to investigate whether entrapment at the carpal tunnel affects these posture-induced changes, an effect that might similarly occur in other compressive or traumatic neuropathies. These applications demonstrate how resolving functional manoeuvres by “dynamic” ultrasound SWE may be utilized for clinical diagnostic assessments.