Masticatory myositis is a neuromuscular disorder that affects dogs and literally means inflammation of the muscles involved in chewing (mastication). With this autoimmune disease, the body starts attacking its own cells for unknown reasons. In the early stages, the muscles involved in chewing can become inflamed and painful. In cases of more progressed disease, the muscles involved in chewing become weak and small and the dog is unable to open its mouth. The head will often have a "sunken in" look.

Tests to help diagnose the disorder may include blood work (chemistry, autoantibody levels), and biopsy with histopath analysis.

Treatment involves drugs that suppress the immune system. If needed, a feeding tube may be placed until the dog is able to eat again. In general, a better response to medication is seen when the condition is treated early.