Myositis (Inflammation of Muscles) in Dogs

Overview of Canine Myositis

Myositis is a condition in which skeletal muscles are damaged by a noninfectious inflammatory process dominated by lymphocytic infiltration that may occur in dogs. Signs of myositis are generally sudden in onset.

General Causes of Myositis in Dogs

• Immune-mediated causes associated with a hyperactive immune system
• Toxoplasma, Neospora or Hepatozoon (parasites)
• Ehrlichia (rickettsia)
• Drug induced
• Paraneoplastic syndrome – clinical signs or illness associated with/secondary to cancerous processes

What to Watch For

Signs of myositis in dogs may include:

• Stiff-stilted gait
• Muscle swelling or atrophy (shrinkage)
• Muscle pain
• Weakness
• Exercise intolerance (tires easily)
• Difficulty swallowing
• Regurgitation, which is the effortless evacuation of fluid, mucus, and undigested food from the esophagus

Diagnosis of Myositis in Dogs

Diagnostic tests may include:

• Baseline tests to include a complete blood count (CBC), biochemical profile, and urinalysis are recommended in all patients, and may be within normal limits. Creatine kinase (muscle enzyme) is often markedly elevated.
• Muscle biopsy is the single most important test for diagnosing myositis. Multiple muscles should be sampled as the disease may have a patchy distribution.
• Serum antinuclear antibody titer (ANA) may be positive, suggesting immune disease
• Titers (blood tests) screening for infectious organisms (Ehrlichia, toxoplamsa, etc.)
• Radiographs (x-rays) of the thorax (chest) that support a dilated esophagus.
• Esophagram (barium swallow)
• Fluoroscopy of the pharynx, esophagus and upper gastrointestinal tract to confirm abnormal motility (movement).
• Electromyographic evaluation (EMG) helps determine the distribution of muscle involvement and the muscles that need biopsy. It also helps differentiate myositis from polyneuropathy (disorder of the nerves) as the cause of weakness.

Treatment of Myositis in Dogs

Most dogs are treated on an outpatient basis. Supportive care may be required to prevent wounds and decubital ulcers (bed sores) in severely affected dogs that cannot walk. In the cases where an underlying infectious disorder has been identified, specific therapy should be instituted. Additional treatment may include:

• Fluid and electrolyte therapy
• Nutritional support, as many pets have a megaesophagus and cannot get food past the malfunctioning organ.
• Immunosuppressive drugs, such as corticosteroids and azathioprine (Imuran®), as this disorder is often immune mediated

Home Care

Administer all medication and diet as directed by your veterinarian. If any change is noted in your dog’s condition, notify your veterinarian. In particular, if coughing or difficulty breathing is observed, contact your veterinarian at once, as this may signal aspiration pneumonia secondary to a malfunctioning esophagus.

Life long therapy may be necessary in these patients.