Masticatory Muscle Myositis in Dogs

German Shepherd Dogs and Cavalier King Charles Spaniels are somewhat predisposed to masticatory muscle myositis, although it can occur in any dog. Difficulties such as swallowing, and therefore, eating and drinking can lead to serious consequences for your pet. If your dog is showing signs of having jaw discomfort, take him to the veterinary clinic for an evaluation.

The masticatory muscles are used in chewing. They include the jaw muscles and the muscles of the temples. Therefore, masticatory muscle myositis is inflammation of the muscles in the jaw and temple. It affects the trigeminal nerve and can lead to atrophy of the muscles and dogs may have a dropped jaw that cannot stay closed.

The symptoms of masticatory muscle myositis can vary depending on the muscles that are affected. If you notice any of these symptoms contact your veterinarian for an appointment to have a full assessment done on your dog.

• Swelling of the muscles on the top of the head
• Progressive muscle loss
• Difficulty moving the jaw
• Difficulty drinking
• Difficulty eating or picking up food
• Unable to open the mouth
• Eyes look sunken
• Eyes look protruding
• Stiff movements
• Weakness
• Regurgitation of food and water
• Difficulty swallowing
• Difficulty breathing

Masticatory muscle myositis can be caused by several things. 

Infection or Virus

Some form of an infectious agent, parasite or virus within the muscle can cause inflammation within the masticatory muscles. 

Abnormal Immune Reaction

An abnormal immune reaction of the body against the muscle is known as immune-mediated myositis. The body produces antibodies that will target parts of the masticatory muscles. This will cause inflammation.

Cancer

Inflammation within the masticatory muscles can develop as pre-cancerous and then turn into cancer. Cancer found within the body can cause an immune reaction, causing masticatory muscle myositis. This is known as a paracancerous effect.

Dogs that have been diagnosed with masticatory muscle myositis produce antibodies against the 2M muscle fibers. There is a blood test that was developed at the University of California at San Diego that will test for these antibodies. 

A biopsy from the temporalis muscle is also recommended to determine the severity of masticatory muscle myositis. There is scarring in the muscle and by examining a biopsy of the muscle, it can be determined how far the disease has progressed and assess your dog’s ability to respond to the recommended treatment.

Your veterinarian will prescribe a treatment plan that is aimed at attempting to counteract the body’s immune system response by suppressing the immune system with immunosuppressant medications. A dose of prednisone, a steroid, is also a common part of the treatment plan. 

The short-term treatment plan will be put in place to return your dog’s immune system to normal by using aggressive medications and high doses. Once the disease is being controlled, the medications are slowly reduced. The long-term goal is to eventually get your dog completely off all medications. Most cases, however, require a continual use of a low dose medication. 

Physical therapy may also be required depending on the severity of the disease. Physical therapy will encourage proper chewing and swallowing. Never force your dog’s jaws open.

Severe cases of masticatory muscle myositis may require surgery to remove a portion of the front jaw, allowing your dog to be able to lap water and food. While surgery may give you more time with your dog, it is a last resort option and the prognosis is extremely guarded.

With early detection, masticatory muscle myositis may be treated successfully, although it is a difficult disease to manage. Be sure to follow the treatment plan that your veterinarian prescribes for your dog to ensure recovery. All medications must be given as prescribed, if you notice any side effects, contact your veterinarian immediately.

Return to your veterinarian for all follow-up visits and post-treatment testing to make certain that the disease is being well controlled. Your veterinarian may adjust the medications being given as the treatments progress. 

Early detection of masticatory muscle myositis is important to a good prognosis. Dogs requiring surgery because of severe muscle scarring will have a guarded prognosis.