Equine Muscle Disorders

What is tying-up?

Exertional Rhabdomyolysis (ER) is the technical term for tying-up. ER is the cramping and spasming of muscle groups including the gluteal (butt) and topline muscles often associated with exercise. Sometimes the neck, shoulder and fore leg muscles are involved as well. The horses are often sweaty, shaking or walking very stiffly, and panting or breathing very shallow. The damage to the muscles that follows an episode of ER can lead to muscle necrosis and kidney damage.

A diagnosis if ER can be made based on clinical signs and testing the blood for elevations in muscle enzymes, CK and AST. Muscle biopsies and genetic tests can help differentiate what has caused the horse to tie-up as there are different conditions associated with similar clinical signs.

A single episode of ER does not necessarily mean a horse has a tie-up condition like those listed below. Sporadic ER can occur when a horse is exercised beyond its fitness level or when it is exercised in extreme weather conditions for instance.

RER (Recurrent Exertional Rhabdomyolysis)

RER is a form of tying-up caused by an abnormality in Calcium regulation in muscle cells. RER is most commonly seen in young, female, excitable Thoroughbreds.  There is currently no known specific genetic cause of RER but the disorder is being researched. RER is diagnosed based on clinical signs and a muscle biopsy. There is no cure or medical treatment for RER but the condition can be managed with changes in diet, management and exercise.

PSSM (PolySaccharide Storage Myopathy)

Horses with PSSM tie-up due to an abnormality in how their muscles handle and store sugars. Type 1 PSSM is caused by an inherited defect in one particular gene (which can be tested for) while the cause for Type 2 PSSM is still unknown.  Type 1 PSSM is a common cause for tie-up signs in Quarter Horses, Paints, Appaloosas, and warmbloods. PSSM can often be managed with diet and exercise changes but there is no cure or medication used to treat it.

HYPP (Hyperkalemic Periodic Paralysis)

HYPP is an inherited muscle disease causing an abnormality in how muscle cells manage electrolytes.  This mismanagement causes sporadic episodes of muscle contraction of paralysis (not always associated with exercise). The genetic defect has been identified primarily in descendents of the stallion, Impressive. There is a test available for HYPP.  HYPP is managed with dietary changes and there are some medications available to help the horse’s body regulate electrolytes.


Shivers is actually considered more of neurological issue but the cause is unknown. Shivers presents as involuntary spasms of the hindlimbs  and tail. The horse may have difficulty backing or holding its hind legs up for farrier work. It is most common in draft horse breeds and warmbloods.  There is no known effective therapy for shivers.

Please call us if you are concerned that your horse may be showing signs of tying-up.

More information: http://www.cvm.umn.edu/umec/lab/home.html

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