Inflamed Chewing Muscles and Eye Muscles in Dogs

Focal Inflammatory Myopathy in Dogs

The term myopathy is a general clinical term for a disorder of the muscles. Focal inflammatory myopathy affects specific muscle groups, in this case the masticatory muscles, which are the facial muscles involved in chewing, and the extraocular muscles, the group of muscles that are adjacent to the eyeball and that control the movements of the eye.

Focal inflammatory myopathy is suspeced to be due to autoantibodies, or antibodies that are known to react against the body’s own tissues. Antibodies are proteins found in the blood and which are used by the immune system to identify and destroy foreign invaders, such as bacteria and viruses. In effect, the antibody has crossed signals, mistakenly attacking the body as though reacting to a pathogen. Focal inflammatory myopathy describes a condition in which these autoantibodies begin targeting the muscles of the affected animal.

A genetically based familial form has been found to occur in cavalier King Charles spaniels, rottweilers, Dobermans, and samoyeds, in which the masticatory muscles are affected. A similar form, affecting the extraocular muscles, has been seen in golden retrievers.

Symptoms and Types

Masticatory Muscles

• Problems with normal jaw movements
• Inability to pick up a ball
• Inability to get and keep food into the mouth
• Jaw pain
• Muscle swelling
• Progressive loss of muscle mass

Extraocular muscles

• Swelling around the eye
• Protrusion of eyeball from the socket

Causes

Immune-mediated

Diagnosis

You will need to give your veterinarian a thorough history of your dog’s health, including a background history of symptoms. After taking a detailed history, your veterinarian will conduct a complete physical examination on your dog.

Your veterinarian will try to open your dog’s mouth, which often proves unsuccessful in these patients. In an attempt to induce pain and swelling of the muscles so that the source of the problem is more evident, your veterinarian may attempt to manipulate your dog’s jaw muscles. Laboratory tests will include a complete blood count (CBC), biochemistry profile, and urinalysis.

The biochemistry profile may indicate higher levels of serum creatine kinase, indicating muscle injury. More specific testing includes taking a muscle tissue sample, especially important in masticatory diseases. This test can help in reaching a confirmatory diagnosis. More advanced testing may include demonstrating the autoantibodies against the muscle fibers. Diagnostic imaging will include X-ray of the jaw bones and ultrasound of the eye orbit to examine the swollen extraocular muscles. Magnetic resonance imaging may also be used to examine the muscle inflammation.