When Your Horse's Muscles Ache
Equine muscle injuries are often elusive, leading to frustration for the rider and a challenging diagnosis for the veterinarian. Since muscle injury can accompany and/or mimic skeletal problems, tendon or ligament injury, or neurological disease, diagnosis can be quite complicated. The best way to prevent muscle injury is to keep horses in regular exercise programs and avoid demanding more of them than their level of conditioning permits. It is also very important to maintain a consistent feeding program appropriate to each horse's level of fitness and energy requirements to avoid metabolic problems such as tying-up.
Assessment of suspected muscle damage should begin with a complete history, including any chronic or recent lameness problems, loss of condition, or attitude change, as well as any abnormalities under saddle noticed by the rider or trainer. A physical examination should follow--this includes listening to the heart and lungs, evaluating musculature symmetry, palpating major muscle groups to locate any focal areas of soreness, and palpating and assessing range of motion of the neck and legs. Depending on clinical signs, a complete blood count and/or serum chemistry might also be done to evaluate muscle enzyme levels and rule out systemic disease, which can secondarily cause discomfort or muscle injury.
A thorough lameness examination--and perhaps diagnostic imaging--will help rule out skeletal, tendon, and ligament injuries as the primary source of pain. The horse should be examined walking and trotting in hand on a straight line and on a circle. Observation under saddle might also be necessary, as discomfort might only be seen during specific movements or over fences.
At the time of examination, it is important for the veterinarian to look for signs of physical and/or dynamic (movement) asymmetry, including gluteal muscle atrophy, a "dropped hip," or variation in the swing phase between forelimbs and/or hindlimbs. Observation of how a horse tracks (the course by which he travels) might be helpful in isolating a muscle injury once other causes of lameness have been ruled out. Subtle alterations in tracking can be better evaluated with slow motion video, a technology readily available to most horse owners and veterinarians.
In many cases of muscle injury, thorough examination and palpation will not reveal a specific injury. Nuclear scintigraphy (bone scan) can be helpful in ruling out skeletal pathology as well as isolating an injury to muscle where it attaches to the bone. This technology is especially useful in evaluating muscle injuries in the back and pelvis regions, where clinical evaluation is more difficult.
Other potentially useful imaging modalities include ultrasonography and thermography. Radiography can help identify areas of mineralization in muscle and assess primary skeletal injuries that can lead to altered locomotion and secondary muscle pain.
We'll divide muscle problems into these categories:
- Muscle wasting secondary to illness or systemic disease;
- Performance/stress-related; and
- Exertional rhabdomyolysis, also called tying-up.
Muscle Wasting, Systemic Disease
Loss of muscle mass can result from any number of systemic diseases or simply from inadequate nutrition or conditioning. Diseases of the gastrointestinal tract, liver, kidney, or endocrine system (e.g., Cushing's disease) can cause generalized weight loss as well as reduced muscle mass. Certain neurologic diseases such as equine degenerative myelopathy (EDM), equine protozoal myeloencephalitis (EPM), or localized nerve damage can also present with distinct muscle atrophy. In these cases, the initial clinical signs might be indicative of a muscle disorder, but a thorough physical and laboratory examination will often reveal an underlying disease process. Treatment of the primary condition (where possible) might result in partial or complete resolution of the muscular signs.
Causes of traumatic muscle injuries can include falling down, being cast for a prolonged period, colliding with another horse, hitting rails or standards while racing or jumping, and trailer accidents. Another type of muscle damage sometimes included in the traumatic category is injection site myopathy, when a horse develops mild to severe swelling at the site of a vaccine or other intramuscular injection. The reaction might be due to infection, but more commonly, the swelling is a sterile inflammation.
Muscle injury due to trauma will present with pain on palpation, local swelling, and elevation of muscle enzymes. Other signs can include lacerations and/or hematoma (bruise) formation. Such injuries are best treated with anti-inflammatories and application of cold to injured muscles during the acute phase of injury (two to three days), followed by heat over the affected muscles during the chronic phase of treatment. Of course, any wounds should be appropriately treated as well.
While traumatic injuries generally involve direct impact, stress-related injuries result from acute or chronic overuse of a muscle or muscle group.
Performance- or stress-related muscle injuries include back soreness, isolated acute muscle strains, and chronic repetitive muscle strains. In any horse with back pain, it's important to have saddle fit professionally evaluated. Horses with unusual conformation might need a custom-made saddle to avoid saddle-related back pain, while other horses might just need padding adjustment.
It is the authors' experience, however, that primary injury of the musculature of the back is rare. Lumbar (lower back) muscle soreness is most commonly secondary to hind end lameness such as degenerative joint disease (arthritis) of the hocks. Joint pain or sacroiliac subluxation can cause a horse to track abnormally with his hind legs, leading to strain of lumbar musculature (muscles of the back behind the saddle area). The sacroiliac joint is the joint between the sacrum (five fused vertebrae between the lumbar vertebrae and tail) and the ilium (the largest part of the pelvis).
Another common cause of back soreness is impingement (rubbing together) of the dorsal spinous processes of the thoracolumbar vertebrae, also known as "kissing spines." This chronic condition resulting from abnormal conformation of two or more adjacent vertebrae can cause skeletal pain and secondary muscle spasm.
If clinical examination along with radiography does not isolate a reason for back soreness, nuclear scintigraphy is indicated to look for any areas of bone remodeling.
Various treatments for back muscle pain have been successful. The standard therapeutic regime of rest, non-steroidal anti-inflammatories (NSAIDs, e.g., phenylbutazone, flunixin meglumine, ketoprofen, naproxen, meclofenamic acid, or carprofen), and muscle relaxants (methocarbamol) is often effective. Therapy options thought to provide additional relief include local anti-inflammatory injections into back muscles, alternating hot and cold compresses, and alternative medicine such as acupuncture, massage, magnetic blankets, and/or chiropractic manipulation. Extracorporeal shock wave therapy is a recently developed technology that has also shown promise.
Chronic muscle strains involve repeated bouts of inflammation and lead to scarring and/or mineralization (calcium deposition) in the affected muscle and its tendinous and ligamentous attachments. This condition--fibrotic myopathy--is most common in hind end muscles (particularly the semitendinosus muscle, which extends the hip and hock). The scarring eventually results in a gait dysfunction featuring a shortened cranial swing phase (with the leg not moving as far forward as usual). Diagnosis is based on clinical presentation and palpation or with ultrasonography and/or radiography. Suggested treatment includes surgical transection (cutting) of the semitendinosus tendon or of the scarred muscle tissue. Physical therapy should follow surgery to stretch muscle fibers and reduce scarring recurrence.
Acute muscle strains can be identified by physical examination. However, if the affected muscle or muscle group is not accessible to palpation, it could be hard to identify. Depending on the severity of the strain, muscle enzyme levels might not be elevated. In such cases, specific diagnosis is nearly impossible, and symptomatic treatment might be the only option. If treatment with anti-inflammatories and muscle relaxants doesn't relieve the pain, nuclear scintigraphy might help focus treatment on a specific muscle group. Although scintigraphy is usually used to diagnose bone problems, it can also show a distinctive pattern in deep muscle injury of the back or gluteal muscles.
The clinical appearance of a "tied-up" horse is marked firmness or cramping of the gluteal muscles, gait stiffness, mild to severe pain on palpation of hind end muscles, profuse sweating, elevated heart and respiratory rates, and in severe cases, inability to move. Due to muscle cell injury, certain contents of these cells such as myoglobin (a protein found in high concentration in muscle) and muscle enzymes including creatine kinase (CK), aspartate aminotransferase (AST), and lactate dehydrogenase (LDH) will be released into circulation. The elevated muscle enzymes can be detected on a serum chemistry profile, while myoglobin is readily detected when it is excreted in the urine, giving it a brown-tinged appearance. Horses which have tied up are often clinically dehydrated.
Treatment of an acutely tied-up horse involves restoring normal hydration and electrolyte balance, improving muscle perfusion (blood flow), controlling pain, and avoiding further muscle damage. In order to reduce the risk of further muscle damage, it's important to move an affected horse as little as possible and to keep him quiet. This can be done by keeping the horse in a stall until clinical signs improve and giving tranquilizers if he is panicky. Acepromazine can help increase circulation to affected muscle groups in addition to its calming effects.
Most horses get adequate rehydration by nasogastric intubation with an electrolyte solution. However, in severely dehydrated horses, intravenous fluids are indicated. Flunixin meglumine (Banamine) along with other NSAIDs provides good pain relief, although it should be used carefully in severely dehydrated horses due to potentially harmful kidney effects. Dantrolene, a muscle relaxant, has also been used with some success in reducing the pain of tying-up.
There are several known causes of tying-up; some aren't fully understood. A common scenario involves an under-conditioned horse asked to work vigorously. A combination of overexertion, dehydration, and electrolyte imbalances can lead to tying-up. Generally, treatment followed by rest and appropriate conditioning will reduce the risk of recurrence in this type of horse.
Some bloodlines of Thoroughbreds, Standardbreds, and Arabians might be genetically predisposed to tying-up. Despite good conditioning, these horses might have chronically elevated muscle enzyme levels in their blood and display chronic intermittent exercise intolerance. This condition is seen most often in excitable Thoroughbred fillies. Long-term management involves a regular exercise routine and attempts to reduce sources of anxiety in addition to providing salt or electrolytes daily. Reducing carbohydrates in the diet and replacing calories with fat has also been helpful in some cases.
Heavy breeds of horses, such as drafts and warmbloods, are prone to a metabolic abnormality known as polysaccharide storage myopathy (PSSM). These horses tend to demonstrate recurrent bouts of tying-up, especially when worked after a few days of rest without a decreased grain ration (thus the nickname Monday morning syndrome). With PSSM horses, carbohydrates are stored in muscle cells as glycogen, but the normal mechanism of breaking down the glycogen is defective. Consequently, glycogen accumulates in muscle cells, and their usual energy source is inaccessible. This results in the same clinical signs as described previously. Definitive diagnosis of PSSM is made based on muscle biopsy. Management involves providing less of the affected horse's caloric requirements as carbohydrates and more with fats, such as by reducing grain and adding corn oil. Replacing some feed with rice bran or other high-fat grains has also been successful.
In conclusion, muscle injury in the horse is often difficult to diagnose and challenging to both the owner and veterinarian. Excellent communication between owner, trainer, and veterinarian is paramount, and patience throughout the treatment and recovery phase is necessary. Proper conditioning and appropriate nutrition are critical for preventing muscle injury and reducing the risk of recurrence. Once a diagnosis is made, rehabilitation should include rest followed by an extensive formal period of controlled exercise agreed upon by the owner, trainer, and veterinarian.
See the Lameness and Ailments and Syndromes sections at www.TheHorse.com.
About the Author
Scott J. Swerdlin, DVM, MRCVS, is President of Palm Beach Equine Clinic, where he has practiced since 1983. Swerdlin has an extensive sport horse practice including dressage, jumpers, and polo ponies, and he has been in the performance horse practice for [Scott Swerdlin] over 20 years. Dr. Swerdlin enjoys playing polo and watching his wife Amy compete in Dressage. Shana E. Chase, DVM, is a former associate veterinarian with Palm Beach Equine Clinic.
POLL: University Equine Hospitals