CLINICAL SYNDROME AND DIAGNOSIS OF HYPERKALEMIC PERIODIC PARALYSIS INQUARTER HORSES

Of the 16 horses studied, 14 belonged to a family previously shown to be susceptible to hyperkalaemic periodic paralysis (HPP), and 8 were s hown to have HPP. Diagnosis of HPP by electromyographic detection of m yotonic discharges or by oral administration of KCl to induce clinical signs and hyperkalaemia had similar reliability and gave the same res ult in 80% of cases. KCl had to be administered at doses up to 0.2 g/k g bodyweight to produce signs in some horses. KCl challenge testing wa s more time consuming than EMG and resulted in one fatality. Overall, electromyography and potassium challenge testing together gave the mos t accurate diagnosis. Horses were more likely to manifest signs of HPP if they were immature. There was no sex predisposition. The most freq uent sign of HPP was muscle fasciculation. Other signs were sweating, muscle spasm, and weakness. Respiratory rate increased greatly during some attacks. Attacks of muscle fasciculation without hyperkalaemia we re observed in 2 HPP-affected horses. Affected horses had a greater fr equency of spontaneous clinical abnormalities, due mainly to trailerin g problems, traumatic abrasions and episodes of continuous muscle fasc iculation. Hyperkalaemia was confirmed in only one of these instances.