The onset of masticatory muscle myositis (MMM) may occur suddenly or gradually. It may appear that the dog is in pain when they open their mouth or when they chew. There may be instances in which they do not open their mouths at all.
What is masticatory muscle myositis?
A masticatory muscle is a muscle that is used for mastication (chewing). The jaw muscles and temple muscles are among the most powerful muscles in the body. Myositis is the term used to describe inflammation of the muscles. In masticatory myositis, no other muscles are affected.
What causes masticatory muscle myositis?
The condition of masticatory muscle myositis is an immune-mediated or autoimmune condition in which your immune system mistakenly attacks certain parts of your body. It is the immune system that protects the body from infectious organisms, such as bacteria or viruses. An autoimmune disease occurs when the immune system attacks and destroys its own organs and tissues. In this case, the immune system targets the muscles of mastication preferentially.
Embryologically, the chewing muscles have a special molecular structure because of the unique motor nerves that serve them. The chewing muscles contain type 2M muscle fibres, which cannot be found anywhere else in the body. The immune system inappropriately attacks the fibres of the 2M muscles when masticatory myositis develops. There is still a lack of understanding as to why the immune system does this.
What are the signs of masticatory muscle myositis?
The average patient age is three years old. There are several breeds of dogs that are most commonly encountered, including German shepherds, Labrador retrievers, Doberman pinschers, Golden retrievers, and Cavalier King Charles spaniels. Patients may be either male or female. It is characteristic of the acute phase of the disease that the masticatory muscles are swollen and the eyes appear to bulge as a result of the swollen pterygoid muscles behind them. When this stage of the disease is present, there may be a fever and swelling of local lymph nodes. In order to achieve the best results, therapy should be initiated at this time; however, unfortunately the problem is often not recognised until the muscles begin to waste away (atrophy) and the jaws are rigidly closed, making eating difficult.
It would be expected that muscle atrophy and pain associated with this condition would be symmetrical, but this is not always the case. Certainly, lack of symmetry does not constitute evidence against MMM.
How do I diagnose masticatory muscle myositis?
An enzyme called creatine kinase (CK) can be detected in the blood through a blood test. Normally, high amounts of this substance are not seen in the blood as it is found in muscle cells. However, if muscles become damaged this muscle enzyme leaks into the blood and can be detected at quite high levels suggesting severe muscle damage. Electromyography (which measures electrical activity in muscle) may also be helpful.
Masticatory myositis patients produce antibodies against the 2M muscle fibres. A blood test can be performed to detect these antibodies. Due to the fact that blood sampling is not a very invasive procedure, it is often used to confirm disease at an early stage of the medical evaluation.
In addition to sending the blood sample, you may also collect a biopsy from the temporalis muscle of the head, since measuring the degree of scarring in the muscle will assist in determining the severity of the disease and assessing the likelihood of the pet responding to treatment. An estimated 15 percent of pets will test negative for antibodies even though they have the disease, and a muscle biopsy helps distinguish these patients from those with a general muscle inflammation.
Can masticatory muscle myositis be treated?
As a general rule, treatment consists of suppressing the immune system with high doses (rather than the lower anti-inflammatory doses that are more commonly used) of corticosteroids, such as prednisolone. It is recommended that high doses be maintained until the jaw appears to open normally. After that, the dose may be gradually reduced over a period of six months. Many times, the drug cannot be completely discontinued.
In cases where prednisone therapy is a concern, azathioprine can reduce the amount of prednisone that is required to achieve remission. Azathioprine acts as both a chemotherapy agent and an immune suppressant and should not be used at low doses. When using this medication for a long period of time, blood tests should be monitored. Additionally, ciclosporin, an immunomodulator, has been used as a supplement to steroid therapy.
Premature discontinuation of therapy can result in relapse. A better prognosis can be achieved if treatment is begun as early as possible. The results will be less satisfactory if too much scarring has been caused by the inflammation. To assess the extent of the scarring, a muscle biopsy is recommended.
If the patient has difficulty opening his or her jaw (i.e. trismus), semi-liquid diets may be necessary. Physical therapy may be enhanced by encouraging the use of chew toys. It is common for dogs with end-stage disease to have so much scarring that they are unable to eat effectively and malnutrition is a significant problem.
In the early stages of the disease, corticosteroid treatment provides the best results. It is possible that there will be no response to treatment if the disease has progressed to an advanced stage before treatment is initiated.
What is the prognosis for masticatory muscle myositis?
In a study of 18 dogs with masticatory myositis, short-term follow-up was available in 14 out of 18 cases. The full range of jaw motion was regained in eight out of fourteen patients treated with immunosuppressive doses of prednisone. Five out of 14 dogs had a partial response, i.e., improved jaw movement, but not full range; immunosuppressive doses of prednisone were given to four out of five dogs, and an anti-inflammatory dose of prednisone was given to one out of five dogs. Only one of the 14 patients receiving low dose dexamethasone showed a response. Following the initial treatment, three of thirteen patients with partial or complete responses experienced a recurrence.
In nine of the 14 cases, long-term follow-up (from five months to seven years) was available. Eight patients had no recurrence and good jaw mobility. Five of the eight patients were off all medications, two of the eight patients died of unrelated causes while still taking prednisone, and one patient was still taking prednisone a year after diagnosis. As for the remaining dog, no improvement was observed – he had shown no response to the treatment.