Recessive Myotonia congenita

Clinical Presentation

Patients typically present in the first decade of life. They will often complain of stiffness especially in their legs. They will often notice the stiffness is worst following a period of rest and may fall over if they move suddenly.

Patients will usually have myotonia of hand grip on examination and this will usually improve with repeated activity (warm-up phenomenon). They may also have significant muscle hypertrophy, especially in the calves.

Genetic Diagnosis

Patients with recessive MC have 2 mutations in their chloride channel gene, CLCN1 which encodes the channel CLC-1 which is expressed only in muscle. This protein is important in maintaining the resting potential of the muscle membrane. When it malfunctions the muscle membrane remains in a depolarised state after a contraction triggering myotonia and difficulty relaxing the muscle.

In recessive MC, mutations cause a loss of function of the chloride channel. A patient needs 2 loss of function mutations to cause clinical and electrical myotonia. This means that patients will not pass on the condition to their children unless their partner also carries a loss of function mutation in their CLCN1 gene.

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