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.

CRAIG STEVENSON

CO-OWNER & MANAGING BROKER GESWEIN FARM & LAND

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