Hyperkalemic periodic paralysis
Muscle twitching, sudden weakness and an apparently normal horse afterwards can be part of HYPP. This inherited disorder changes the electrical signals involved in muscle contraction. Some episodes are subtle, while severe attacks can threaten breathing and life. DNA testing identifies a specific variant; it cannot tell an owner when the next episode will happen or how serious it will be.
Dominant inheritance
HYPP disrupts the electrical signals that control muscles
Muscle cells move charged particles, including sodium and potassium, to contract and relax. The SCN4A gene provides instructions for a sodium-channel protein. This protein forms a passage that controls sodium entering through the membrane, the outer covering of the cell. The HYPP variant changes this channel, making muscle electrical activity unstable. Involuntary contractions may be followed by weakness or paralysis as normal signalling becomes disrupted.
Hyperkalaemic refers to increased potassium in the blood, which can occur during an episode. It does not mean that SCN4A makes a potassium channel. Changes in potassium and other circumstances can help trigger episodes in susceptible horses. Molecular research published in 1992 identified the causative sodium-channel variant and established a basis for targeted DNA testing.
HYPP muscle attacks can be far apart and vary in severity
Signs can include twitching or trembling muscles, weakness, paralysis and collapse. Noisy breathing may develop when airway muscles are involved. Severe episodes can cause respiratory failure or sudden death. The duration, severity and combination of signs vary, including between attacks in the same horse. A mild previous episode does not guarantee a mild future one.
Some horses appear normal between attacks and have only a few recognised episodes in their lives. Others have repeated problems. Stress, feeding changes, fasting, illness, anaesthesia and changes in exercise can contribute. Signs are often noticed around the start of intensive training but may first become apparent later. An attack-free age does not establish that the variant is absent.
HYPP has been documented in Quarter Horses, Paint Horses and Appaloosas
HYPP is documented in Quarter Horses and related breeds, including Paint Horses and Appaloosas. The known variant is historically associated with descendants of the stallion Impressive. This illustrates how extensive use of a popular sire can spread a variant through related populations, rather than a disease arising because a horse belongs to a particular breed.
Not every descendant of a one-copy horse inherits the variant. Heavy muscling also does not prove HYPP, and a different appearance does not rule it out. Family history, conformation and genetic status are separate pieces of information. A laboratory result correctly linked to the horse is needed to establish its status for the tested variant.
Weakness or twitching alone cannot establish an HYPP diagnosis
Weakness or twitching may be confused with tying-up, an episode in which muscle fibres are damaged. Colic, choke, where the oesophagus becomes blocked, and diseases of the nervous system can also cause similar signs. During an HYPP attack the horse generally remains conscious, and it may look normal afterwards. These observations help a veterinarian assess possibilities; they are not reliable instructions for an owner to diagnose a collapse without an examination.
Assessment combines the episode history, examination, appropriate laboratory findings and genetic testing. A blood potassium measurement depends on timing: sampling between attacks is not a substitute for DNA analysis. Breathing difficulty, collapse or marked weakness requires urgent veterinary attention whatever the suspected cause. Establishing that the horse has the variant does not explain every future illness automatically.
One copy of the HYPP variant can cause muscle attacks
The test targets the SCN4A variant responsible for the known form of equine HYPP and distinguishes zero, one and two copies. One copy can produce effects: inheritance is autosomal dominant, and both males and females can be affected. Horses with two copies usually have more frequent or more severe signs than one-copy horses.
Copy number cannot precisely predict when an individual will have episodes. A one-copy horse that has never shown an obvious problem is not an unaffected recessive carrier. A negative result means the targeted variant was not detected; it does not exclude every other cause of twitching, weakness or collapse. Clinical findings still need their own explanation.
Veterinary care for HYPP aims to reduce attacks and prepare for emergencies
Long-term management aims to reduce triggers and establish a plan for episodes. A veterinarian can review feeding, consistency of routine, activity and the circumstances preceding previous attacks. The horse’s nutritional needs and actual feed composition matter more than a simple universal list of allowed and forbidden ingredients. A plan should be fitted to that animal.
Management does not remove the variant from the DNA. Some horses have few signs with appropriate care, while others remain at risk of serious attacks. Possible collapse also affects handler and rider safety. The horse’s HYPP status should be shared with its veterinarian, especially before anaesthesia, and with the people responsible for its daily care.
A mare or stallion can pass on the HYPP variant without having visible attacks
A parent with one copy of the variant responsible for HYPP has a 50% chance of passing that copy to a foal at each conception. Breeding it to a horse without the variant can therefore produce one-copy foals that are themselves susceptible to HYPP. Whether the parent has experienced obvious attacks does not change that DNA transmission probability.
If the mare and stallion each have one copy of the HYPP variant, every pregnancy has a 25% chance of a foal without this variant, a 50% chance of a foal with one copy and a 25% chance of a foal with two copies. A parent with two copies of the HYPP variant passes one copy to every foal. These percentages describe genotypes, not a guaranteed number or severity of attacks. Breeding decisions need both parents’ results and consideration of future foal welfare.
The three possible DNA test results for HYPP
N/H and H/H are common HYPP test notations. H denotes a version of SCN4A with the HYPP variant; N denotes a version without it. The gene itself is present in both cases.
Questions about the condition: Hyperkalemic periodic paralysis
Can the first recognised HYPP attack occur in an adult horse?
Yes. A horse may have few signs or its first recognised episode as an adult. Age and a normal appearance between episodes cannot replace a genetic test result.
Is HYPP the same as tying-up, an episode of muscle fibre damage?
No. HYPP disturbs muscle excitability; rhabdomyolysis involves damage to muscle fibres. Some outward signs can overlap, which is why an examination is needed to determine the cause.
Scientific sources about the condition: Hyperkalemic periodic paralysis
This article brings together the research and the university and laboratory resources listed below. Numbers after each section link to the references used. Sources consulted in November 2026.
- Hyperkalemic Periodic Paralysis (HYPP)
UC Davis Veterinary Genetics Laboratory · Test documentation
- Hyperkalemic periodic paralysis: clinical signs and management
UC Davis Center for Equine Health · University resource
- Rudolph et al. (1992) — A sodium-channel mutation in Quarter Horses
Nature Genetics · Research
- HYPP: clinical signs, genetics and inheritance
Laboratoire de génétique vétérinaire — Labgenvet · Test documentation