Dominic Stone
Masticatory muscle myositis (MMM) is an immune-mediated inflammatory condition that affects the muscles involved in chewing. The disease causes these muscles to become swollen and painful, making it difficult for dogs to open their mouths, eat, or chew. Masticatory muscle myositis is characterised by the immune system attacking the unique Type 2M muscle fibres found only in the muscles of the head and jaw. The condition can be acute or chronic, with the acute phase often going unnoticed or confused with other conditions due to vague symptoms. The average age of dogs with MMM is 3 years, but cases have been seen in puppies as young as 3 or 4 months.
| Characteristics | Values |
|---|---|
| Definition | An immune system disorder where the dog's immune system identifies the dog's muscle tissue as foreign and attacks it. |
| Affected Muscles | Masticatory muscles, i.e., muscles involved with chewing. |
| Symptoms | Swollen and painful masticatory muscles, inability to open the mouth, inability to eat, excruciating face pain, lethargy, drooling, vocal changes, gagging, exophthalmos, inappetance, reduced play behaviour, fever, painful swelling of the temporalis and masseter muscles, potentially lymphadenopathy. |
| Diagnosis | Blood test to detect circulating autoantibodies that attack the muscle fibres (2M antibody ELISA assay), muscle biopsy to determine the level of inflammation and severity of fibrosis, diagnostic imaging, immunohistochemistry, serum titer of 2M fiber antibodies, complete blood count, chemistry panel, electromyography. |
| Treatment | Immunosuppressive doses of oral corticosteroids like prednisone, Azathioprine, Cyclosporine, Mycophenolate, Leflunomide, manual opening of the mouth under anaesthesia, feeding soft or liquid food. |
| Prognosis | Depends on the degree of inflammation and response to therapy. Relapses are common, and the disease can be fatal if untreated or in end-stage. |
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What You'll Learn
- Masticatory muscle myositis (MMM) is an immune system disorder
- MMM causes swollen and painful muscles, preventing dogs from eating
- The disease affects Type 2M muscle fibres, unique to the head and jaw
- MMM can be diagnosed through a blood test or muscle biopsy
- Treatment involves steroids and immunosuppressive drugs
Masticatory muscle myositis (MMM) is an immune system disorder
MMM can affect any breed and both males and females, with an average age of 3 years, although cases have been found in puppies as young as 3 or 4 months old. The disorder is characterised by inflammation, necrosis, and atrophy of the masticatory muscles. The acute phase of MMM may include vague symptoms such as lethargy, facial pain, drooling, vocal changes, gagging, and reduced play behaviour. The chronic phase may include muscle shrinkage as the bones of the head become more prominent.
The exact cause of MMM is unknown, but it is believed to be triggered by bacterial or viral infections, vaccinations, stress, exposure to allergens or toxins, reactions to medication, or even cancer. Diagnosis of MMM is done through a blood test that searches for circulating autoantibodies that attack the Type 2M muscle fibres. A muscle biopsy may also be performed to determine the level of inflammation and the severity of fibrosis.
The traditional treatment for MMM is an immune-suppressive dose of corticosteroid medication, usually prednisone, given over several months. The prognosis is good if dogs are treated aggressively, and jaw function and muscle mass can be regained. However, MMM can be recurrent, and relapses may be more difficult to treat.
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MMM causes swollen and painful muscles, preventing dogs from eating
Masticatory muscle myositis (MMM) is an immune system disorder that causes a dog's immune system to identify its own muscle tissue as foreign and attack it. The disorder causes the muscles involved in chewing to become swollen and painful, preventing dogs from eating.
MMM affects the Type 2M muscle fibres, which are unique to the temporalis and masseter muscles of the head and jaw. The disease causes inflammation, necrosis, and atrophy of these masticatory muscles, resulting in extreme pain and preventing the dog from opening its mouth to eat, chew, or play with toys. The average age of dogs with MMM is 3 years, but there have been cases in puppies as young as 3 to 4 months old.
The acute phase of MMM may include vague symptoms such as lethargy, facial pain, drooling, vocal changes, gagging, exophthalmos (protrusion of the eyeball), inappetence, and reduced play behaviour. As the disease progresses to the chronic phase, symptoms may include muscle shrinkage as the bones of the head become more prominent, anorexia, weight loss, and difficulty in opening the jaw.
The diagnosis of MMM is made through a blood test that detects circulating autoantibodies attacking the 2M muscle fibres or a muscle biopsy to determine the level of inflammation and fibrosis. Treatment for MMM typically involves immunosuppressive doses of corticosteroid medication, usually prednisone, given over several months. The prognosis for MMM is good if the dogs are treated aggressively, and jaw function and muscle mass can be regained. However, relapse is common, and MMM can be fatal if left untreated or if the disease progresses to the point that most muscle fibres of the jaw are replaced by fibrous tissue.
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The disease affects Type 2M muscle fibres, unique to the head and jaw
Masticatory muscle myositis (MMM) is an immune-mediated inflammatory condition that affects the muscles of mastication, or chewing. These muscles are involved in the movement of the jaw during the process of chewing. The disease is characterised by inflammation, necrosis, and atrophy of the masticatory muscles, causing them to become swollen and painful.
The disease specifically affects Type 2M muscle fibres, which are unique to the temporalis and masseter muscles of the head and jaw. These muscle fibres are not found elsewhere in the body. The immune system identifies these fibres as foreign and attacks them, causing inflammation. This results in extreme pain and prevents dogs from opening their mouths to eat, chew, or play with toys.
The Type 2M muscle fibres are composed of a unique myofibre that is not present in other skeletal muscles. The immune system's attack on these fibres leads to the development of autoantibodies, which are specific antibodies directed against the 2M fibres. These autoantibodies can be detected through a blood test, which is used to help diagnose MMM.
The presence of autoantibodies against Type 2M muscle fibres is seen in 85-90% of dogs with the acute phase of MMM. However, they may not be present in dogs with the chronic phase of the disease. As the disease progresses, the chronic form of MMM may include muscle shrinkage as the bones of the head become more prominent.
The diagnosis of MMM can be confirmed through a muscle biopsy, which involves the removal and examination of a small section of the affected muscle. This helps to determine the level of inflammation and the extent of fibrous tissue formation due to muscle deterioration. Treatment for MMM typically involves the use of immunosuppressive medications, such as steroids or corticosteroids, to reduce inflammation and manage the immune response.
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MMM can be diagnosed through a blood test or muscle biopsy
Masticatory Muscle Myositis (MMM) is an immune-mediated inflammatory condition that affects the muscles involved in chewing. It causes these muscles to become swollen and painful, making it difficult for dogs to open their mouths to eat, chew or play with toys.
MMM can be diagnosed through a blood test that measures circulating autoantibodies that attack the muscle fibres (2M fibres). This test must be carried out before any treatment is administered, as steroids can cause a false negative result. A muscle biopsy may also be required if autoantibodies are not detected by the blood test. This involves removing a small sample of the affected muscle to determine the level of inflammation and the severity of fibrosis.
The 2M fibres are unique to the temporalis and masseter muscles of the head and jaw and are not found elsewhere in the body. The immune system identifies these fibres as foreign and attacks them, resulting in inflammation. This inflammation can cause the bones of the head to become more prominent, and in some cases, scar tissue can form within the muscles, causing permanent problems.
The blood test used to detect 2M antibodies is called a 2M antibody ELISA assay. A muscle biopsy is needed to determine the progression of the disease and subsequent prognosis. This can reveal histologic changes such as lymphocytic-plasmacytic cellular infiltrates, muscle atrophy, and fibrosis.
The early diagnosis of MMM is important to improve the prognosis and the likelihood of successful treatment. If left untreated, MMM can be fatal as the dog may be unable to eat or drink.
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Treatment involves steroids and immunosuppressive drugs
Masticatory muscle myositis (MMM) is an immune system disorder that causes a dog's immune system to attack its own masticatory muscles, which are the muscles involved in chewing. The disease causes these muscles to become swollen and painful, preventing the dog from opening its mouth to eat, chew, or play.
MMM can be treated with steroids and immunosuppressive drugs. Steroids are typically the treatment of choice, but they can interfere with the 2M antibody test, which is used to diagnose MMM, as they decrease the quantity of circulating antibodies in the blood and may result in a false negative. Therefore, it is important to conduct the blood test before beginning steroid treatment. The current recommended traditional treatment for MMM is an immunosuppressive dose of corticosteroid medication, usually prednisone, given over a period of months. Prednisone is typically prescribed at a dose of at least 1 mg/kg per 12 hours and is tapered by 25% to 50% every 2 to 8 weeks based on clinical signs, medication side effects, and repeat 2M fiber antibody titers. Other immunosuppressive drugs, such as azathioprine, cyclosporine, cytarabine, cyclophosphamide, or mycophenolate, may be prescribed in combination with prednisone to help combat the inflammation. The goal of treatment is to aggressively halt the immune system's attack on the muscle and return the dog to normal as quickly as possible, improving prognosis and preventing permanent damage to the muscles.
It is important to note that chronic steroid use can lead to muscle wasting, and dogs may need other pain medications until the steroids reduce inflammation and circulating antibodies. Additionally, inappropriate doses of immunosuppressive medications or an inappropriate length of time of medication administration can lead to a relapse or worsening of the condition. Therefore, it is crucial to give all medications as prescribed and to work closely with a veterinarian.
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Frequently asked questions
Masticatory muscle myositis (MMM) is an immune-mediated inflammatory condition that affects the muscles involved in chewing. It causes these muscles to become swollen and painful, making it difficult for dogs to open their mouths, eat, or chew.
Symptoms of masticatory muscle myositis include lethargy, facial pain, drooling, vocal changes, gagging, exophthalmos, reduced play behaviour, and an inability to open the mouth. In its acute phase, the symptoms may be vague and confused with other conditions. The chronic phase includes more prominent symptoms such as muscle shrinkage and weight loss.
Masticatory muscle myositis is diagnosed through a blood test that detects circulating autoantibodies attacking the 2M muscle fibres. A muscle biopsy may also be performed to determine the progression of the disease. Treatment typically involves immunosuppressive doses of corticosteroid medications like prednisone, given over several months with a gradual taper.
The prognosis for dogs with masticatory muscle myositis depends on early diagnosis and treatment initiation. If treated promptly and aggressively, dogs can regain normal jaw function and live a normal life with the disease. However, MMM can be recurrent, and relapses may be more difficult to treat. In advanced cases, where most muscle fibres have been replaced by fibrous tissue, the disease becomes incurable.
