Page 478 - The Veterinary Care of the Horse

HYPERKALAEMIC PERIODIC PARESIS (HYPP)

VetBooks.ir This is a muscle disorder affecting Quarter Horses and their crosses, American Paint horses

and Appaloosas. It is an inherited trait and the gene responsible has been identified in
descendents of the Quarter Horse sire ‘Impressive.’ In most horses, intermittent clinical signs

begin by 2 to 3 years of age with no apparent abnormalities between episodes.

Causes

• The genetic defect results in a functional abnormality of skeletal muscle cell membranes

leading to periods with high serum potassium levels and muscle weakness.

• Diets high in potassium (such as alfalfa and molasses) may induce signs along with
sudden dietary changes.

• Episodes may be triggered by stress including transport, sedation, fasting and
anaesthesia.

• Exposure to cold temperatures can precipitate an attack.

Clinical signs

• Affected horses experience episodes of sweating, muscle tremors and weakness, which
may produce collapse.

• Facial muscle spasm, difficulty swallowing and prolapse of the third eyelid may occur.

• The episodes typically persist for 15–60 minutes.

• Respiratory distress may occur in some cases.
• Heart abnormalities may be seen on ECG.

Diagnosis

A definitive diagnosis can be made by a DNA test from tail hairs or a blood sample.

Treatment

• Low-grade exercise may stop the muscle trembling if an episode is detected early.

• Feeding grain to reduce potassium levels can help.

• Severe cases may require medical treatment including intravenous fluids.
• However, most cases are manageable, particularly when the owners are aware of the