Hyperkalemic Periodic Paralysis (HYPP) is something that Quarter Horse breeders know all about.  This condition is also known as Impressive Syndrome, as the beautiful halter stallion was a sufferer and passed it onto many of his offspring.

This condition is an inherited disorder that affects the sodium channels in the muscle fibers.  These channels regulate the potassium levels in the blood stream.  The condition is characterized by muscle twitching, extreme muscle weakness and periods of sudden paralysis.  Many owners first discovered the condition in their own horses when he horse suddenly collapsed beneath them in the warm-up pen at horse shows.  The condition is a dominant disorder.  This means that heterozygote bred horses bred to normal horses will produce the condition 50% of the time.

Impressive was a halter stallion that produced extra large, bulky muscled horses.  His offspring did well in the show pen because of this and the judges favored it.  So, of course, if the judges are favoring a certain type of horse, breeders are going to breed to the stallion that produces it.  Impressive was a star and heavily bred to.  As of 2003, there were approximately 55,000 living Impressive offspring registered with the American Quarter Horse Association.  These bulky horses also tend to be homozygous (H/H) for the gene or heterozygous (N/H).

In 1994, the University of Pittsburgh was provided with a grant from several equine organizations so that they could isolate this trouble-causing gene.  The university was then able to develop a blood test that breeders could use to determine whether or not their foals were:

• Homozygous (H/H): These horses will pass the disease on and as of 2007, are not registerable with the AQHA.
• Heterozygous (N/H): These horses have the gene and will pass the disease on 50% of the time.
• Or, the horse does not have the gene and will not pass it on.  These horses are denoted with a (N/N).