What is the definition of Potassium-Aggravated Myotonia?

Potassium-aggravated myotonia is a disorder that affects muscles used for movement (skeletal muscles). Beginning in childhood or adolescence, people with this condition experience episodes of sustained muscle tensing (myotonia) that prevent muscles from relaxing. Myotonia causes muscle stiffness that worsens after exercise. In this disorder, episodes of myotonia may also be triggered (aggravated) by eating foods that are high in the mineral potassium, such as bananas and potatoes. During these episodes, stiffness occurs in skeletal muscles throughout the body.

Potassium-aggravated myotonia ranges in severity from mild episodes of muscle stiffness (myotonia fluctuans) to severe, disabling disease with frequent attacks (myotonia permanen). Unlike some other forms of myotonia, potassium-aggravated myotonia is not associated with episodes of muscle weakness.

What are the causes for Potassium-Aggravated Myotonia?

Potassium-aggravated myotonia is caused by variants (also known as mutations) in the SCN4A gene. The SCN4A gene provides instructions for making a protein that is critical for the normal function of skeletal muscle cells. For the body to move, skeletal muscles must tense (contract) and relax in a coordinated way. Muscle contractions are triggered by the flow of positively charged atoms (ions), including sodium, into skeletal muscle cells. The SCN4A protein forms channels that control the flow of sodium ions into these cells.

Variants in the SCN4A gene alter the usual structure and function of sodium channels. The altered channels cannot properly regulate ion flow, increasing the movement of sodium ions into skeletal muscle cells. When excess potassium is present in the body, which occurs after eating potassium-rich foods, even more sodium ions flow into skeletal muscle cells in order to maintain a proper balance of calcium and potassium. The influx of extra sodium ions triggers prolonged muscle contractions, which are characteristic of myotonia.

How prevalent is Potassium-Aggravated Myotonia?

This condition appears to be rare; it has been reported in a small number of individuals and families worldwide.

Is Potassium-Aggravated Myotonia an inherited disorder?

Potassium-aggravated myotonia is inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder. In some cases, an affected person inherits a variant in the SCN4A gene from one affected parent. Other cases result from new variants in the gene.  These cases occur in people with no history of the disorder in their family.

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