Hyperkalemic periodic paralysis

Hyperkalemic periodic paralysis (HYPP, HyperKPP) is a genetic disorder which occurs in both humans and horses, where it is also known as Impressive Syndrome. It 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 may be called Gamstorp episodic adynamy. It is characterized by muscle hyperexcitability or weakness which, exacerbated by potassium or cold, can lead to uncontrolled shaking followed by paralysis. Onset in humans usually occurs in the twenties.

The mutation which causes this disorder is dominant on SCN4A with linkage to the sodium channel expressed in muscle. The mutation causes single amino acid changes in parts of the channel which are important for inactivation. In the presence of high potassium levels, including those induced by diet, sodium channels fail to activate properly.

Equine hyperkalaemic periodic paralysis occurs in 1 in 50 quarter horses and can be traced to a single ancestor, a stallion named Impressive.

Symptoms and presentation
This inherited disease is characterized by violent muscle twitching and substantial muscle weakness or paralysis among affected horses. HYPP is a dominant genetic disorder; therefore heterozygotes bred to genotypically normal horses have a statistic probability of producing clinically affected offspring 50% of the time.

Horses affected with HYPP can be treated with some possibility of reducing clinical signs, but the degree that medical treatment helps varies from horse to horse. There is no cure. Horses with HYPP often lose muscle control during an attack.

Some horses are more affected by the disease than others and some attacks will be more severe than others, even in the same horse. Symptoms of an HYPP attack may include:


 * Muscle trembling
 * Prolapse of the third eyelid - this means that you may note the third eyelid flickering across the eye, or covering more of the eye than normal
 * Generalized weakness
 * Weakness in the hind end - the horse may look as though it is 'dog-sitting'
 * Complete collapse
 * Abnormal whinny - this is because the muscles of the voicebox are affected as well as other muscles
 * Death - In a severe attack the diaphragm is also paralyzed and the horse can suffocate.

HYPP attacks occur randomly and can strike a horse standing calmly in a stable just as easily as during exercise. Following an HYPP attack, the horse appears normal and is not in any pain which helps to distinguish it from Equine Exertional Rhabdomyolysis (ER), commonly known as "Azoturia," "Monday Morning Sickness" or "tying up." Horses that are tying up usually suffer attacks in connection with exercise and may take anywhere from 12 hours to several days to recover. Muscle tissue is also damaged in an attack of ER, and the horse will be in pain during and following an attack. A blood test will reveal elevations in certain muscle enzymes after an episode of ER and so the two diseases, while superficially similar, are easily distinguished from one another in the laboratory.

Unlike with seizures, horses with HYPP are fully conscious and lucid during an attack. Horses may suffocate during an HYPP attack due to paralysis of the respiratory system. Horses which collapse during an episode are also clearly distressed as they repeatedly struggle to get to their feet again. If this occurs while the horse is being ridden or otherwise handled, the human handler or rider may be at risk of being injured by the movement of the horse.

Inheritance and prevalence
The disease is linked to the bloodline of the famous American Quarter Horse stallion Impressive, who has over 55,000 living descendants. Although the disease is primarily limited to the American Quarter Horse breed and closely related breeds such as American Paint Horses and Appaloosas at this time, cross-breeding has begun to extend it to crossbreds recognized by other breed registries as well as grade horses. Until the AQHA restricted the registration of animals with the condition, the spread of the disease was perpetuated by the favorable placings given to affected horses in halter competition at horse shows, because a secondary characteristic associated with N/H and H/H horses is heavy, bulky muscling that is favored by stock horse judges, a trend that began with Impressive and predates the modern understanding of the disease. Some stock horse breeds with Quarter Horse bloodlines have yet to restrict registration in order to limit the perpetuation of HYPP.

Regulation
Recently, horse organizations have begun instituting rules to attempt to eliminate this widespread disease. The American Quarter Horse Association (AQHA) now mandates testing for the "Impressive" mutation and will no longer register homozygous (H/H) foals as of 2007, with discussion of heterozygous (N/H) foals pending. The Appaloosa Horse Club (ApHC) will no longer accept homozygous foals as of 2008. It is believed that both primary palomino registries will exclude any foal carrying the "Impressive" mutation as of 2007. The main organization affected by HYPP that has not yet taken action is the American Paint Horse Association (APHA), although many other smaller organizations are also affected.

Disease in humans
Although much less publicized, Hyperkalemic Periodic Paralysis has also been observed in humans. In humans the disorder causes episodes of extreme muscle weakness, with attacks often beginning in infanthood. Depending on the type and severity of the HyperKPP, it can increase or stabilize until the fourth or fifth decade where attacks may cease, decline, or, depending on the type, continue on into old age. Factors that can trigger attacks include rest after exercise, potassium-rich foods, stress, fatigue, weather changes, certain pollutants (e.g.: Cigarette smoke) and periods of fasting. Muscle strength often improves between attacks, although many affected people may have increasing bouts of muscle weakness as the disorder progresses (abortive attacks). Sometimes with HyperKPP 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, dependent on the type of myotonia. (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 (normakalemia). Ingesting potassium can trigger attacks in affected individuals, even if blood potassium levels do not rise in response.

Genetics
In 1994, researchers at the University of Pittsburgh, with a grant from various horse organizations, isolated the genetic mutation responsible for the problem and developed a blood test for it. Using this test, horses may be identified as:
 * H/H, meaning they have the mutation and it is homozygous. These horses always pass on the disease.
 * N/H, meaning they have the mutation and it is heterozygous. These horses are affected to a lesser degree, and pass on the disease 50% of the time.
 * N/N, meaning they do not have the mutation and cannot pass it on, even if they are descendants of Impressive.

In humans, the most common underlying genetic cause is one of several possible point mutations in the gene SCN4A. This gene codes for a voltage-gated sodium channel Nav1.4 found at the neuromuscular junction. 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.

Action potentials from the central nervous system cause end-plate potentials at the NMJ which causes sodium ions to enter via Nav1.4 and depolarise the muscle cells. This depolarisation triggers the entry of calcium from the sarcoplasmic reticulum to cause contraction (tensing) of the muscle. To prevent the muscle from being perpetually contracted, the channel contains a fast inactivation gate which plugs the sodium pore very quickly after it opens. This prevents further entry of sodium. In time, potassium ions will leave the muscle cells, repolarising the cells and causing the pumping of calcium away from the contractile apparatus to relax the muscle.

Mutations altering the usual structure and function of this sodium channel therefore disrupt regulation of muscle contraction, leading to episodes of severe muscle weakness or paralysis. Mutations have been identified in residues between transmembrane domains III and IV which make up the fast inactivation gate of Nav1.4. Mutations have also been found on the cytoplasmic loops between the S4 and S5 helices of domains II, III and IV, which are the binding sites of the inactivation gate.

In patients with mutations in SCN4A, therefore, the channel is unable to inactivate, sodium conductance is sustained and the muscle remains permanently tense. Since the motor end plate is depolarised, further signals to contract have no effect (paralysis). The condition is hyperkalemic because a high extracellular potassium ion concentration will make it even more unfavourable for potassium to leave the cell in order to repolarise it to the resting potential, and this further prolongs the sodium conductance and keeps the muscle contracted. Hence, the severity would be reduced if extracellular (serum) potassium ion concentrations are kept low.

In the case of the horse Impressive, the muscles were always contracting which was equivalent to a constant work-out. Thus the development of an impressive musculature.

In contrast to HyperKPP, Hypokalemic Periodic Paralysis (noted in humans) refers to loss-of-function mutations in channels that prevent muscle depolarisation and therefore are aggravated by low potassium ion concentrations.

Treatment

 * Glucose or other carbohydrates can be given during an attack and may reduce the severity.
 * Intravenous calcium decreases activity of sodium channels. It may stop sudden attacks.
 * Diuretics such as furosemide may be needed to stop sudden attacks. acetazolamide and thiazide diuretics such as chlorothiazide are also effective.
 * Intravenous glucose and insulin stimulates potassium uptake into the cell by the Na-K ATPase and may reduce weakness without a loss of total body potassium.
 * A high-carbohydrate diet may be recommended.
 * Avoidance of other known attack triggers.