Demystifying Muscle Disease

Muscle diseases are a hot topic for horse owners right now. And the more we learn, the more confusing it seems to get. Find out what you need to know to about muscle diseases that can impact your horse’s health.

The note on my desk said, “Client wants to buy a horse with PSSM. What do you think?” Good question! Equine muscle diseases are one of the hottest topics around these days and are the subject of a lot of active research—meaning we learn more every day. Any time I’m asked a question about one of these disorders, I turn to the most current research to learn what’s new. And every time I do, it seems more and more confusing. Between the alphabet soup of acronyms (PSSM, MH, RER to name just a few) and the similar-but-different explanations, it’s easy to get bogged down.

If I’m confused, chances are you are, too. In this article, I’m going to do my best to demystify the confusing world of equine muscle diseases by honing in on what you need to know, beginning with a description of signs you’ll see that might point toward a muscle disease. Next, we’ll focus our attention on the steps your vet is likely to recommend if you think your horse has a muscle problem, including how to make a diagnosis and what to do about it.

I’ll include some examples to help you see how all of this information plays out in you and your horse’s real life. Finally, to help you keep things straight, I’ve also compiled a comprehensive reference chart to provide you details about the diseases behind all of those letters.

Does Your Horse Have Signs?

Symptoms of a muscle disorder range from something as vague as undiagnosable low-grade lameness to a full-blown tie-up episode. Look for the following signs:

Muscle atrophy: Atrophy, also called muscle wasting, refers to loss of muscle mass. Most commonly, this might be described as lack of muscling along the topline in a performance horse. Specific muscles or groups of muscles might also exhibit atrophy—a classic example would be the condition known as sweeny that results in a visible loss of muscle over a horse’s shoulder blade because of damage to the nerves. Muscle atrophy also results in weakness, which can lead to complaints of poor performance.

Muscle pain: Think about how you feel when you go to the gym and lift just a few too many weights. Your horse can experience the same kind of muscle pain as a result of muscle disease. He may also experience cramping in some circumstances, such as a tie-up episode. Your vet may identify painful muscles during a physical exam, either by how your horse reacts to palpation or by how he moves.

Muscle fasciculations: Your horse may exhibit intermittent signs of twitching or muscle fasciculations when tiny sections of muscle contract independently. These signs often develop due to electrolyte misfunctions.

Rhabdomyolysis or a “tie-up” episode: During a tie-up episode, your horse will experience full-body cramping and pain. He’ll have an elevated heart rate, will paw, breathe hard, and sweat profusely due to the pain he feels. Most of the time he’ll refuse to move at all. In a worst-case scenario, he may even go down and be unable to get back on his feet. Tying-up can be in response to exercise (exertional) or unrelated to an exercise session (non-exertional). A tie-up can also occur as an isolated event in a horse that doesn’t have an underlying muscle disorder—typically when exercise demands outstrip energy availability. If your horse ties up repeatedly, however, it’s likely there’s an underlying reason.


Making a Diagnosis

If you suspect your horse may have a muscle disease, there are several steps that’ll help to make a diagnosis, beginning with a call to your veterinarian. When you call, be prepared to answer a lot of questions, as a thorough history can provide important clues. If possible, you should know your horse’s pedigree, be able to provide details about diet and exercise schedules including turnout and supplements, and report on what symptoms you’ve observed and under what conditions—especially if signs only show up during or after exercise. Your vet will perform a thorough physical exam, evaluate for signs of lameness, and is likely to want to watch your horse work. You’ll have the best results if you come prepared.

Basic lab tests: Your vet will look for muscle atrophy, weakness, and pain that could indicate a muscle abnormality, and will run basic lab tests that may help identify a muscle problem. Specifically, they’ll look for elevations in muscle enzymes (including Creatine Phosphokinase or CK/CPK, and Aspartate Aminotransferase or AST) that indicate muscle breakdown has occurred. If your horse has recently tied-up, these enzymes will be elevated. CK levels peak approximately 4 to 6 hours after a tie-up episode and return to normal fairly quickly, while AST levels increase after 24 to 48 hours and can stay elevated for as long as 10 days. Enzyme levels can give you an estimation of the severity of the episode, and can also be used to help determine a safe schedule for return to work.

If your horse is between episodes when your vet performs the initial exam, enzymes are likely to be in normal range. In this situation, your vet may recommend an exercise test with blood drawn four to six hours after 15 minutes of work at a walk and trot. A mild elevation of CK under these conditions may indicate an underlying muscle abnormality.

While identifying muscle enzyme elevation after work may be useful for diagnosing muscle disease, it’s important to recognize that some muscle disorders rarely lead to muscle breakdown. This means muscle disease can’t be completely ruled out just because muscle enzymes remain in normal range.

Vitamin E/Selenium tests: Your veterinarian may also recommend checking your horse’s blood levels of vitamin E and selenium. Vitamin E is an antioxidant that works in conjunction with selenium to maintain normal nerve and muscle function. Vitamin E deficiencies are common in horses—especially if they aren’t turned out on green pasture for at least 12 hours each day. A number of muscle diseases have been linked to low vitamin E levels, including Equine Motor Neuron Disease (EMND) and Vitamin E responsive Myopathy (VEM). If your horse’s vitamin E levels are low, supplementation with a natural form of vitamin E will be recommended. (Note: Recommended will be d-alpha tocopherol, NOT DL-alpha tocopherol which is the synthetic form.) Selenium supplementation will also be suggested based on test results.

Genetic testing: If your vet suspects an underlying genetic disease, genetic testing will be the next step. A number of muscle disorders, including Polysaccharide Storage Myopathy 1 (PSSM1), Myosin Heavy Chain Myopathy (MYHM), Malignant Hyperthermia (MH), and Hyperkalemic Periodic Paralysis (HYPP) have all been linked to a specific gene mutation and can be definitively diagnosed with a genetic test. Others, such as Polysaccharide Storage Myopathy 2 (PSSM2) and Recurrent Equine Rhabdomyolysis (RER), are believed to have a genetic component, but a specific gene mutation has yet to be identified. These conditions are the subject of ongoing research, and more is being learned every day in hopes that genetic testing might be available eventually. One word of caution—if you plan to pursue genetic testing, work closely with your veterinarian when selecting a lab to perform the tests. To make sure you get accurate results, it’s wise to be wary of commercial laboratories offering genetic tests that have not been well validated.

Muscle biopsy: If genetic tests are negative and your horse continues to have problems, a muscle biopsy might be needed to make a diagnosis. This will require your veterinarian to surgically remove a sample of muscle and submit it to a lab where it’ll be examined under a microscope for signs of abnormalities that are characteristic of the different diseases. Your vet will select the location and technique for collecting the sample depending on the suspected disease.

How It Works

Now that you understand the basics about recognizing signs of muscle disease and the steps required to make a diagnosis, let’s take a look at how this might work in a specific situation with you and your imaginary mare, Beauty.

You have been having some training issues with Beauty, who is often crabby and reluctant to go forward. One day she experiences an acute tie-up episode that prompts an emergency call to your veterinarian, who administers a tranquilizer to help relax muscles, and intravenous fluids to help flush out potentially damaging muscle breakdown products from Beauty’s system. Lab tests confirm that the tie-up was severe, with extreme elevations in both CK and AST. Your vet recommends that you take steps to reduce the carbohydrate levels in Beauty’s diet, and advises you to make sure she gets very regular, daily exercise. They also draw blood to check for vitamin E and selenium levels, which are in normal range.

Things seem slightly better for a month or two, until Beauty ties up again…twice…in a period of several weeks. At this point, your vet expresses concern that Beauty could have an underlying muscle disorder and recommends genetic testing for PSSM1, one of the diseases most likely to be associated with the signs that Beauty has been exhibiting. Your vet is surprised when the test comes back negative, and suggests a muscle biopsy. Sure enough, the biopsy confirms the presence of abnormal glycogen in Beauty’s muscles that is characteristic of PSSM2. You and your vet get serious about establishing a management plan for Beauty that includes a carefully regulated low-carbohydrate/high-fat diet, and you commit yourself to a daily exercise regimen. Beauty’s performance is much improved, and it’s been six months since she last tied up. Because you have a confirmed diagnosis, you know how important it will be to stick to this careful management plan for the rest of her life.

What If?

• The genetic test had come up positive for PSSM1? Management recommendations would have been the same, but you would not have needed to pursue the muscle biopsy to confirm your diagnosis. You might opt out of considering Beauty as a broodmare prospect due to the risk of passing on the gene.

• The biopsy had shown changes consistent with RER instead of PSSM2? The careful dietary management and regular exercise schedule you committed to for PSSM2 would be similar for RER. In addition, RER seems to be particularly associated with stress and anxiety, so taking steps to reduce Beauty’s stress levels, including judicious use of tranquilizers, might be helpful.

• Beauty was crabby and reluctant to work, but never exhibited signs of tying up? Her vitamin E levels came back critically low. Your vet would have recommended vitamin E supplementation and if Beauty responded well, would have made a presumptive diagnosis of VEM that could be confirmed with a muscle biopsy.

Caring For Your Horse

It’s easy to see why muscle disorders are confusing. There are so many similar-yet-different diseases that it’s hard to keep track. Yet in many cases, identifying the specific underlying cause of your horse’s symptoms is crucial for formulating an effective management plan, and managing your expectations for him. It’s important to understand the nature and severity of his disease.

So did my client buy that horse? She did. The little mare only had one copy of the PSSM1 gene, and had never exhibited any signs of disease. She had a great temperament, was willing to work, and showed no other signs of lameness or underlying health concerns. My client decided she was worth the risk, knew she might have to adjust her future plans based on the mare’s condition, and was fully prepared to provide the careful management she’d require for the rest of her life. 

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