Muscle Disorders in Dogs

Type II muscle fiber deficiency is a muscle disease of unknown cause that is present at birth in some Labrador Retrievers. The condition appears to be inherited and has been known to occur in both yellow and black Labradors. Signs become noticeable at less than 5 months of age and include wasting away of muscle, stunted growth, and weakness. These signs progressively worsen until the animal reaches maturity (between 6 and 12 months), when they stabilize. Animals may have a normal life span. A definite diagnosis usually requires a urine test, muscle biopsy, and electromyography. There is no effective treatment.

Fibrotic myopathy is an uncommon progressive disorder that leads to degeneration of the thigh muscles. The cause is not known, but German Shepherds appear to be predisposed to developing the condition. Affected muscles are characterized by a permanent, abnormal tightening. Normal tissues are replaced by thick connective tissue. Other signs include lameness without pain. Surgery is only sometimes helpful. The outlook for recovery is guarded because signs can recur.

Myositis ossificans is a muscle disorder in which noncancerous bony deposits appear in the muscles and connective tissue. The cause is unknown. The disorder frequently affects tissues near the hip joint in Doberman Pinschers. It may be related to a bleeding disorder (von Willebrand’s disease) in these dogs. Surgical removal of the bony mass is usually helpful.

Polymyositis is an inflammatory muscle disorder in adult dogs that affects the entire body. It may be associated with immune-mediated disorders such as lupus erythematosus or myasthenia gravis. The condition can have a sudden onset, or it may be recurring and progressive. Signs include depression, lack of energy, weakness, weight loss, lameness, muscle tenderness or pain, and wasting away of muscle. Corticosteroids are generally recommended for treatment; however, other drugs that suppress the immune system may also be used. The outlook for recovery is favorable, although signs sometimes reappear.

Masticatory myositis is an inflammatory condition that affects the muscles used to chew. The exact cause is unknown, although the body’s own immune response plays a role. In acute cases, muscles are swollen and the dog has difficulty opening the jaw. In chronic cases, signs include persistent loss of appetite, weight loss, difficulty opening the jaw, and wasting away of muscle. Blood tests, electromy-ography, and a muscle biopsy may help to confirm the diagnosis. Although the condition sometimes improves on its own, treatment with corticosteroids given by mouth is recommended in most cases. Relapses are common, and longterm medication may be required.

Malignant hyperthermia is a disorder of skeletal muscle usually brought on by certain types of inhaled anesthesia and stress. It is characterized by an abnormal increase in metabolic rate. Although the condition is most common in pigs, it is also known to occur in some heavily muscled dogs (particularly Greyhounds).

Signs include rapid heartbeat, increased breathing rate, fever, muscle tightness and rigidity, and heart and lung failure. Signs develop between 5 and 30 minutes after exposure to the anesthetic agent. Treatment requires immediately stopping the anesthesia and administering oxygen. Fluid injections, corticosteroids, ice packs, and muscle relaxants are also used. The outlook is poor in severe cases.

This muscle disorder of racing Greyhounds and working dogs is caused by overuse. It appears to be triggered by an inadequate supply of blood to the muscle after exercise or excitement. This can cause kidney disease and destruction of muscle cells. Signs include muscle pain and swelling that becomes noticeable 24 to 72 hours after racing or overuse. In severe cases, stiffness, deep or fast breathing, collapse, and kidney failure may occur. Urine tests are used to confirm the diagnosis. Treatment includes supportive care such as fluid injection, bicarbonate, body cooling, rest, and muscle relaxants. The outlook for recovery depends on the severity of the case.

Injury (trauma) of the muscles and the associated tendons can cause many forms of myopathy in dogs.

Tumors that originate in the skeletal muscle can be benign or cancerous (malignant). Malignant tumors can spread and invade nearby muscle. They can also spread to other parts of the body.

Signs include localized swelling and lameness. The diagnosis is confirmed by taking a small tissue sample called a biopsy. The tumor generally must be surgically removed or the limb amputated. Chemotherapy and radiation may be used depending on the type of tumor. See also Introduction to Cancer and Tumors

Also see professional content regarding muscle disorders in dogs.