Masticatory Muscle Myositis (MMM) is an immune mediated disease where the body starts attacking the specific muslces of mastication (chewing). They're of a different embryologic origin than other muscles, which is why only these muscles are affected and not all muscles.
From my internal medicine book:
In the acute form, hypertrophy of the temporal and masseter muscles is observed with myalgia (painful eating, i think). The animal is reluctant to open its mouth. It has difficult eating and may dribble saliva. Sometimes the jaw remains open because complete closing is impossible. Fever, inflammation of tthe tonsils, and local adenitis may also be found. In most cases myositis of the masticatory muslces is a chronic condition with severe, progressive muscle atrophy accompanied by fibrosis, resulting in a reduced ability to open the mouth and trismus (lockjaw).
The disorder is generally bilateral. Exopthalmos (bulging eye) linked to enlargement of the temporal muscles sometimes can be the cause of an optic neuritis and can result in vision disoders. Biochemical examinations are not specific, but the serum CK level may b be increased and leukocytosis (high white blood cell count) is sometimes found. Electromyography (EMG) reveals abnormal spontaneous activity. Muscle biopsy shows sites of necrosis (death) and phagocytosis (inflammatory cells eating them up) of type 2M fibers (the type particular to these muscles), with perivascular infiltration of mononucleate cells (meaning, white blood cells are coming out of the vessels to attack the 2M fibers because the body is recognizing them as foreign).
Adminstration of immunosuppressive doses of corticosteroids is the only advisable therapy. Clinical recovery and usually rapid and complete if treatment is begun early. Should a relapse occur, the maximum dose should be administered again. Sometimes a prolonged course of treatment is required, with small doses administered on alternate days. Therapy is ineffective if administered to an animal that has already experienced several inflammatory episodes that have resulted in extensive fibrosis.
Textbook of Veterinary Internal Medicine
, Ettinger and Feldman
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