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HYPP - Still With Us

A recent journal article not only spoke about hyperkalemic periodic paralysis (HYPP) as a finding in adult Quarter horses, but also recently implicated it as a differential in foal deaths as well. Why?

Hyperkalemic periodic paralysis (HYPP) is a muscle disease which has been reported in certain lines of registered Quarter horses, Appaloosas, and Paints, Affected horses often display well developed muscles, and are usually highly rated when shown in halter classes in the show ring. This condition has no sex predilection, although it is mostly seen in males, and no breed restrictions. The disease is very similar to one known to exist in humans called adynamicaepisodicahereditaria or hyperkalemic periodic paralysis (HPP). In horses, lineage of affected individuals has been traced back to a single sire.Impressive, known for his greatness in halter showing. Over 50,000 registered Quarter horses are related to known carriers of this disease. This disease has been shown to be genetically transmitted as an autosomal dominant trait. This also means that the heterozygotes have the potential to manifest HYPP as well. However, those offspring that are homozygotes tend to be more severely affected.

HYPP is commonly characterized by signs such as muscle spasm, weakness, recumbency, and high serum potassium concentrations. Research shows that these signs are a result of a permanent defect in ion transport across the skeletal muscle cell membrane, resulting in increased sodium permeability, reduced polarization, and thus changes in muscle excitability. Its association with certain alleles of the gene coding for skeletal muscle sodium channels has led researchers to believe that a point mutation results in a single amino acid in the alpha subunit of the sodium channel protein. Normal channels close once the muscle membrane has depolarized. Defective channels remain open and allow excessive influx of sodium resulting in greater efflux of potassium, and continuous membrane depolarization. In severe attacks, weakness can be seen due to the abolition of membrane action potentials. The hyperkalemia that is seen is secondary to the influx of sodium, and the release of muscle potassium as potassium channels open to repolarize the membrane.

Factors that may trigger clinical attacks can be attributed to a variety of conditions such as chilling, transportation, recovery from anesthesia, or any stressful event such as showing. HYPP has been misinterpreted as a wide variety of other diseases, such as colic, "tying-up" syndrome, and recumbency for one reason or another. The earliest clinical signs of muscle fasciculations can progress to muscle group spasms, retraction of the lips and eyelids (risussardonicus), stiff jaw, prolapse of the tired eyelid, increased respiratory rate, inspiratorystridor, and stiff gait in more severe cases. Whether standing or recumbent, most horses will sweat profusely during an episode. Horses have been known to die during an attack from respiratory paralysis, or cardiac failure secondary to hyperkalemia. Horses usually appear normal after an attack, whereas horses with "tying-up" syndrome walk with a stiff gait and have painful, firm muscles of the hindlimbs, rump or back. HYPP is also not usually associated with exercise, but seen at rest, during feeding or following stressful events.

Testing for HYPP can be done in several ways. In the field, the test of choice is the gene probe for HYPP-type sodium channel DNA. This is the most sensitive test for detecting the mutation in susceptible   horses,   and   requires   whole anticoagulated(EDTA) blood. Diagnosis can also be made based on clinical signs of HYPP together with an elevated serum potassium concentration. To avoid problems with potential potassium leakage due to leakage from red blood cells, one should separate plasma soon after the blood has clotted. Testing with electromyography, which measures the electrical activity present in selected muscles, can also be done to further support a diagnosis. When necropsies are performed, tissues can be used for both gene probe testing, as well as histopathological interpretation. Histologically, one can see vacuolization in type 2B muscle fibers with mild degenerative changes. Electron microscopy shows dilatations of the sarcoplasmicreticulum of myofibers. Diagnosis based on genetic testing of whole blood is particularly useful in prepurchaseexams, and in deciding whether or not to use a particular horse for breeding.

Recently, four foals presented to clinics were reported to have episodes of upper airway stridor while being restrained for physical examination. Endoscopic examination revealed laryngeal spasms and partial pharyngeal paresis in each one. Electromyography  showed  abnormal  muscle activity in the foals as well as their dams. Two of the foals had to have emergency tracheostomiesperformed to restore an airway after it collapsed due to the over-excitement. On the basis of clinical signs, electromyographic interpretation, response to treatment, and increased serum potassium levels, a diagnosis ofHYPP was made in each case. For this reason, HYPP should be considered as a differential diagnosis for upper airway stridor, and/or sudden death possibly due to asphyxiation attributable to laryngeal collapse.

This disease has been recognized for several years now, but its importance is changing from one of scarcity, to one of diversity, from one of novelty, to one of an important genetic disease. Without prompt action, the breeding of affected individuals will greatly increase the expression of this serious disease. Veterinarians can help prevent this from happening by testing suspect horses and educating the public about this condition.

-Kevin R.Cawood, Class of 1996

-Edited by JaniceSojka,DVM




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