Hyperkalemic periodic paralysis

Hyperkalemic periodic paralysis (HYPP), also known as Impressive Syndrome, is an inherited autosomal dominant disorder which affects sodium channels in muscle cells and the ability to regulate potassium levels in the blood. It is most commonly associated with horses, but also occurs in humans, where it is also called Gamstorp episodic adynamy.

Disease in humans
Although much rarer, hyperkalemic periodic paralysis has also been observed in humans. In humans the disorder causes episodes of extreme muscle weakness, usually beginning in the second decade and depending on the type and severity of the HYPP will increase or stabilize until the fourth or fifth decade where attacks usually decline and can altogether stop. Factors that can trigger attacks include rest after exercise, potassium-rich foods, stress, fatigue, certain pollutants (eg: Cigarette smoke) and periods of fasting. Muscle strength improves between attacks, although many affected people may have increasing bouts of muscle weakness as time goes on (abortive attacks). Sometimes with HYPP those affected may experience degrees of muscle stiffness and spasms (myotonia) in the affected muscles. This can be caused by the same things that trigger the paralysis, dependant on the type of mytonia. (See also paramyotonia).

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.

The most common underlying cause is one of several possible point mutations in the gene SCN4A. 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.

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.