Dominant Myotonia congenita

Clinical Presentation

Patients typically present in the 1st decade of life with stiffness most commonly in the arms and hands. This usually will improve with repeated use (warm-up phenomenon). They are often less severely affected than patients with recessive myotonia congenita. They will usually have other affected family members as the condition is dominantly inherited.

On examination, patients will have hand grip myotonia which improves with repeat hand closure. They do not usually have any weakness or muscle hypertrophy. 

Genetic Diagnosis

Patients will usually have a single mutation in their chloride channel gene, CLCN1. Only specific mutations can cause dominant MC. They act by affecting the function of the chloride channel homodimer causing what is known as a dominant-negative effect therefore inhibiting both normal and abnormal channels when bound together.

To confirm whether a particular mutation can cause dominant myotonia congenita, we will usually express it in the lab in an expression system to see whether it can cause dominant-negative inhibition of the chloride channel homodimer. This helps us differentiate between dominant and recessive chloride channel mutations.

As the condition is dominant, there is a 50% chance of passing it on to any children. 

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