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.