The masticatory muscles are those of the jaw and temporal muscles; they are responsible for chewing. Muscular infections may occur for a variety of reasons -- tetanus, eye abscesses, dental infections and foreign body injuries, for instance -- the most common condition that affects the masticatory muscles is masticatory myositis. While technically not a true infection, masticatory myositis triggers an immune response in the body similar to its response to an infection.
The actual cause of masticatory myositis is unknown. It is an inflammatory disorder whereby the body’s immune system targets type "2M" muscle fibers found only in the masticatory muscles. Similar to how the immune system targets foreign bodies in infections, such as bacteria or viruses; masticatory myositis causes the immune system to attack the muscle fibers, leading to inflammation of the muscles.
Symptoms of masticatory myositis include muscle swelling in the face and forehead, eye bulging, prolapsed inner eyelids, drooling, muscle loss and fever. Pain and inflammation in the jaw muscles prevent your dog from opening his mouth or chew food, leading to weight loss.
While masticatory myositis can affect any dog of any breed or age, certain breeds show a greater risk of occurrence. German shepherds, Doberman pinschers and retrievers are at greatest risk. Typically, the condition surfaces in young and middle-age dogs more often than in senior dogs.
Diagnosis begins with a clinical examination by a veterinarian to rule out other underlying causes of symptoms, such as tooth infections or lodged foreign bodies. Blood tests, including a 2M antibody test, X-rays, CT scans and electromyography help to confirm a diagnosis of masticatory myositis.
Treatment consists of immunosuppressive drugs and corticosteroids to reduce inflammation and suppress the immune system. If eating is difficult or impossible, transitioning to soft foods or even tube-feeding may be required. In many cases, treatment must continue for the life of the dog. Often soft toys aid in jaw movement and physical therapy for the muscles. Early diagnosis and treatment help to reduce scarring caused by extended muscle inflammation. Extensive scarring contributes to a poor prognosis.