Rare disorders with a prevalence of 1:100,000.
Caused by mutations in skeletal muscle chloride and sodium channels with myotonia without muscle wasting.
Myotonia associated with stiffness, pain, fatigue and weakness.
No known accepted and safe treatment.
Mexileitine class 1b anti arrhythmic drug used and compared to placebo resulted in improved stiffness over 4weeks (Statland JM et al).