Myotonic disorders are a group of rare, heterogeneous, inherited, neuromuscular disorders characterized by a shared symptom called myotonia. Myotonia can be described as an inability to relax a contraction of skeletal muscle which originates from a voluntary muscular contraction such as shaking someone’s hand and blinking, or everyday activities such as walking across a street and climbing stairs.
Non-dystrophic myotonias (NDM) are a sub-set of rare (prevalence of 1:100,000 1 ), inherited, myotonic disorders which are caused by mutations within ion channels in the sarcolemma membrane of skeletal muscles. Non-dystrophic myotonias exhibit both sodium and chloride channelopathies, resulting in altered membrane excitability. For patients with NDM, myotonia is the most prominent symptom and demonstrates different phenotypes in subgroups of NDM disorders, and can affect different parts of the body, such as legs, arms or facial muscles, more severely.
Myotonia symptoms in NDM patients has an onset in childhood, which is often perceived by patients to persist, and even increase in severity, over their lifetime, impacting daily life 2,3. Myotonia is described by patients in a variety of ways, and is often referred to as stiffness, cramps, pain, difficulty releasing a fist, or difficulty swallowing or eating. This can contribute to substantial delays in diagnosis and treatment, leading to decreased patient quality-of-life and often significant disability.
1. Emery AE, et al. Neuromuscular Disorders 1991
2. Trivedi JR, et al. Exp Neurol 2014
3. Trip J, et al. J Neurol 2009