Paramyotonia congenita

Clinical Presentation

Patients with Paramyotonia congenita (PMC)  typically present in early childhood. Parents will often mention that their eyes may get stuck shut whilst crying or sneezing. It commonly causes stiffness of the eyes and face more than other muscles and is significantly worse in the cold and therefore patients are often more severely affected in the winter. The myotonia worsens with exertion hence the term 'para' myotonia. Patients may also have episodes of weakness with the myotonia. Occasionally patients may find the weakness and myotonia worsens with elevated potassium.

Sodium channel myotonia is another term used for patients with myotonia and mutations in the sodium channel gene. This used to also be termed "potassium-aggravated myotonia" or "pure myotonia" as patients would have myotonia without weakness but were often sensitive to potassium. Because of the significant clinical overlap with all of these syndromes we now name them all PMC.

Genetic Diagnosis

PMC is caused by mutations in the SCN4A gene which encodes the sodium channel expressed in muscle, NaV1.4. Mutations tend to cause increased flow of sodium ions into the muscle causing sustained depolarisation, increasing the firing of action potentials and triggering myotonia. If the depolarisation is prolonged, it can trigger inactivation of the sodium channels making the muscle membrane inexcitable and leading to weakness.

PMC is inherited dominantly which means there is a 50% chance of the patient passing on the condition to each of their children. 

Common mutations in the sodium channel NaV1.4

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