Simple and economical HandClench Relaxometer device for reliable and sensitive measurement of grip myotonia in myotonic dystrophy.

Grip myotonia and weakness are attractive treatment response biomarkers in clinical trials of myotonic dystrophy type 1 (DM1). There is a need to develop simple, patient-friendly and reproducible methods of quantifying grip myotonia in multisite trial settings. We designed a HandClench Relaxometer (HCR) that measures grip myotonia and strength. In contrast with the existing quantitative myometry (QMA) setup, the HCR is portable, economical, can be used with any laptop and generates automated command prompts. We demonstrate the feasibility and reliability of HCR device in twenty DM1 individuals and ten age-matched controls; patients returned for follow up within two months. The device showed excellent day to day reproducibility (ICC >0.80) in patients. The HCR device detected myotonia in milder muscle disease and measured longer myotonia duration than QMA indicating enhanced sensitivity for quantifying myotonia in DM1. The reaction time to the relax but not squeeze command was delayed and showed warm up similar to myotonia in DM1. HCR outcomes were correlated with key pinch strength, hand dexterity test, and fat replacement in the MRI of the long finger flexor muscles. Use of the HCR is warranted for grip myotonia and strength measurements in longitudinal observational and interventional studies of DM1.