HYPERKAELIMIC PERIODIC PARALYSIS

Hyperkalemic periodic paralysis  is a genetic disorder that occurs in horses and humans, where it is also known as Impressive syndrome.


HYPP

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 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. 

Causes

Hyperkalemic periodic paralysis is congenital. This means it is present at birth. In most cases, it is passed down through families (inherited) as an autosomal dominant disorder. In other words, only one parent needs to pass the gene related to this condition on to you in order for you to be affected.

Occasionally, the condition may be the result of a genetic problem that is not inherited.

It is believed that the disorder is related to problems with the way the body controls sodium and potassium levels in cells.

Risk factors include having other family members with periodic paralysis. It affects men more often than women.

Symptoms 

• Muscle trembling
• Prolapse of the third eyelid — this means that the third eyelid flickers across the eye or covers 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 — because the muscles of the voicebox are affected as well as other muscles
• Death — in a severe attack the diaphragm is 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 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 that collapse during an episode are clearly distressed as they repeatedly struggle to get to their feet. 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 as of 2003. 

Although the disease is primarily limited to the American Quarter Horse 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.


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 aschlorothiazide 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.                 

Prevention

A medicine called acetazolamide prevents attacks in many cases. Other medicines may also be prescribed. A low potassium, high carbohydrate diet may help prevent attacks. Avoiding fasting, strenuous activity, or cold temperatures also may help.

 References

Chinnery PF. Muscle diseases. In: Goldman L, Schafer AI, eds. Goldman’s Cecil Medicine. 24th ed. Philadelphia, Pa: Elsevier Saunders; 2011:chap 429.

Mount DB, Zandi-Nejad K. Disorders of potassium balance. In Taal MW, Chertow GM, Marsden PA, et al., eds.Brenner and Rector’s The Kidney. 9th ed. Philadelphia, Pa: Elsevier Saunders; 2011:chap 17.