09/16/2025
An excellent & easily accessible resource for equine muscle myopathies ๐ด๐ฆ
๐ ๐๐ก๐๐ง ๐๐ฎ๐ฌ๐๐ฅ๐๐ฌ ๐๐จ๐๐ค ๐๐ฉ: ๐๐ฑ๐ฉ๐ฅ๐จ๐ซ๐ข๐ง๐ ๐๐ฒ๐ข๐ง๐ ๐๐ฉ ๐ข๐ง ๐๐จ๐ซ๐ฌ๐๐ฌ ๐ด
When your horse ties up, stiffens after work, or seems off in their muscle performance, it can feel like a mystery with too many possible answers. Is it PSSM? RER? Or something else entirely? Myopathies in horses often look alike on the surface, but the underlying causes, management, and outcomes can be very different. Knowing how to recognize the clues can make all the difference in keeping your horse comfortable, safe, and performing at their best.
I was able to work with Dr. Katie Young on this post, inspired by her guest appearance on the Scoop and Scale podcast hosted by Dr. Clair Thunes and Jill Jackson, as well as Dr. Stephanie Valberg, an international leader in diagnosing and treating equine neuromuscular disorders. For this post, I wanted to dive into different forms of exertional myopathies often related to โtying upโ in horses.
Similar to colic, tying up is a broad term used to describe several different underlying conditions that present with similar clinical signs, all involving muscle damage. These disorders may stem from factors such as diet, overexertion, or genetic mutations. Some are classified as exertional myopathies (RER, PSSM, etc.), where episodes are triggered during or after exercise, while others fall under nonexertional myopathies (MYHM, IMM, etc.), which can develop independent of activity and are often linked to nutrition, toxins, or immune causes. To make things even more complex, certain myopathies show the classic markers of exertional rhabdomyolysis (tying up), such as high CK and AST on bloodwork, while others cause weakness, stiffness, or muscle loss without those obvious changes in serum CK activity. In this post, weโll focus specifically on exertional myopathies.
Each form of tying up has its own distinct trigger and management strategy, what works for one horse could make things worse for another. Thatโs why pinpointing the exact type is so important. While this post will walk you through key differences to help you spot the signs, only a veterinarian can give you a definitive answer through diagnostics including a thorough clinical history, blood tests, genetic testing, and potentially muscle biopsies. When it comes to muscle disorders, guessing isn't good enough, successfully managing your horseโs condition depends on getting it right.
๐๐ฉ๐จ๐ซ๐๐๐ข๐ ๐๐ฑ๐๐ซ๐ญ๐ข๐จ๐ง๐๐ฅ ๐๐ก๐๐๐๐จ๐ฆ๐ฒ๐จ๐ฅ๐ฒ๐ฌ๐ข๐ฌ (๐๐)
Often called azoturia, Monday morning disease, or simply the โclassicโ form of tying up, this sporadic type can affect any horse. Itโs most common in horses that arenโt properly conditioned for the level of work theyโre asked to perform. Triggers can include overexertion, electrolyte imbalances, heat, or an inconsistent training schedule, making it a frustrating but preventable challenge for many horse owners. Horses with this condition should have free access to or be supplemented with salt or electrolytes to meet requirements, and selenium and vitamin E status should be evaluated and the diet balanced for nutrients.
๐๐๐๐ฎ๐ซ๐ซ๐๐ง๐ญ ๐๐ฑ๐๐ซ๐ญ๐ข๐จ๐ง๐๐ฅ ๐๐ก๐๐๐๐จ๐ฆ๐ฒ๐จ๐ฅ๐ฒ๐ฌ๐ข๐ฌ (๐๐๐)
This form of tying up is most commonly seen in highly excitable horses, particularly young, fit Thoroughbreds, Standardbreds, and Arabians, especially those in race or endurance training with high energy demands. This condition is thought to stem from abnormal calcium regulation within muscle cells. Since calcium plays a key role in muscle contraction, this disruption can lead to painful muscle damage during or after exercise. Clinical signs include unwillingness to move, firm muscles, heavy sweating, labored breathing, muscle tremors, dark brown urine, and elevated serum creatine kinase (CK) and aspartate aminotransferase (also known as aspartate transaminase; AST) levels. If these signs are observed, a vet should be contacted immediately.
While a specific gene has not been directly linked to RER, research suggests itโs influenced by both genetics and environment. Risk factors include a nervous temperament, being female, limited turnout, inconsistent exercise, and high-stress environments (Valberg et al., 1999). While these horses arenโt as sensitive to high starch and sugar in the diet, a low to moderate nonstructural carbohydrate and moderate to high fat diet can help manage this condition, along with regular turnout and consistent work. Veterinarians may prescribe a drug called dantrolene to help manage the disease.
๐๐จ๐ฅ๐ฒ๐ฌ๐๐๐๐ก๐๐ซ๐ข๐๐ ๐๐ญ๐จ๐ซ๐๐ ๐ ๐๐ฒ๐จ๐ฉ๐๐ญ๐ก๐ฒ ๐ (๐๐๐๐๐)
This condition is linked to a mutation in the glycogen synthase 1 (๐๐ ๐1) gene, which results in excessive glycogen storage in the muscle. Unfortunately, horses with PSSM1 are not efficient at converting stored glycogen to energy, especially when consuming a high-sugar or starch diet. This may result in lethargy or episodes of tying up. Clinical signs are similar to RER but may also include lack of energy, gait abnormalities, or mild colic and are not related to stress and excitement.
This form is commonly found in draft and stock-type horses but has been identified in many breeds to date, excluding Thoroughbreds, Standardbreds, and Arabians. Thankfully, PSSM1 is well understood and can be identified with genetic testing. Management includes forage with
