Muscle Disorders in Dogs; Myopathies and Myositis in Dogs

A disease condition damaging the muscle tissue is referred to as a myopathy. Some of the myopathies affecting dogs are congenital while others are acquired through injuries, nutritional deficiencies or toxins introduced into the body.

Myositis refers to a disease condition resulting in the inflammation of muscles. Infections, both bacterial and viral, parasitic infestations and auto immune diseases are the usual causes of myositides.

Type II Muscle Fiber Deficiency

This disease characterized by the deficiency of muscle fibers is a congenital condition sometimes observed in Labrador Retrievers. The exact cause is not known, but it is generally considered an inherited disease. Black as well as yellow Labradors are equally affected. Even though the condition is present at birth, the typical symptoms become apparent when the puppies are about 5 months old. Wasting away of the muscles is the primary effect of the disease, and as the disease progresses steadily, it often results in weakness and stunted growth. It eventually stabilizes when the puppy has become mature, usually towards the end of the first year.

Electromyography and urine tests may help in the diagnosis, and the veterinarian may do a biopsy of the muscle to confirm the disease. However, no specific treatment has been found to be effective. The disease does not affect the lifespan of the dog in spite of the growth retardation.

Fibrotic Myopathy

This progressive myopathy affects the muscles in the thigh, causing them to deteriorate over time. This disease is not very common, but German Shepherds are more prone to it. Permanent tightening of the muscles result as a thicker connective tissue develops in the place of the normal muscle fibers. The dog may become lame, but the condition is not painful. Surgical intervention has only limited success as the disease may recur even after the surgery.

Myositis Ossificans

The occurrence of bony deposits in the muscles as well as in the associated connective tissue is the primary effect of this disease. Doberman Pinschers are usually affected by this condition in the muscles close to their hips. This breed is prone to another bleeding abnormality called von Willebrand’s disease; and some relation is suspected between the two disorders. Surgery is the main treatment for Myositis ossificans. Removing the affected portion of the muscles may lead to recovery.

Polymyositis

This inflammatory disease affects all the muscles in the body. It mainly occurs in adult dogs. This type of myositis is related to certain auto immune diseases such as myasthenia gravis and lupus. The inflammation may appear suddenly or it can be a chronic or recurrent condition that steadily becomes worse.

The usual symptoms are lethargy, weakness, loss of appetite and weight loss. The dog may become lame and depressed. The widespread inflammation makes the muscles tender and painful to touch. There may be loss of muscle mass too.

Polymyositis is treated with corticosteroids and other immune system suppressors to reduce the autoimmune reactions of the body. The treatment is usually effective, but there is the risk of the disease recurring.

Masticatory Myositis

As the name indicates, this is a disease affecting the muscles involved in mastication, or the chewing of food. It is not clear why the inflammation occurs, but the overreaction of the immune system is implicated. When the inflammation is severe, the dog may find it difficult to open its mouth due to the swollen muscles. Dogs with chronic masticatory myositis display symptoms such as lack of appetite, and refusing to eat because of the difficulty in chewing, and consequent loss of weight and muscle mass.

To make a definitive diagnosis, the veterinarian may require a biopsy of the affected muscle and an electromyography, in addition to blood tests. In some cases, the inflammation may eventually subside by itself, but oral administration of corticosteroids is often required to relieve the symptoms. They may have to be continued for a long time as there is a risk of the disease recurring.

Malignant Hyperthermia

This life threatening condition occurs due to an adverse reaction to anesthesia administered through inhalation. This is more often found in pigs, but certain breeds of dogs with high muscle mass, such as the Greyhounds, are also affected.

The skeletal muscles suddenly become rigid, accompanied by other symptoms such as rapid breathing and abnormally high heart rate. The body temperature rises sharply due to a sudden rise in metabolic rate. These adverse reactions occur within 5-30 minutes of inhaling the anesthetic. Unless promptly treated, the dog’s condition can deteriorate rapidly, culminating in lung failure and heart attack.

When the typical symptoms of malignant hyperthermia are observed, the anesthesia should be stopped and supplemental oxygen should be given. Corticosteroid drugs and muscle relaxants to relieve the rigidity are given and fluids are administered intravenously while the icepacks are used to bring down the body temperature. If the reaction has been severe, the dog may succumb to it.

Rhabdomyolysis (Exertional Myopathy)

Overexertion is the cause of rhabdomyolysis which usually affects some breeds of working dogs and Greyhounds used for racing. Continuous activity requiring high energy expenditure may exhaust the muscles as the blood supply becomes insufficient to meet the demand. This may starve the muscle fibers of oxygen and they may die. Kidney disease also develops.

Pain and inflammation of the muscles that appear 1-3 three days after a race or other over activity is the initial symptom. Muscle stiffness, rapid and deep breathing are signs of severe disease. Kidney failure and related symptoms may follow.

When the symptoms appear, the dog should be made to rest and muscle relaxants are administered. Cooling the body and intravenous administration of fluids and bicarbonate may help the dog recover. The prognosis depends on the extent of myopathy.

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