Muscle relaxation time as outcome measure for myotonia in patients with non-dystrophic myotonic syndromes (NDMs)

Non dystrophic myotonic syndromes (NDMs) form a heterogeneous group of rare
diseases caused by either chloride or sodium channelopathies that exclusively
affect skeletal muscles. The key symptom of NDMs is myotonia, i.e., a delayed
muscle relaxation after voluntary or evoked muscle contraction.

Recently the neurology department of the Radboud University Nijmegen Medical
Centre (RUNMC) defined the Dutch genotypes and phenotypical characteristics of
NDMs, by conducting a nation wide survey. They concluded that a good quantative
outcome measure for myotonia does not exist yet.

There is no evidenced based, effective therapy for NDMs patients. Therefore,
the only drug treatment option left for NDMs patients is prescription of off
label drugs, for example mexiletine, a sodium channel blocker. There are
serious plans for setting-up a mexiletine trial using the N of 1 methodology.
For determination of the effectiveness of mexiletine, a reliable and validated
outcome measure is needed which can quantify myotonia. In this study we want to
investigate if relaxation time (RT) measured by handgrip myometry and muscle-
elastography is a reliable outcome measurement for the quantification of
myotonia in NDM patients.

In this study we want to investigate if relaxation time (RT) measured by
handgrip myometry and muscle- elastography is a reliable outcome measurement
for the quantification of myotonia in NDM patients. Mean RT values en standard
deviations of NDM patients measured in this study, will be used in sample size
calculations for a future double blind, randomized, placebo controlled, cross
over intervention study with Mexiletine versus placebo. In this study the
therapeutic effect of Mexiletine will be evaluated by measurement of the RT of
the fingerflexors, as a new method for quantification of myotonia.

observational study without invasive interventions.

There are no direct advantages for the participating subjects. Disadvantages
for participating subjects: the study wil cost the subjects two times 45
minutes, 3 days prior to the measurments they will have to follow a potassium
low diet and they have to fast 2 hours prior to the measurement.
Risks are minimal, some extra muscle stiffness or muscle pain of the right hand
due to the measurement of muscle strength of the right hand can occur.