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What It Is
Masticatory muscle myositis (MMM) is a neuromuscular disease in which the muscles used by the dog to chew – the jaw and temporal muscles, which are called masticatory muscles – are inflamed, and it is painful and difficult or impossible for the dog to open its mouth.
MMM is an immune-mediated disease which is reportedly well known in the Cavalier King Charles spaniel as being hereditary. Other breeds in which MMM is most common are German shepherds, Labrador retrievers, Doberman pinschers, and Golden Retrievers.
“Myositis” is a general term for inflammation of the muscles. “Immune-mediated” diseases are conditions in which the immune system may over-react or start attacking the body. MMM has also been known as “eosinophilic myositis” or “atrophic myositis”. The inability to open the mouth is called “trismus”.
This condition is limited to the masticatory muscles because they have a molecular structure, called 2M muscle fibers, which are found nowhere else in the dog’s body. Masticatory myositis results when the immune system’s antibodies specifically target these 2M muscle fibers.
The disorder is known to have an early onset in Cavalier King Charles spaniels, having occurred in Cavalier litter mates as young as twelve weeks of age, and several other CKCSs before six months. The reported typical age of onset is about three years
In a 2005 study, Dr. Diane Shelton, board certified in veterinary internal medicine, concluded:
“In a search of the database of the Comparative Neuromuscular Laboratory over the years 2001-2005, masticatory muscle myositis was confirmed in 11 young Cavalier King Charles Spaniels with onset at less than 6 months of age. In several cases, onset followed within 10 days of vaccinations. Although long-term information is not yet complete, there was resolution of clinical signs of masticatory muscle myositis in most cases. Of interest, other complications such as early onset hypothyroidism and allergies were reported. Additional information will be provided as more is learned about this early-onset masticatory muscle myositis in this breed.”
Masticatory muscle myositis may start suddenly or appear gradually. The main clinical sign is difficulty in opening the mouth (called “trismus”). In the initial (acute) stage, there may also be swelling of the jaw and temporal muscles, pain in the jaw, and bulging eyes due to the swollen muscles behind them. There may be a fever and a swollen local lymph node. The dog will seem to in pain when he tries to open his mouth or attempts to chew and will be reluctant to eat.
If MMM is not noticed early enough, it develops into the chronic stage, in which damaged muscle fibers are replaced with fibrotic connective tissue which results in further restriction of opening the mouth. In the chronic stage, there is noticeable atrophy of the masticatory muscles and inability to open the mouth due to the fibrosis. Instead of appearing swollen, the dog’s jaw and temple muscles will hollow. The fibrosis can become so severe that the mouth can not be opened even under general anesthesia.– a very serious form of lockjaw. Unfortunately, many owners do not recognize that their dogs are having a problem until they reach the chronic phase.
You may expect your Cavalier to endure a battery of diagnostic tests, including complete physical and neurologic examinations, general and specialized blood tests – there is a unique blood test for masticatory muscle myositis called the “2M antibody” test – as well as a muscle biopsy, x-rays, urinalysis, and electromyography.
The 2M antibody blood test detects circulating antibodies against masticatory muscles’ 2M fibers. Only the masticatory muscles have the molecular structure called 2M fibers. Masticatory myositis results when the immune system’s antibodies specifically target these 2M fibers.
The 2M antibody test is an “immunocytochemical” test, in which positives for MMM reportedly have been proven to be 100% accurate, and false negatives for MMM have occurred only 10% to 15% of the time.
The test is conducted by the Comparative Neuromuscular Laboratory at the University of California, San Diego. Serum samples should be sent to the laboratory; its turn around time for reporting results is five to seven week days. The laboratory’s director is Dr. G. Diane Shelton, ACVIM board certified in internal medicine. Contact information is on this website: Comparative Neuromuscular Laboratory
The purpose of some of the tests is to rule out other possible causes of the symptoms. For example, the physical and neurologic examinations are to confirm that clinical signs are limited to the masticatory muscles. Polymyositis is a more generalized muscle inflammation of the masticatory muscles and other muscles but otherwise is difficult to distinguish from masticatory myositis. Other possible causes of such symptoms include temporomandibular joint disorders, and endocrine disorders, such as Cushing’s syndrome and hypothyroidism.
General anaesthesia may be called for to get the dog to open its mouth to check for other possible causes of the pain, such as broken teeth or bones or a dislocated jaw. In advanced chronic cases, the dog’s jaws may not open even under anaesthesia.
If the dog’s jaw or temporal muscles have started to atrophy (a symptom in the chronic stage of MMM), the examining veterinarian should find out if the dog has been treated with lengthy (seven days or longer) corticosteroid therapy for another disorder, since corticosteroids have been known to cause these muscles to atrophy.
The owner should tell the veterinarian about any recent vaccinations of the dog. Dr. Diane Shelton, board certified in veterinary internal medicine, recently reported that several young Cavaliers developed MMM within ten days of vaccinations.
It is important to begin therapy of the MMM-affected dog as early as possible, so that the acute stage does not progress to the chronic stage. However, treatment should not begin before thorough testing and diagnosis.
The standard course of treatment consists of heavy, lengthy doses of corticosteroids, such as prednisone, until the dog’s jaw seems to open normally. Then, the doses may be lowered gradually over six to nine months. If the medication is discontinued too early, the dog may be expected to relapse. So, sufficient dosages of corticosteroids need to be given and for a sufficient period of time. MMM is known to respond well initially to corticosteroid therapy, but a relapse usually will occur if the treatment is discontinued prematurely. In some cases, the drug cannot ever be completely stopped.
Prednisone is a synthetic corticosteroid which serves as an “immunosuppressant” intended to suppress the dog’s immune system. Prednisone may have some harmful side effects. Patients on long term prednisone will drink and urinate excessively. For dogs not able to tolerate the side effects of corticosteroids, another immunosuppressive drug, azathioprine (Azasan, Imuran, Azamun, Imurel) may be prescribed in addition to corticosteroid treatment.
Other drugs which have been considered for treatment of MMM include dexamethasone, which is approximately ten times stronger than prednisone, and cyclosporine, which is another immunosuppressive drug, and colchicine, which is used for its anti-fibrotic properties in liver disease.
The dog may require a semi-liquid diet during initial recovery. Its mouth should not be forced open, but the dog should be encouraged to chew its toys, as a form of physical therapy.
Prognosis of recovery from masticatory muscle myositis is determined by the degree of fibrosis present and the dog’s response to treatment. The prognosis is good if: (a) MMM is detected early, in the initial “acute” stage; and (b) the treatment is appropriate, meaning mainly aggressive immunosuppressive therapy comprised of high doses of medication given long enough to avoid a relapse. “Good” prognosis means that a full or significant partial range of jaw motion is regained.
Dogs which are not treated until they are in the “chronic” stage of masticatory muscle myositis may be expected to have a less favourable prognosis.