Masticatory muscle myositis (MMM) is an autoimmune disease in which a dog's immune system attacks its own chewing muscles. The classic sign is a dog who suddenly can't or won't open its mouth, often with swollen, painful muscles on the top and sides of the head. Left untreated, those muscles shrink and scar, and the jaw can stay stiff for good.
A specific blood test (the 2M antibody test) confirms it, and immune-suppressing treatment from your veterinarian works best when it starts early. Most dogs treated in the early phase do well, although relapses are common.
If your dog can't eat or drink, or an eye suddenly bulges, call your vet now. The full when-to-call list is below.
What Is Masticatory Muscle Myositis in Dogs?
"Masticatory" means chewing, and "myositis" means muscle inflammation. MMM is not one muscle but a group. The temporalis on top of the head, the masseter on the cheek and the pterygoid muscles deep behind the eye close the jaw. The front part of the digastricus, under the jaw, helps open it.
Why only these muscles? They are built from a special fiber type, called type 2M, that the leg and body muscles do not have. In MMM the immune system makes antibodies against 2M fibers, so the attack stays on the head and the limbs are spared. If "autoimmune" is new to you, our guide to autoimmune diseases in dogs explains the idea.
MMM is a dog disease first. It has been reported only rarely in cats.
Why Does MMM Happen?
Nobody knows exactly. One theory is molecular mimicry: the immune system reacts to an infection, and the antibodies or T cells (white blood cells that direct immune attacks) it makes also recognize the 2M fibers because the two look alike. That is still a theory.
Some dogs seem genetically more prone to immune-mediated disease. Suspected triggers include infection, vaccination, stress, allergens, reactions to medication and environmental toxins. In most dogs the trigger is never found, and that does not change the treatment.
Signs of Masticatory Muscle Myositis in Dogs
MMM has two phases, and they look quite different.
Acute phase (early) | Chronic phase (later) | |
|---|---|---|
Head shape | Swollen muscles on top of the head and cheeks | Sunken temples; the bony ridge on top of the skull and the cheekbones stand out |
Jaw | Can't open the mouth, or yelps when it opens | Still can't open the mouth fully; scar tissue can lock it |
Eating | Drops food, refuses hard food and chews, won't pick up toys | Poor appetite, weight loss |
Eyes | Sometimes bulging eyes | Sometimes sunken eyes |
Other | Sometimes fever and swollen lymph nodes in the first weeks | Muscle wasting on both sides of the head |
The acute pain can be severe: many dogs stop eating, stop carrying toys and hate having their face touched.
Watch the eyes. The pterygoid muscles sit right behind them, so when they swell they can push the eyes forward. Bad swelling can stretch the optic nerve and threaten vision, which is why a bulging eye is a call-now sign.
Most owners don't notice a problem until the chronic phase, when the muscles have already shrunk. A dog with a newly "bony" head and a mouth that won't open wide has often had MMM for a while.
MMM usually starts in young adult dogs. The average age at diagnosis is about 3 years, but it has been reported in puppies as young as 12 weeks.
When to Call Your Vet
Call now
- Your dog can't eat or drink at all
- One or both eyes are bulging, or your dog seems to have trouble seeing
- Severe pain: crying, hiding, won't let you near the face
- Weak, flat or dehydrated after days of not eating
Call today for an appointment
- Your dog can't open its mouth fully, or yelps when opening it, yawning or chewing
- Swollen muscles on top of the head or on the cheeks
- Dropping food, refusing kibble or treats, no longer picking up toys
- Temples that look newly sunken, or a head that looks bonier than before
Already on treatment? Call your vet if
- Swelling, pain or a stiff jaw comes back while the medicine is being lowered
- Your dog is eating less or losing weight
- Side effects bother you, such as heavy thirst and urination or accidents in the house
Never force or pry your dog's mouth open to check. Forced opening can fracture the jaw or dislocate the jaw joint, and it does not help.
Which Dog Breeds Are Predisposed to MMM?
MMM can happen in any breed and in both males and females. Large breeds and young adults are affected more often. Breeds that show up repeatedly in veterinary reports include:
- German Shepherds
- Golden Retrievers
- Labrador Retrievers
- Doberman Pinschers
- Rottweilers
- Weimaraners
- Cavalier King Charles Spaniels, which appear to have a genetic predisposition and can be affected as young puppies
A breed on this list does not mean your dog will get MMM. It means that if your dog shows the signs above, MMM belongs high on your vet's list.
How Is Masticatory Muscle Myositis in Dogs Diagnosed?
Speed matters here, because muscle that has turned into scar tissue does not come back.
Exam first. Your vet does a full physical and neurological exam to see whether the problem is limited to the chewing muscles. A proper look inside the mouth often needs heavy sedation or anesthesia in these dogs.
Bloodwork. Routine tests include creatine kinase (CK), an enzyme that leaks out of damaged muscle. CK is often high in the acute phase and often normal in the chronic phase, so a normal CK does not rule MMM out.
The 2M antibody test. This blood test looks for the antibodies that attack 2M fibers. Signs that fit MMM plus a positive test confirm the diagnosis. According to a review from the Comparative Neuromuscular Laboratory at UC San Diego, the test picks up about 85 to 90 percent of dogs with MMM and essentially never flags a dog that doesn't have it.
Why the test comes before steroids. If a dog has already had immune-suppressing steroids for about a week or more, the test can come back falsely negative. Dogs in the very late, scarred stage can also test negative. Tell your vet about every medicine your dog has had, including anything another clinic gave for the "sore mouth."
Muscle biopsy. A small sample, usually from the temporalis, shows how much inflammation and scarring there is. It helps when the antibody test is negative and predicts how much jaw function can come back. It needs anesthesia, which carries extra risk in these dogs (see below).
Imaging. X-rays, CT or MRI rule out jaw fractures and joint problems and can show inflamed muscles early.
Infections. Your vet may also test for infections that can look similar, such as Neospora and Toxoplasma, before starting immune-suppressing treatment.
What Else Can Stop a Dog Opening Its Mouth?
Problem | What you might see | How vets tell it apart |
|---|---|---|
Masticatory muscle myositis | Can't open the mouth; swollen, painful head muscles, then wasting on both sides | 2M antibody test, biopsy |
Trigeminal neuritis | The opposite: the jaw hangs open and the dog can't close it; usually not painful | Exam, MRI, spinal fluid |
Trigeminal nerve sheath tumor | Wasting on one side of the head only | MRI |
Jaw joint injury, fracture or old fused joint | Often after trauma; mouth won't open or close normally | X-rays or CT |
Abscess or mass behind the eye | Pain opening the mouth, sometimes a bulging eye or swelling behind the back teeth | Mouth exam under sedation, imaging |
Tetanus | Stiff jaw that won't open | Exam and history |
Polymyositis | Weakness, stiff gait or muscle loss in the body and legs, not just the head | 2M test is negative; limb muscle biopsy |
Long-term steroid treatment can also thin the chewing muscles on its own, so muscle loss in a dog already on steroids is not automatically MMM getting worse.
How Vets Treat MMM
Treatment calms the immune attack before scar tissue replaces the muscle, so vets push for early, steady treatment instead of a short course.
The main treatment is an immune-suppressing steroid, usually prednisone or prednisolone. Your vet sets the dose and the taper. It starts at an immune-suppressing level and stays there until the jaw moves well and bloodwork such as CK has settled, then comes down slowly, usually over months.
That slow taper is the whole game. MMM often responds quickly, then relapses if treatment stops too soon or drops too fast, and dogs that relapse are less likely to go back into remission. Some dogs can eventually stop medicine; others need a low dose for life. Never stop or change the dose yourself.
If steroids alone don't control MMM, or the side effects are hard to live with, your vet may add another immune-suppressing drug, with regular rechecks and bloodwork. Pain medicine may be needed early on; only give what your vet prescribes.
Steroid side effects to expect and report:
- Drinking and urinating more
- Bigger appetite and weight gain
- Higher risk of infections and stomach ulcers with long use
- Cushing's-like changes with long-term use (read more about Cushing's disease in dogs)
- Some loss of chewing muscle from the steroids themselves
Jaw stretches don't help either. Forcing the jaw open, even under anesthesia, risks fractures and joint dislocation without improving the outcome, and physical therapy is generally not needed: in early cases, treating the inflammation brings back normal or near-normal jaw movement.
Expect regular rechecks of jaw movement, weight and bloodwork. Your vet may repeat the antibody test to track progress.
Feeding a Dog Who Can't Open Its Mouth
A stiff jaw makes eating hard, so your vet will build feeding into the plan. The usual options are:
- Softened food. Kibble soaked in warm water until soft, or canned food, may be enough for a dog who can open its mouth a little.
- A gruel. For a dog with very little jaw movement, food blended into a thick, smooth gruel is often the best option.
- A log. Note how much your dog eats and drinks and its weight, and bring it to rechecks. Call your vet if your dog can't drink or keeps losing weight.
If your dog still can't take in enough, your vet has backups: nutrition through an IV line in severe early cases, or a feeding tube in long-standing cases. A feeding tube sounds dramatic, but it keeps the dog fed and hydrated while the medicine works.
Before any sedation or anesthesia
Tell every vet who sees your dog that it has MMM, especially before dental work or any procedure under anesthesia. A jaw that won't open can make placing a breathing tube difficult and raises the risk of inhaling vomit or saliva. Dangerous tongue swelling during anesthesia has also been reported. The anesthesia team can plan for it if they know.
Prognosis of Masticatory Muscle Myositis in Dogs
The outlook is generally good when MMM is caught in the acute phase and treated firmly. Most dogs treated early regain normal or near-normal jaw movement. In one UK referral study of dogs treated with a corticosteroid, 93% had improved within two weeks of starting treatment and all of them by ten weeks.
The outlook is less certain once scar tissue has formed. Dogs treated in the chronic phase can still improve, but the jaw may not open fully again and the head muscles may stay thin.
Relapses are common, and each one tends to be harder to treat, which is why vets taper slowly and keep up rechecks. A muscle biopsy gives the most accurate forecast: if it shows mostly scar tissue and few remaining muscle fibers, a full return of jaw movement is unlikely.
MMM Questions From Vet Teams
What does the 2M antibody test need?
A serum sample sent to a specialist neuromuscular laboratory, such as the Comparative Neuromuscular Laboratory at UC San Diego, following that lab's current submission instructions. Draw it before any immune-suppressing steroids if you can.
How accurate is the 2M antibody test?
The test is an ELISA (serum in wells coated with masticatory muscle antigen; bound antibody gives a measurable color change) and has largely replaced the older tissue-staining test with the same accuracy.
- Sensitivity (dogs with MMM that test positive): about 85 to 90%, so a negative result does not fully rule MMM out, especially after steroids or in end-stage disease.
- Specificity (dogs without MMM that test negative): reported as 100%, so a positive result with compatible signs confirms MMM.
Do you need a muscle biopsy as well as the 2M test?
Not always for the diagnosis, but it shows the fibrosis that shapes the prognosis. Biopsy the temporalis, not the overlying frontalis, which is not affected, and weigh the anesthesia risk in a dog with trismus.
Originally written by Dr. Catharina Hjorth. Updated by the I Love Veterinary editorial team.
Sources
- Merck Veterinary Manual: Masticatory Myositis in Dogs and Cats
- Merck Veterinary Manual (pet owner version): Muscle Disorders in Dogs
- Melmed C, Shelton GD, Bergman R, Barton C. Masticatory Muscle Myositis: Pathogenesis, Diagnosis, and Treatment. Compendium, 2004 (UC San Diego Comparative Neuromuscular Laboratory)
- VCA Animal Hospitals: Masticatory Myositis in Dogs
- Pitcher GD, Hahn CN. Atypical masticatory muscle myositis in three cavalier King Charles spaniel littermates. J Small Anim Pract, 2007
- Foreman M, Cherubini GB. Dexamethasone for masticatory muscle myositis in dogs. Top Companion Anim Med, 2021
- Nanai B, et al. Life threatening complication associated with anesthesia in a dog with masticatory muscle myositis. Vet Surg, 2009
- Cauduro A, et al. Use of MRI for the early diagnosis of masticatory muscle myositis. J Am Anim Hosp Assoc, 2013
- Southeast Veterinary Neurology: Masticatory Muscle Myositis, Trigeminal Neuritis, and Trigeminal Nerve Sheath Tumors in Dogs
- Barnes Veterinary Neurology: Masticatory Muscle Myositis
This article is for general information and is not a substitute for advice from your veterinarian.
