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Clarity Equine Nutrition

2 weeks ago

💡 When Stiffness Isn’t Simple: Making Sense of Equine Muscle Disorders

When your horse shows muscle stiffness, reluctance to move, or poor performance, it can be overwhelming trying to figure out the cause. Muscle disorders like PSSM, RER, and MFM can present with similar outward signs, but each requires a different management approach.

This flow chart, created in collaboration by Dr. Michelle DeBoer from Equine, PhD and Dr. Katie Young at Kentucky Equine Research, both past guests on the Scoop and Scale podcast, is an excellent tool to help horse owners sort through the complexities of equine muscle disorders. It’s not a substitute for veterinary diagnostics, but it’s a very helpful guide to better understand where your horse might fit and what next steps to consider.

🎧 Want to dive deeper? Check out our podcast episodes covering these conditions:

👉 Ep. 44: Understanding PSSM1 & Nutritional Management
scoopandscale.com/ep-44-understanding-pssm1-nutritional-management-for-your-horse/

👉 Ep. 47: Decoding Muscle Myopathies, Part 1 (RER, MFM, PSSM2-ER)
scoopandscale.com/ep-47-decoding-muscle-myopathies-part-1-understanding-rer-mfm-mfm-er-mfm-wb-pss...

👉 Ep. 48: Decoding Muscle Myopathies, Part 2 (Management Strategies)
scoopandscale.com/ep-48-decoding-muscle-myopathies-part-2-managing-rer-mfm-mfm-er-mfm-wb-pssm2-er/

📌 Save this flow chart for quick reference, and always consult your veterinarian for an accurate diagnosis and individualized plan.
#equinenutrition #pssm #pssm2 #tyingup #musclemyopathy #equinepodcast #HorseCare #horsefeed #horsemanagement
www.facebook.com/share/p/1BNwNyMM5A/🔒 𝐖𝐡𝐞𝐧 𝐌𝐮𝐬𝐜𝐥𝐞𝐬 𝐋𝐨𝐜𝐤 𝐔𝐩: 𝐄𝐱𝐩𝐥𝐨𝐫𝐢𝐧𝐠 𝐓𝐲𝐢𝐧𝐠 𝐔𝐩 𝐢𝐧 𝐇𝐨𝐫𝐬𝐞𝐬 🐴

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 “tying up” 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, we’ll 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. That’s 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 horse’s condition depends on getting it right.

𝐒𝐩𝐨𝐫𝐚𝐝𝐢𝐜 𝐄𝐱𝐞𝐫𝐭𝐢𝐨𝐧𝐚𝐥 𝐑𝐡𝐚𝐛𝐝𝐨𝐦𝐲𝐨𝐥𝐲𝐬𝐢𝐬 (𝐄𝐑)
Often called azoturia, Monday morning disease, or simply the “classic” form of tying up, this sporadic type can affect any horse. It’s most common in horses that aren’t properly conditioned for the level of work they’re 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.

𝐑𝐞𝐜𝐮𝐫𝐫𝐞𝐧𝐭 𝐄𝐱𝐞𝐫𝐭𝐢𝐨𝐧𝐚𝐥 𝐑𝐡𝐚𝐛𝐝𝐨𝐦𝐲𝐨𝐥𝐲𝐬𝐢𝐬 (𝐑𝐄𝐑)
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 it’s 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 aren’t 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.

𝐏𝐨𝐥𝐲𝐬𝐚𝐜𝐜𝐡𝐚𝐫𝐢𝐝𝐞 𝐒𝐭𝐨𝐫𝐚𝐠𝐞 𝐌𝐲𝐨𝐩𝐚𝐭𝐡𝐲 𝟏 (𝐏𝐒𝐒𝐌𝟏)
This condition is linked to a mutation in the glycogen synthase 1 (𝘎𝘠𝘚1) 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 <12% NSC, a low starch/sugar and high fat diet, and regular, consistent exercise.

𝐏𝐨𝐥𝐲𝐬𝐚𝐜𝐜𝐡𝐚𝐫𝐢𝐝𝐞 𝐒𝐭𝐨𝐫𝐚𝐠𝐞 𝐌𝐲𝐨𝐩𝐚𝐭𝐡𝐲 𝟐 – 𝐄𝐱𝐞𝐫𝐭𝐢𝐨𝐧𝐚𝐥 𝐑𝐡𝐚𝐛𝐝𝐨𝐦𝐲𝐨𝐥𝐲𝐬𝐢𝐬 (𝐏𝐒𝐒𝐌𝟐 – 𝐄𝐑)
Some horses show clinical signs consistent with PSSM1 but test negative for the known mutation. Yet their muscle biopsies reveal abnormal glycogen accumulation similar to PSSM1. These horses respond well to the same dietary and management strategies; they just don’t carry the 𝘎𝘠𝘚1 mutation. This form, often called PSSM2-ER, is mainly seen in Quarter Horse-related breeds but can occur in other breeds.

𝐌𝐲𝐨𝐟𝐢𝐛𝐫𝐢𝐥𝐥𝐚𝐫 𝐌𝐲𝐨𝐩𝐚𝐭𝐡𝐲 (𝐌𝐅𝐌)
Previously grouped under PSSM2, myofibrillar myopathy (MFM) is now recognized as its own subtype with distinct pathology. Unlike PSSM1, this disorder is characterized by abnormal protein aggregates, specifically desmin, within muscle fibers (Valberg et al., 2016; 2017).

Muscle damage is believed to be linked to a deficiency in a cysteine-derived antioxidant essential to preventing cellular damage from oxidative stress. Thus, a moderate NSC, low-fat diet is recommended to limit oxidative stress. Providing high-quality protein through supplements or ration balancers can be beneficial. Specifically, supplementation with N-acetyl cysteine, the only absorbable form of cysteine, supports the synthesis of glutathione, an essential antioxidant. Other antioxidants that may provide benefits include coenzyme Q10 and, in cases of deficiency, natural vitamin E. Because management of MFM differs from PSSM1 and PSSM2-ER, accurate diagnosis is essential. This condition is most often seen in warmbloods with exercise intolerance and endurance Arabians tying up after long endurance rides.

𝐌𝐅𝐌 – 𝐖𝐁
This subtype is seen in warmbloods and their crosses, often with a gradual onset. These horses may have promising athletic careers but begin to decline in performance between ages 6–8. Clinical signs include unwillingness to go forward with normal or slight elevations in CK, and poor performance signs such as exercise intolerance, soreness, vague hindlimb lameness, and reluctance to collect or canter. Since these signs overlap with other conditions, including back pain, gastric ulcers, or poor saddle fit, a full veterinary evaluation is essential.

Diagnosis requires a muscle biopsy. Common recommendations include a few days of rest each week for muscle recovery (a 3 day on and 2 day off schedule works well for many of these horses), long-and-low warm up, and lots of turn out.

𝐌𝐅𝐌 – 𝐄𝐑
Arabians, especially those in endurance disciplines, are most commonly affected. Clinical signs may be subtle, mild stiffness but with markedly high CK. Arabians who tie up near the end of a ride are more likely to have MFM-ER, while those affected earlier are often diagnosed with RER. MFM-ER horses benefit from daily exercise, which can be supported with a hot walker or exerciser.

𝐓𝐞𝐬𝐭𝐢𝐧𝐠 𝐚𝐧𝐝 𝐃𝐢𝐚𝐠𝐧𝐨𝐬𝐢𝐬
PSSM1 can be easily diagnosed via hair-root DNA testing, but diagnosing PSSM2 and MFM is more complex. While some commercial labs offer genetic panels, this genetic testing has not been validated.

Dr. Valberg, a leading researcher on equine myopathies, conducted studies comparing genetic testing and muscle biopsy results and found no consistent correlation (Valberg et al., 2021; 2023). Therefore, muscle biopsy remains the gold standard for diagnosing PSSM2 and MFM. Horses under 8 years of age may start showing signs of MFM but the desmin aggregates in young horses may not yet be visible under the microscope. In such cases a 4 to 6 week treatment trial for MFM may be recommended.

𝐂𝐨𝐧𝐜𝐥𝐮𝐬𝐢𝐨𝐧
Understanding the different forms of muscle diseases is essential for providing the right care and preventing future episodes. What looks like one problem on the surface could have very different roots beneath, and each demands a unique strategy. Always work with your veterinarian to determine the specific type through appropriate diagnostics. Hopefully, this flow chart can help guide you through the process. With the right combination of diet, exercise, and management, many horses with tying up can stay comfortable, capable, and successful.

Have you ever managed a horse with tying up? What helped most in your experience?

Cheers,
Dr. DeBoer

Valberg SJ, Mickelson JR, Gallant EM, MacLeay JM, Lentz L, De La Corte F. Tying up in Quarter Horses and Thoroughbreds: separate diseases with common clinical signs. Proceedings. American Association of Equine Practitioners, 45: 311. 1999;313.

Valberg SJ, McKenzie EC, Eyrich LV, Shivers J, Barnes NE, Finno CJ. Suspected myofibrillar myopathy in Arabian horses with a history of exertional rhabdomyolysis. Equine veterinary journal. 2016 Sep;48(5):548-56.

Valberg SJ, Nicholson AM, Lewis SS, Reardon RA, Finno CJ. Clinical and histopathological features of myofibrillar myopathy in Warmblood horses. Equine veterinary journal. 2017 Nov;49(6):739-45.

Valberg SJ, McKenzie EC, Eyrich LV, Shivers J, Barnes NE, Finno CJ. Suspected myofibrillar myopathy in Arabian horses with a history of exertional rhabdomyolysis. Equine veterinary journal. 2016 Sep;48(5):548-56.

Valberg SJ, Henry ML, Herrick KL, Velez‐Irizarry D, Finno CJ, Petersen JL. Absence of myofibrillar myopathy in Quarter Horses with a histopathological diagnosis of type 2 polysaccharide storage myopathy and lack of association with commercial genetic tests. Equine veterinary journal. 2023 Mar;55(2):230-8.

Pagan JD, Valberg SJ. The Role of Nutrition in Managing Muscle Disorders. Veterinary Clinics: Equine Practice. 2025 Jan 27.

Valberg SJ, Finno CJ, Henry ML, Schott M, Velez‐Irizarry D, Peng S, McKenzie EC, Petersen JL. Commercial genetic testing for type 2 polysaccharide storage myopathy and myofibrillar myopathy does not correspond to a histopathological diagnosis. Equine veterinary journal. 2021 Jul;53(4):690-700. ... See MoreSee Less