Masticatory Muscle Myositis (MMM) in Dogs

Overview of Masticatory Muscle Myositis in Dogs

Masticatory muscle myositis (MMM) is an inflammatory condition involving the muscles of mastication or chewing in the dog. MMM is caused by an immune-mediated process targeted against specific muscle fibers; the immune system attacks the body’s own muscles.

MMM occurs in all breeds of dog, but appears to be more common in German shepherds, Doberman pinschers, and retrievers. Young and middle-aged dogs are most commonly affected. MMM has not been reported in the cat.

The disease occurs in both acute (sudden) and chronic forms, and the signs may vary with each form. The chronic form is seen more commonly.

What to Watch For

Symptoms of masticatory muscle myositis in dogs may include:

• Swelling associated with facial and forehead muscles
• Fever
• Pain upon opening the mouth
• Reluctance to eat or chew
• Excessive salivation
• Bulging of the eyes
• Prolapse (protrusion) of the third eyelids
• Shrinkage of head muscles
• Inability to open the mouth normally
• The latter two signs are recognized the most often, as they are associated with the chronic form of the disease.

Diagnosis of Masticatory Muscle Myositis in Dogs

• History and physical exam
• Complete blood count (CBC)
• Biochemical profile
• Serum creatine kinase
• Serum type 2M myosin antibody titer
• Muscle biopsy and immunocytochemistry
• Electromyography (EMG)
• Skull and dental radiographs (X-rays) or other imaging modality such as CT scan

Treatment of Masticatory Muscle Myositis in Dogs

• Immunosuppressive doses of corticosteroids
• Additional immunosuppressive drugs if clinically indicated
• Dogs that are severely affected and not immediately responsive to medical management may require a feeding tube.

Home Care and Prevention

• Recheck examination every 3 to 4 weeks during the first several months of therapy is important to monitor response to treatment and allow the veterinarian to taper the medications appropriately. In many cases, long-term or even lifelong therapy may be necessary.
• Feeding soft foods at home until the dog’s signs have improved is imperative. Home management of a feeding tube may be required as well.
• Avoidance of rawhides or other chew toys is advisable.
• There are no preventative measures to reduce risk of MMM.

In-depth Information on Masticatory Muscle Myositis in Dogs

MMM is an inflammatory condition which specifically affects the muscles of mastication or chewing. These include the masseter (cheek muscle), temporalis (muscle on top of the head), and pterygoid muscles. These muscles are composed of a unique type of fiber that is not found in other skeletal muscles. The disease is believed to be an immune-mediated or autoimmune process (these terms are synonymous) because the immune system inappropriately makes antibodies directed against these special muscle fibers. The end result of the antibody production is destruction of the muscle fibers, resulting in inflammation and cell death.

Historically, two distinct diseases were thought to exist, but the current belief is that one disease process exists, and that it may be seen clinically as either acute or chronic disease. The chronic disease is much more commonly recognized, and dogs are usually presented with the complaint of inability to open the mouth normally or sometimes even at all. Despite limited jaw mobility, these dogs are not usually in pain.

Owners may also notice atrophy of the head muscles, which may cause a pointy or skull-like appearance to the head and a sunken appearance to the eyes. Otherwise, these dogs are bright, alert and active. Dogs with the acute form of the disease may be lethargic, have a fever, react painfully to touching the head, experience excessive salivation and be reluctant to chew or eat. They may also have enlargement of local lymph nodes. Although the chronic disease may be a sequela to episodes of acute disease, dogs with signs of chronic disease rarely have a history of previous episodes consistent with acute disease.

Other diseases that may cause signs similar to MMM include:

• Traumatic, inflammatory and cancerous conditions affecting the mouth, teeth, skull or eyes. This might include fractures, tooth root abscesses, oral ulcers or stomatitis (inflammation in the mouth), oral, nasal or retrobulbar (behind the eye) tumors, and luxation (dislocation) of the temporomandibular joint (where the jaw attaches to the skull). These conditions would likely mimic the signs seen with the acute form of MMM.
• Polymyositis (PM). PM is a more widespread muscle disease that affects multiple muscles throughout the body. It is also believed to be the result of an immune-mediated process. Muscle atrophy of the muscles of mastication is usually an important feature of this disease; however, the dogs have clinical signs of weakness, and possible lameness, as well as reluctance to exercise.
• Trigeminal neuropathy. This disease is caused by loss of normal nerve function of the trigeminal nerve, which supplies nerve branches to the muscles of mastication. Loss of normal input from the nerve causes atrophy of the muscles; however, this results in a dropped jaw or inability to close the mouth, rather than inability to open it.
• Infectious myositis. Myositis can also be caused by infectious organisms including: protozoal infections (Toxoplasmosis, Neosporosis, Hepatozoonosis), infections with spirochetes (Leptospirosis), and heartworm infection. These also cause diffuse muscle disease rather than disease localized to the muscles of mastication.

In-depth Information on Diagnosis

A thorough history and physical exam are imperative to making an appropriate diagnosis. Although there are many disease processes that may have signs similar to acute MMM, there are fewer causes of signs consistent with chronic MMM. Physical exam findings are important in order to determine the appropriate course of action with respect to the diagnostic work-up. Tests may include:

• Complete blood count. The CBC evaluates both the red and white blood cells. In acute cases of MMM, it is not uncommon to see mild anemia (low red blood cell count), a neutrophilia (elevated neutrophil count) and sometimes an eosinophilia (elevated eosinophil count). The neutrophils and eosinophils are two different types of white blood cells. The CBC is usually within normal limits in chronic cases of MMM.
• Biochemical profile. The biochemical profile is an appropriate way to assess other organ systems, including the liver and kidneys. It is useful in ruling out systemic disease, as may be seen with some of the infectious causes of myositis. It also evaluates the proteins in the blood, which are composed of albumin and globulin. Elevated globulin levels may sometimes be seen with acute MMM, and can be a marker of inflammation and/or antibody production.
• Serum creatine kinase (CK). CK measurements may be elevated in dogs with the acute form of the disease. This is a muscle enzyme that increases in response to muscle damage. A lack of CK elevation does not rule out the disease, but a high CK level may support the diagnosis.
• Serum type 2M myosin antibody titer. The 2M antibody titer is a test which detects circulating antibodies against the specific muscle fibers (2M) that are damaged in MMM. This is a very useful test, as 81 percent of dogs with MMM will have positive test results. However, the absence of a positive test does not rule out the disease. Additionally, the test may become negative if dogs have been treated with corticosteroids prior to obtaining the serum sample.
• Muscle biopsy. Muscle biopsy is the gold standard for diagnosing MMM. The acute form is characterized by specific inflammatory cell infiltrates in the muscle and resultant muscle cell death. The chronic form of the disease can also demonstrate inflammatory cells, but there is also muscle fibrosis (scarring) seen, due to the prolonged nature of the condition. Muscle biopsy can help distinguish MMM from infectious causes of myositis as well as neurologic causes of muscle atrophy.
• Immunocytochemistry. This special staining technique is performed on the biopsy tissue to evaluate for the presence of antibodies against the 2M muscle fibers. It is similar to the 2M antibody titer, but it looks for antibodies at the tissue level rather than in the blood. As many as 85 percent of dogs with MMM have positive test results with immunocytochemistry.
• Electromyography (EMG). EMG studies evaluate the muscles by looking at their electrical activity. This test can be useful for determining if muscle disease is limited to one set of muscles, as is the case with MMM. It can also help distinguish between primary muscle disease and disease caused by neurologic dysfunction of the nerves supplying the muscles in question. It is not useful, however, for distinguishing between different causes of muscle disease.
• Radiography or CT Scan. Radiographs or CT scan images of the head and/or teeth may be useful in cases where traumatic or cancerous causes of the dog’s clinical signs are suspected. They may not be particularly useful in chronic cases of MMM.

In-depth Information on Treatment

The therapy of MMM, whether acute or chronic, is centered around suppressing the immune system. Because the disease is the result of an immune-mediated process, the goal of therapy is to inhibit the immune system, thereby halting the antibody induced destruction of the muscles. Immunosuppressive therapy is not a benign process, as it makes the body more susceptible to infection. Therefore, it is important that a confirmed diagnosis be established prior to starting this type of treatment.

• Corticosteroids at high doses are the mainstay of treatment for MMM. Corticosteroids are widely used in veterinary medicine, and different doses have differing effects. High dose steroid therapy must be used in order to achieve suppression of the immune system. Most dogs show clinical improvement quite rapidly following institution of this therapy. Most common is prednisone.
• Additional immunosuppressive drugs may be required if the patient does not show adequate response with steroid therapy alone or if there are adverse side effects seen with the steroid use. A drug in this class is azathioprine (Imuran®). This also suppresses the immune system, but does so via a different mechanism than corticosteroids.
• Feeding tubes may occasionally be required in dogs that do not show rapid improvement and are not able to open their mouths far enough to eat. There are various styles of feeding tubes available. The least invasive form of tube feeding involves placement of a small bore tube through the nose into the esophagus. Tubes can also be surgically placed or placed with the aid of an endoscope into the esophagus or stomach for feeding purposes.

Follow-up Care for Dogs with Masticatory Muscle Myositis

Optimal treatment for your dog involves a combination of home and professional veterinary care. Follow-up can be critical, especially if your dog does not rapidly improve.

Administer all prescribed medications as directed. Alert your veterinarian if you are having problems treating your dog. Follow-up after the initial 3 to 4 weeks of therapy is very important, as drug dosages may be gradually decreased depending on the pet’s clinical response to therapy. Corticosteroids must be very slowly tapered over a several month period, and should never be abruptly discontinued, as this can make your dog quite ill. Some dogs need to be treated with lifelong therapy if disease relapses occur.

Because of the increased risk of secondary infection when being treated with immunosuppressive drugs, owners must watch for problems. Infections are most commonly seen in the urinary and respiratory tracts, as well as the skin. If you notice straining to urinate or blood in the urine, red, irritated or itchy skin, or coughing and/or nasal discharge, these could all be signs of infection. Contact your veterinarian if any of these symptoms arise.

Certain side effects of high dose corticosteroid therapy are to be expected: Dogs will drink and urinate more than normal, may pant, and may have an increased appetite. Adverse effects of this therapy may include gastrointestinal upset and even bleeding or ulcer formation. This may manifest as vomiting, diarrhea, dark and tarry stools, or general weakness. Any of these adverse effects should be brought to the attention of your veterinarian immediately.