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Polymositis and Dermatomyositis in Dogs

Polymositis and dermatomyositis are both generalized disorders which involve the inflammation of the dog's muscles. More specifically, polymyositis involves skeletal muscle damage due to inflammation, but with no pus formation, whereas dermatomyositis is a form of polymyositis in which characteristic skin lesions are also seen.

Various breeds of dog, including Newfoundland and boxer, may be affected by polymositis, while dermatomyositis is typically reported in rough-coated collies, Shetland sheepdogs, and Australian cattle dogs.

Symptoms and Types

• Stiff-stilted gait
• Muscle swelling
• Muscle weakness
• Muscle pain (especially when muscles are touched)
• Exercise intolerance
• Enlarged esophagus (megaesophagus)
• Regurgitation
• Skin lesions (in dermatomyositis)

Causes

• Immune-mediated infections
• Drugs
• Cancer

Diagnosis

You will need to give a thorough history of your dog’s health, including the onset and nature of the symptoms. The veterinarian will then conduct a complete physical examination as well as a biochemistry profile, urinalysis, complete blood count, and tests to evaluate levels of creatine kinase enzyme -- normally found in the brain, muscles, and other tissues -- to assess muscular damage.

He or she will also collect a sample of the muscle to send to a veterinary pathologist for further evaluation. This is the single most important test for diagnosing polymyositis.

In dogs with regurgitation, thoracic X-rays will help evaluate the esophagus for dilatation or identify tumor(s) within the esophagus. Surgery may be required if tumor(s) are found.