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Date Posted: 11:11:33 07/04/09 Sat
Author: Michelle Convis
Subject: Re: Masticatory muscle myositis
In reply to: Samantha 's message, "Masticatory muscle myositis" on 12:22:24 07/03/09 Fri

A member of my Dogue de Bordeaux club experienced this with one of their dogues. When it happened, I researched it to get info about it, to understand it better. Here's what I found:

Masticatory Muscle 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. In time, the muscles around the head (particularly the temple region) hollow out giving the dog a thin faced look. What is happening? There are plenty of possibilities but here is one that must be ruled out.

What is “Trismus?”

In short, trismus is the inability to open the 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:

A foreign body 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
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), and radiographs to assess the tempero-mandibular (jaw) joints and jaw bones themselves. General blood tests are also drawn including a special test for Masticatory Myositis (the “2M antibody” test). In more advanced cases of Masticatory myositis the patient’s jaws will not open even under general anesthesia.

Polymyositis is difficult to distinguish from Masticatory myositis. Poly myositis 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?

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 pterigoid 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. 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 pterigoid 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 symmetrical atrophy and pain but this is often not the case. Lack of symmetry certainly is not evidence against Masticatory Myositis.

The 2M Antibody Blood test

Thanks to Dr. G. Dian Shelton at the University of California at San Diego, there is a blood test to make the diagnosis of Masticatory Myositis. Positive results are felt to be accurate in all cases (no false positives have been seen with this test) though up to 15% of patients will falsely test negative. In these patients further testing is needed to reach the correct diagnosis.

Specimens for submission are generally sent directly to Dr. Shelton’s lab. For details on preparing samples plus the appropriate forms, one may refer one’s veterinarian to:

http://medicine.ucsd.edu/vet_neuromuscular/

Treatment

In short, treatment is suppression of the immune system usually through high doses (rather than the more commonly used lower “anti-inflammatory” doses) of 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. For details click here.

If therapy is discontinued prematurely, relapse is common.

Semi liquid diets may be needed to feed the patient with trismus. It is important not to try to force the jaws open but encouraging chewing of toys can be helpful physical therapy.

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

J Am Anim Hosp Assoc 28[4]:300-306 Jul/Aug'92 26 Refs

Margi A. Gilmour, DVM, DipACVO; *Rhea V. Morgan, DVM, DipACVO; Frances M. Moore, DVM, DipACVP

Dept. of Urban Practice; College of Veterinary Medicine; University of Tennessee; Knoxville, TN 37901

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Diseases of the Muscles of the Face: Masticatory Muscle Myositis (MMM) and Extraocular Myositis (EOM)

What are masticatory muscle myositis and extraocular myositis?

Myositis is the medical term for inflammation of the muscles. Masticatory muscle myositis (MMM) is an inflammatory disease that affects the muscles of the jaw. "Masticatory" refers to chewing. Extraocular myositis (EOM) is an inflammatory disease that selectively affects the muscles that surround the eye. Extraocular myositis does not affect the muscles of the jaw. Neither type of muscle inflammation affects the muscles of the limbs. Masticatory muscle myositis and extraocular myositis are seen in dogs. Golden retrievers may be more likely to develop extraocular myositis than other breeds.

What causes masticatory muscle myositis and extraocular myositis?

Masticatory muscle myositis and extraocular myositis are considered immune-mediated diseases. An immune-mediated disease is a malfunction of the body's immune defense mechanism in which the immune system is turned against the body rather than its normal function of protecting the body. The immune system actually attacks part of the body, in this case, the muscles.

What are the signs of masticatory muscle myositis or extraocular myositis?

Typical signs associated with masticatory muscle myositis include sudden (acute) or long-term (chronic) pain when the jaw is opened and inability to pick up a ball or put food into the mouth. The muscles may be suddenly (acutely) swollen or the muscles may waste away over time (known as "progressive muscle atrophy"). Typical signs associated with extraocular myositis are sudden (acute) swelling of the muscles of the eye, leading to bulging of the eyes out their sockets. In long-standing cases, the eyes may be sunken within the sockets.

How is masticatory muscle myositis or extraocular myositis diagnosed?

Masticatory muscle myositis or extraocular myositis is diagnosed by medical history, physical examination, and laboratory testing. The veterinarian may suspect masticatory muscle myositis or extraocular myositis after examining the pet, but several tests are needed to confirm the diagnosis. Routine blood tests and urinalysis typically are normal. A blood test that measures a muscle enzyme (serum creatine kinase [CK]) may be abnormal. The serum creatine kinase may be normal or slightly elevated. A muscle biopsy (surgical removal and microscopic evaluation of a sample of the muscle) is required to confirm the diagnosis. Radiographs (X-rays) are performed to identify abnormalities of the jaw that may cause difficulties in jaw movement. An ultrasound examination (visualization of deep tissues by recording ultrasonic waves) of the region surrounding the eye may be performed to help identify any potential masses that could be causing the eyes to bulge out of their sockets.

How is masticatory muscle myositis or extraocular myositis treated?

Reducing the immune system's attack on the muscles treats masticatory muscle myositis or extraocular myositis. High doses of corticosteroids are required to suppress the immune system and arrest the disease process. Improved movement of the jaw, decreased pain, and a return to normal of previously elevated blood tests indicates a positive response to the medication in dogs with masticatory muscle myositis. Decreased swelling of the muscles of the eyes indicates a positive response to medication in dogs with extraocular myositis. Medication, at a low dose, should be continued for a minimum of 6 months.

What is the prognosis for animals with masticatory muscle myositis or extraocular myositis?

The prognosis (outcome) for dogs with masticatory muscle myositis is good if treated early with adequate doses of corticosteroids. Jaw mobility should return to normal unless the disease process has been long standing and significant scar tissue is interfering with jaw function. The prognosis for dogs with extraocular myositis typically is good as the disease is responsive to corticosteroid therapy.

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