Myopathies are diseases that primarily cause damage to muscles or muscle tissues. They may be present at birth or occur due to nutritional imbalances, injury, or ingestion of a poisonous substance. Myositides are diseases that produce a mainly inflammatory reaction in muscle. Common causes include infections, parasitic diseases, and immune--mediated conditions.
Type II Muscle Fiber Deficiency
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.
Exertional Myopathy (Rhabdomyolysis)
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.
Muscular or Tendon Trauma
Injury (trauma) of the muscles and the associated tendons can cause many forms of myopathy in dogs.
Abnormal Contracture of the Shoulder Muscle
Abnormal tightening or contracture of the shoulder muscle (infraspinatus) is a muscle disease that affects one or both shoulders. It usually occurs after trauma in hunting or working dogs. Signs include noticeable lameness, pain, and swelling in the shoulder region. The lameness is temporary, but an abnormal gait develops 2 to 4 weeks after injury as the muscles thicken and tighten. Other signs include the pulling in of the elbow towards the body, pulling away of the foreleg from the body, and external rotation of the carpus and paw. The limb is moved in a circular motion with each stride of the leg. Treatment consists of the surgical removal of a portion of the muscle, including cutting of a tendon. Limb and joint functions usually improve immediately following surgery, and the outlook for full recovery is excellent.
Inflammation of the Biceps Brachii Tendon and Its Covering
Inflammation of the biceps brachii tendon and its covering (sheath) due to injury can occur in one or both forelimbs. It usually affects mature, large dogs. The injury may be direct, indirect, or due to overuse or migration of bone and cartilage fragments called joint mice (see Bone, Joint, and Muscle Disorders of Dogs: Osteochondrosis).
Persistent lameness that may worsen over time is one sign of this condition. The lameness also worsens after exercise and improves with rest. The shoulder joint's range of motion is reduced, and the shoulder muscles may waste away. Applying pressure to the biceps tendon while bending and extending the shoulder joint causes severe pain. X-rays and ultrasonography are often used to help confirm the diagnosis. Surgical inspection using an endoscope can help your veterinarian determine the extent of the injury.
In mild cases, the recommended treatments are rest and nonsteroidal anti--inflammatory drugs given by mouth. Severe cases can be treated with injections of an anti-inflammatory agent and rest. Chronic cases that are resistant to treatment of multiple corticosteroid injections or cases involving joint mice are treated through surgery. The outlook for recovery is good, although severe degeneration in longterm cases may cause lameness to persist after treatment.
Quadriceps Contracture (Stiff Stifle Disease)
This serious thickening and tightening of the quadriceps muscles develops after improperly performed surgical operations on young dogs to repair fractures of the femur. The bone, the connective tissue surrounding the bone, and the quadriceps muscles bond together. This leads to extension and disuse of the limb, porous and easily broken bones (osteoporosis), osteoarthritis, and bone and joint deformations. The affected limb is overextended and often displaced. Surgery is usually required to remove the fibrous tissues that connect the bone and muscles and to restore motion in the stifle joint. Reconstruction of bone and soft tissues should be followed by application of bandages that restrict mobility of the joint. Physical therapy is usually required after surgery. The outlook for recovery is guarded. Careful surgical repair of bone fractures can help prevent this condition.
Achilles Tendon Disruption (Dropped Hock)
This injury to the common Achilles tendon most often afflicts fully grown working and athletic dogs. It is usually the result of trauma. The tendon can be partially or completely ruptured or torn away from the bone. Signs include a severe lameness that keeps the animal from putting weight on the leg, overextension of the tarsus, and a stance in which the heel touches the ground. Swelling, pain, and torn or thickened tendon ends may also be seen. X-rays are useful for diagnosis and may reveal fragments torn from the bone.
Surgical treatment can repair the torn ends and reattach the tendon to the bone. External splints are generally used after surgery to immobilize the joint for several weeks. The outlook for recovery varies, based on the duration of the injury, the success of the surgery, and the dog's expected performance when healthy.
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 Cancer and Tumors: Introduction to Cancer and Tumors
Last full review/revision July 2011 by Russell R. Hanson, DVM, DACVS, DACVECC; Dale A. Moore, MS, DVM, MPVM, PhD; Joerg A. Auer, DrMedVet, Dr h c, MS, DACVS, DECVS; Joseph Harari, MS, DVM, DACVS; Sheldon Padgett, DVM, MS, DACVS