We see many diseases and disorders that involve our pet’s legs, hips, and other muscles, but what if they prevented your pet from eating? What if the disease prevented your dog from even opening his mouth? Masticatory muscle myositis (MMM) is the inflammation of the muscles in the jaw and head that involve chewing. It is also known as eosinophilic myositis or atrophic myositis. There are many kinds of myositis, which literally means inflammation of the muscle tissue ("myo" meaning muscle; "itis" meaning inflammation). Masticatory muscle myositis affects mainly large breeds, but Cavalier King Charles Spaniels and German Shepherds are especially predisposed to the disease. MMM is an auto-immune disease that is caused by the dog's immune system attacking the muscles of the jaw. The presence of 2M fibers (that are not found elsewhere in the dog's body) have a structure similar to that of proteins found on some bacteria, and so the body illicits an immune response to attack the "foreign" substance. A similar disease is found in the eye muscles of Golden Retrievers
MMM can be acute or chronic. Symptoms of acute MMM include swelling in the jaw, drooling, pain in opening the mouth, and anorexia. The eyes can also be affected, as some muscles are connected to both parts of the face. Opthamalic symptoms include third eyelid protrusion, red eyes, and protruding eyeballs. Chronic MMM problems include an atrophy of the jaw muscle (causing the skull and bones of the head to become prominent), malnutrition, weight loss, dehydration, and an inability to open the mouth (trismus). Masticatory muscle myositis causes no neurological or gastrointestinal symptoms, but your dog could exhibit dizziness, vomiting, diarrhea, and other problems if the malnutrition or dehydration is severe.
MMM is diagnosed with a biopsy of the muscle fibers themselves, or a blood test looking for 2M antibodies, or both (the antibody test can have a false negative if in the end stages). Your veterinarian may also have some x-rays taken to rule out problems with the bones or joints of the skull and jaw. It is usually treated with corticosteroids like Prednisone, which calm the body's inflammation response. Pets should also be fed soft or liquid food, and supplementary nutrition and fluids may be given intravenously. Often, the vet will prescribe a high dose of steroids, then attempt to taper off the drugs gradually while monitoring the dog for a relapse.
MMM is by no means fatal, and the prognosis is usually good if diagnosed early on. Sometimes the pet will get "better" (no symptoms present) for a few months, then have a relapse. If the muscles have atrophied and begun to scar, that damage in itself is usually not repairable. The dog may have to remain on a limited diet or liquid food permanently. So if your dog stops eating or shows an abnormality in their face and head, check them out as soon as possible!
Sources: Small Animal Veterinary Association, Textbook of Veterinary Internal Medicine, Merck Veterinary Manual, McFarland Animal Hospital, Davies Veterinary Specialists, www.vetneuromuscular.ucsd.edu