Hyperkalemic periodic paralysis

Hyperkalemic periodic paralysis is a condition that causes episodes of extreme muscle weakness, usually beginning in infancy or early childhood. Most often, these episodes involve a temporary inability to move muscles in the arms and legs. Episodes tend to increase in frequency until about age 25, after which they may occur less frequently. Factors that can trigger attacks include rest after exercise, potassium-rich foods, stress, fatigue, and periods without food (fasting). Muscle strength improves between attacks, although many affected people continue to experience mild stiffness (myotonia), particularly in muscles of the face and hands.

Some people with hyperkalemic periodic paralysis have increased levels of potassium in their blood (hyperkalemia) during attacks. In other cases, attacks are associated with normal blood potassium levels (normokalemia). Ingesting potassium can trigger attacks in affected individuals, even if blood potassium levels do not go up.

Hyperkalemic periodic paralysis affects an estimated 1 in 200,000 people.

Mutations in the SCN4A gene cause hyperkalemic periodic paralysis.

The SCN4A gene provides instructions for making a protein that plays an essential role in muscles used for movement (skeletal muscles). For the body to move normally, these muscles must tense (contract) and relax in a coordinated way. Muscle contractions are triggered by the flow of certain positively charged atoms (ions), including sodium, into muscle cells. The SCN4A protein forms channels that control the flow of sodium ions into these cells.

Mutations in the SCN4A gene alter the usual structure and function of sodium channels. The altered channels cannot properly regulate the flow of sodium ions into muscle cells, which reduces the ability of skeletal muscles to contract. Because muscle contraction is needed for movement, a disruption in normal ion transport leads to episodes of muscle weakness or paralysis.

Read more about the SCN4A gene.

This condition 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 most cases, an affected person has one parent with the condition.