It may start suddenly one day or come on gradually. The dog seems to be in pain when his mouth opens or when he attempts to chew. Perhaps he will not open his mouth at all. There are many possible explanations for this situation, and masticatory myositis is only one of them. In fact, most possibilities are far less exotic than the immune-mediated muscle disease that is the subject of this article, so a step-by-step approach is important.

What is Trismus?

In short, trismus is the inability to open your mouth. Regardless of whether or not it’s painful to attempt, the mouth simply cannot be opened. Of course, a dog with a painful mouth may be unwilling to open his jaws and there is no way to ask the dog if it simply hurts to open his mouth or if it is not possible to open it. Possible explanations are:

• A foreign body (such as a bone, stick, or part of a broken toy) may be stuck in the soft tissues of the mouth.
• Tetanus infection.
• An abscess behind the eye (which almost always stems from an injury inside the mouth).
• Dislocation of the jaw (the jaw can actually be fused closed if there is enough arthritis).
• Polymyositis (a general muscle inflammation).
• Muscular dystrophy.
• Craniomandibular osteopathy (a jaw bone growth abnormality).
• Masticatory myositis.
• Painful dental disease.

Polymyositis is difficult to distinguish from masticatory myositis. Polymyositis is a more generalized muscle inflammation involving other muscles beyond those of mastication. Polymyositis patients will be negative on the 2M antibody test but so are up to 15% of patients with masticatory myositis. A muscle biopsy may be necessary to distinguish these conditions. Electromyography, which measures electrical activity in muscles, may also be helpful.

What are Masticatory Muscles?

Graphic by Dr. Wendy Brooks

The masticatory muscles are the muscles used in mastication (chewing). They include the powerful jaw muscles and temple muscles. The word myositis means muscle inflammation. No other muscles are affected in masticatory myositis.

What Makes These Muscles So Unique That a Disease Process Would Affect Only Them? 

The chewing muscles have a special molecular structure because of the unique nerve branches that serve them. Chewing muscles contain what are called type 2M muscle fibers, which occur nowhere else in the body. Masticatory myositis arises when the immune system inappropriately attacks these 2M muscle fibers. What causes the immune system to do this is still unknown.

Profile of the Masticatory Myositis Patient

The average patient age is three years old. The most common breeds are German shepherds, Labrador retrievers, Doberman pinschers, Golden retrievers, and Cavalier King Charles spaniels. Patients can be of either gender. In the acute phase of the disease, the masticatory muscles are swollen and the eyes appear to bulge due to the swollen pterygoid muscles behind them. There may be a fever and local lymph node swelling at this stage. Results are best if therapy is initiated at this point, but unfortunately, many owners do not notice the problem until the muscles begin to atrophy and the jaws are rigidly closed, making eating difficult.

You would expect the muscle atrophy and pain of this condition to be symmetrical, but this is not always the case. Lack of symmetry certainly is not evidence against masticatory myositis.

Diagnosis of Masticatory Myositis

Patients with masticatory myositis produce antibodies against the 2M muscle fibers, and thanks to Dr. G. Diane Shelton and her lab at the University of California at San Diego, it is possible to test for these antibodies with a blood test. Since blood sampling is not very invasive, this is often done to confirm the disease early in a medical workup.

That said, Dr. Shelton recommends collecting a biopsy from the temporalis muscle in addition to sending the blood sample as, the degree of scarring in the muscle will be helpful in staging the severity of the disease and in assessing the patient's ability to respond to treatment. Approximately 15 percent of patients will test negative for antibodies even though they have the disease, and the muscle biopsy also helps sort these patients out from those with more general muscle inflammation.

Specimens for submission can be sent directly to Dr. Shelton’s lab.

Treatment

In short, treatment is the suppression of the immune system, usually through high doses (rather than the more commonly used lower anti-inflammatory doses) of corticosteroids such as prednisone or dexamethasone. High doses should be maintained until the jaw seems to open normally, a process that can be as short as one month.

After that, the dose may be gradually tapered off over 6 months. In many cases, the drug cannot ever be completely stopped.

Patients on long-term prednisone will drink and urinate excessively. Screening for latent bladder infection is important. See more details on chronic prednisone therapy.

If prednisone therapy is problematic, azathioprine can be used to spare the amount of prednisone necessary to achieve remission. Azathioprine is an agent of chemotherapy as well as an immune suppressive agent and is not used lightly. Monitoring blood tests is recommended for long-term use. Alternatively, cyclosporine, an immunomodulator, has been used to supplement the steroid treatment.

If therapy is discontinued prematurely, relapse is common. The earlier treatment begins, the better the prognosis. If the inflammation has caused too much scarring, the results are not as good. As mentioned, a muscle biopsy is helpful in assessing the extent of the scarring.

Semi-liquid diets may be needed to feed the patient with trismus. Do not try to force the jaws open, as this can dislocate or even break the jaw. Encouraging the use of chew toys, however, can be helpful in physical therapy. Dogs with end-stage disease have so much scarring that they cannot eat effectively, and malnutrition is a big problem. Surgery may be useful to remove a portion of the front jaw to allow the dog to be able to lap food with the tongue. Alternatively, a feeding tube can be placed so that a liquid diet can be directly instilled into the stomach or esophagus. It is important to treat the disease when it is in an early stage, and the prognosis is good so as to avoid these procedures.

In a study of 18 dogs with Masticatory Myositis, short-term follow-up was available in 14 out of those 18 dogs. Complete response, i.e., full range of jaw motion regained, was seen in eight out of the 14 dogs, with all eight of them having been treated with immunosuppressive doses of prednisone. Partial response, i.e., improved but not full range of jaw movement, was seen in five out of 14 dogs--immunosuppressive doses of prednisone were given in four out of those five dogs, and an anti-inflammatory dose of prednisone was given in one of those five dogs. No response was seen in one dog who was treated with low-dose dexamethasone. Recurrence following initial treatment was seen in three out of 13 dogs who had partial or complete responses initially. Long-term (five months to seven years) follow-up was available in nine out of 14. Eight had no recurrence and good jaw mobility, five out of eight were off all medication, two out of eight died of unrelated causes while still on prednisone, and one was still on prednisone 1-year postdiagnosis. The remaining dog was the one who had shown no response--no improvement was seen.

Long-term (five months to seven years) follow-up was available in nine of the 14. Eight had no recurrence and good jaw mobility, five of eight were off all medication, two of eight died of unrelated causes while still on prednisone, and one was still on prednisone one year after diagnosis. The remaining dog was the one who had shown no response and no improvement was seen.