or EOSINOPHILIC MYOSITIS
It may start suddenly one day or come on gradually. The dog seems to be painful when his mouth opens or 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 one's mouth. Regardless of whether or not the act is 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 simply hurts to open the mouth or if it is not possible to open the mouth. Here are the possible explanations:
Sorting these out will require general anesthesia to get the mouth open and check for painful oral conditions (broken teeth, oral foreign body, growths inside the mouth). It may be necessary to take radiographs to assess the temporo-mandibular (jaw) joints and jaw bones themselves. If nothing is found in the mouth to explain the problem, tests for masticatory myositis should be considered.
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 muscle) may also be helpful.
WHAT ARE “MASTICATORY” MUSCLES?
(original graphic by marvistavet.com)
The masticatory muscles are the muscles used in mastication (chewing). They include the powerful jaw muscles and muscles of the temples (the temporalis muscles, the masseter muscles, the pterygoid muscles and the rostral digastricus muscles). The word “myositis” literally means “muscle inflammation.” No other muscles are affected in Masticatory Myositis.
The masticatory muscles are all innervated by the Mandibular branch of the Trigeminal nerve.
The masticatory muscles are all innervated by the Mandibular branch of the Trigeminal nerve. Any disease that affects the Trigeminal nerve will lead to marked atrophy of the muscles of mastication. These patients, however, have dropped jaw that cannot stay closed rather than trismus.
WHAT MAKES THESE MUSCLES SO UNIQUE THAT A DISEASE PROCESS WOULD AFFECT ONLY THEM?
Embryologically, the chewing muscles have a special molecular structure because of the unique motor nerve branches that serve them. Chewing muscles contain what are called type 2M muscle fibers, which occur no where else in the body. Masticatory myositis arises when the immune system inappropriately attacks these 2M muscles fibers. What causes the immune system to do this is still unknown.
PROFILE OF THE MASTICATORY MYOSITIS PATIENT
The average patient age is 3 years. 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.
One 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 medical work up.
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% 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 a more general muscle inflammation.
Specimens for submission can be sent directly to Dr. Shelton’s lab.
In short, treatment is 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. After that, the dose may be gradually tapered 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. For more details on chronic prednisone therapy click here.
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 are recommended with long term use. Alternatively, cyclosporine, an immunomodulator, has been used to supplement the steroid treatment.
If therapy is discontinued prematurely, relapse is common. Prognosis is better the earlier treatment begins. If too much scarring has been caused by the inflammation, 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 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 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.
Results of corticosteroid treatment are best early in the course of the disease.
In a study of 18 dogs with Masticatory Myositis: Short term follow up was available in 14/18 dogs. Complete response, i.e., full range of jaw motion regained, was seen in 8/14 with 8/8 treated with immunosuppressive doses of prednisone. Partial response i.e., improved but not full range of jaw movement, was seen in 5/14 dogs--immunosuppressive doses of prednisone were given in 4/5 and an antiinflammatory dose of prednisone given in 1/5. No response was seen in 1/14 who was treated with low dose dexamethasone. Recurrence following initial treatment was seen in 3/13 with partial or complete response initially.
Long term (5 mos to 7 years) follow up was available in 9/14. Eight had no recurrence and good jaw mobility and 5/8 were off all medication, 2/8 died of unrelated causes while still on prednisone, and 1 was still on prednisone 1 year postdiagnosis. The remaining dog was the one who had shown no response--no improvement was seen.
Masticatory myopathy in the dog: A retrospective study of 18 cases
Page last updated: 9/26/2019