Myositis in Dogs

There are various types of myositis, each with different causes that range from heredity to an immune-mediated response. Such is the case with masticatory myositis, caused by antibodies targeted to attack the masticatory muscles only. As with many types of myositis, without early and aggressive treatment, muscle loss can cause severe atrophy and muscle dysfunction. This can result in conditions like blindness, an inability to eat, and an inability to walk. Lesions can occur that involve swelling, hemorrhage, atrophy, and fibrosis.

Myositis refers to a group of diseases that all share an inflammation of the muscle tissue. This condition can affect only one muscle, or an entire group of muscles, such as the masticatory muscles and the eye muscles, and can cause debilitating symptoms.

Symptoms of myositis can vary greatly, and are dependent on which muscle groups are affected.

Masticatory myositis

• Difficulty moving jaw
• Inability to open mouth
• Swelling of jaw
• Loss of muscles
• Sunken eyes
• Pain in jaw
• Problems eating and drinking
• Muscle atrophy
• Jaw fixation
• Anorexia
• Weight loss
• Blindness

Extra-ocular myositis

• Protrusion of the eyeball
• Swelling around eye
• Impaired vision
• Blindness

Polymyositis

• Stiff walking
• Muscle pain
• Muscle weakness
• Swelling in muscles
• Loss of muscle
• Difficulty swallowing
• Regurgitation 
• Breathing problems

Dermatomyositis

• Skin lesions
• Muscle atrophy
• Muscle pain
• Abnormal gait
• Lesions on feet, ears, face and tail

Types 

There are different types of myositis found in dogs, and each is associated with a specific muscle group.

Masticatory muscle myositis – Affects the muscles found on the sides and the top of the head that are used for chewing. There is a progressive destruction of these muscles that eventually lead to a jaw fixation, and is believed to be immune-mediated. This type of myositis is further divided into Eosinophilic myositis, which often affects German Shepherds, Labradors and Golden Retrievers, and Doberman Pinschers, and Atrophic myositis that affects mainly long-nosed breeds. 

Extra-ocular muscle myositis – Affects the muscles involved in eyeball movement.

Polymyositis – Describes a general myositis. Is believed to be caused by an infectious organism, sometimes Ehrlichia canis, a bacterium often contracted from a brown deer tick. Lesions are found on the esophagus, forelimb muscles and masticatory muscles.

Dermatomyositis – Considered a hereditary disease that affects Shetland Sheepdogs, Australian Cattle Dogs, and other rough coated breeds. Skin lesions develop along with muscle atrophy.

There are a many conditions that can cause myositis. These include:

• Parasitic infection, such as from Toxoplasma gondii parasite
• Bacterial infection, such as from Ehrlichia canis
• Viral infection 
• Immune-mediated response causing an attack against the muscle
• Systemic lupus erythematosus
• Reaction from drugs or toxins, such as penicillamines 
• Cancer
• Genetic disposition
• Trauma

To come to a diagnosis, a complete physical, neurological, and oral exam is conducted, depending on the muscles that seem to be affected. Relate any history or evidence of trauma, as it can often be a cause of the myositis.

A muscle tissue sample is generally examined to look for inflammatory cells, an indicator of myositis. Masticatory muscle myositis can be directly diagnosed through a blood test that measures the level of antibodies; a 2M antibody test, and electromyography, which can highlight any abnormal electrical activity in muscles that are affected. Other tests can be performed to rule out infections or cancer, namely blood tests, and X-rays and ultrasounds of the chest and abdomen.

Once a positive diagnosis of myositis is reached, treatment attempts to counteract the immune system activity against the muscles by using immunosuppressive medications to return the system to normal. High doses of steroids, such as prednisolone, may be prescribed, as well as immunomodulators like cyclosporine, cyclophosphamide, cytarabine, azathioprine, or mycophenolate.

When the myositis is controlled, doses are decreased slowly in the hopes there is not a relapse. The goal is to eliminate all drugs, but generally a low dose of medication is continually needed to keep the symptoms from returning.

The side effects of immunosuppressive drugs can include the increased risk of infection, vomiting, diarrhea, anorexia, liver disease, bone marrow suppression, and pancreatitis. Corticosteroids can cause the muscles to atrophy, even if the dog is improving, and may not be prescribed if that is a concern.

If the underlying cause of the myositis is discovered and treated successfully, medication may be withdrawn completely. If cancer or infection is found to be the cause of the myositis, therapy is aimed to treat these conditions. Treatment for toxoplasmosis includes oral antibiotics and antiprotozoal medications over several weeks.

It may take many weeks to see signs of recovery and the prognosis is fair. Dogs with masticatory muscle myositis may have severe attacks for several weeks that then subside for weeks to years, and can be controlled with corticosteroids.

You may also be prescribed medications for other types of myositis to administer to your dog at home. During treatment, your veterinarian will need to monitor your dog’s progress, as well as ensure there aren’t any unwanted side effects. Be sure to notify your veterinarian if you notice any signs of these.