Selection of quarter horses affected with hyperkalemic periodic paralysis by show judges.

Thirty offspring of a Quarter Horse sire, affected by hyperkalemic periodic paralysis (HPP), were examined electromyographically. On the basis of the detection of or lack of spontaneous activity with high frequency myotonic or pseudomyotonic discharges, the horses were diagnosed as being affected (14 horses) or unaffected (16 horses) with HPP. The show performance of these horses was evaluated for the first 3 to 9 years of their life by use of American Quarter Horse Association records. Horses affected with HPP performed significantly (P < 0.01) better in halter classes than did unaffected horses; mean halter points for the 2 groups were 11.9 and 0.4, respectively. The mean total performance points were not significantly different. None of the offspring had a successful racing record.