A quantitative measure of handgrip myotonia in non‐dystrophic myotonia

Abstract

Introduction Non-dystrophic Myotonia (NDM) is characterized by myotonia without muscle wasting. A standardized quantitative myotonia assessment (QMA) is important for clinical trials. Methods Myotonia was assessed in 91 individuals enrolled in a natural history study using a commercially available computerized handgrip myometer and automated software. Average peak force and 90% to 5% relaxation times were compared to historical normal controls studied with identical methods. Results 30 subjects had chloride channel mutations, 31 sodium channel mutations, 6 DM2, and 24 no identified mutation. Chloride channel mutations were associated with prolonged 1st handgrip relaxation times, and warm up on subsequent handgrips. Sodium channel mutations were associated with prolonged 1st handgrip relaxation times and paradoxical myotonia or warm-up, depending on underlying mutations. DM2 subjects had normal relaxation times but decreased peak force. Sample size estimates are provided for clinical trial planning. Conclusion QMA is an automated, non-invasive technique for evaluating myotonia in NDM.

Heterozygous CLCN1 mutations can modulate phenotype in sodium channel myotonia

“…Warm-up phenomenon also occurs in SCM but seems to be less frequent than in MC and depends on underlying mutations. It has not been documented with the specific SCN4A mutations we found [1,3,5,14,15]. In our experience a great improvement of stiffness after warm-up is suggestive of MC.…”

Quantitative myotonia assessment with a commercially available dynamometer in myotonic dystrophy types 1 and 2

“…Intraclass correlation coefficients (ICC; two‐way random, average measures) were used to assess the inter‐trial variability of the first squeezes in each group. Coefficients of variation (the ratio of standard deviation to the mean of 3 trials) were calculated to allow comparison of our simplified method to the original method because these variables had been used in previous studies . In consideration of the lack of data normality, a Kruskal–Wallis test with a post hoc series of Mann–Whitney tests with Bonferroni corrections were employed to analyze the differences in RT and maximal force among DM1, DM2, and healthy controls.…”

“…The dynamometer produced an analog signal that was sent to a computer via an analog‐to‐digital transducer, and, subsequently, the RT was analyzed by customized software. This method was then used by the same authors to demonstrate the effect of mexiletine in a clinical trial and tested in nondystrophic myotonias . A later study (with lamotrigine) employed the Myotonia Behaviour Scale (MBS), which consists of 6 statements from which the patient has to choose 1, ranging from asymptomatic (score 0) to severe (score 5) myotonia.…”

Isometric skeletal muscle force measurement in primary myopathies

“…Handheld dynamometers or pinch grip devices are also reliable tools to determine volitional muscle strength, even in myopathy patients, and do not require clinical judgement for assigning a score. However, they still require the patient to be alert and cooperative.…”