The aim of this project is (1) to investigate whether or not structural muscle abnormalities could be a consequence of the disorder and (2) to provide further clinical description of this rare phenotype. To do so, the investigators will (1) use Dixon MRI to quantify fatty infiltration in muscle tissue and compare it to muscle strength measurements from isometric dynamometry in order to access contractility and (2) describe the myotonic phenotype with simple squeeze test and questionnaires.

Non-dystrophic myotonias are rare genetic diseases in which the membrane excitability is altered by mutations in genes encoding muscle ion channels.

Patients suffer from myotonic stiffness, pain, fatigue and sometimes paralysis. Non-dystrophic myotonia is distinct from myotonic dystrophies with the absence of muscle degeneration.

Paramyotonica congenita is characterzied by paradoxial myotonia, which, in contrast to the more common myotonia congenita, is myotonic stiffness that worsens with activity. Typically, the first few contractions seem normal, whereas repetition leads to severe stiffness. Our hypophysis is that these patient might also suffer from muscle degeneration.

Inclusion Criteria:

• 18 years of age
• Diagnosed with non-dystrophic myotonia

Exclusion Criteria:

• MRI contraindications

Paramyotonia congenita (prevalence 50-100)