Masticatory muscle myositis (MMM) in dogs is an autoimmune condition in which the dog becomes unable to open the mouth due to the immune system targeting the chewing muscles (muscles of mastication). Integrative veterinarian Dr. Julie Buzby explains how to recognize the symptoms of MMM, the steps your vet will take to reach a diagnosis, and what treatment entails. Then she ends by discussing the outlook for dogs with MMM.
If your dogs are food-motivated, the thought of filling their bowls with food and watching them NOT gobble up dinner seems impossible, right? Unfortunately, it isn’t. Many situations can affect even the most food-focused dog’s ability or desire to eat.
For example, with the immune-mediated disorder masticatory muscle myositis, dogs may become unable to open their mouths without significant pain. This understandably leads to them not eating well or not eating at all.
What is masticatory muscle myositis (MMM) in dogs?
Masticatory muscle myositis (MMM) is an immune-mediated disorder in dogs that causes inflammation and damage to the muscles responsible for chewing and biting (masticatory muscles). These muscles, including the masseter, pterygoid, and temporalis, contain a unique type of muscle fibers (myofibers) called 2M fibers. Interestingly, the 2M fibers are not found anywhere else in the body.
When the immune system starts seeing the 2M fibers as foreign, it triggers an inflammatory response affecting the muscles of mastication. As a result, dogs develop an inability to open their mouths (trismus), have trouble chewing, and experience significant facial pain.
What causes MMM in dogs?
MMM is often considered immune-mediated. However, veterinarians also suspect infection (Toxoplasma gondii, Neospora caninum), stress (poor diet, concurrent illnesses, hormone imbalances), medications, vaccine history, and genetics may play a role in its onset.
While once considered rare, the incidence of masticatory muscle myositis diagnosis in dogs is increasing with improved diagnostic techniques. MMM is now considered one of the more common neuromuscular disorders in dogs.
Which dogs are most prone to developing MMM?
While it occurs in puppies, too, MMM tends to be most common in young adult (around three years old) large breed dogs. Some predisposed breeds include:
- Doberman Pinschers
- German Shepherds
- Golden Retrievers
- Labrador Retrievers
- Rottweilers
- Staffordshire Bull Terriers
- Weimaraners
Interestingly, Cavalier King Charles Spaniels are also at risk despite being considered a small-breed dog.
What are the symptoms of MMM?
The symptoms of masticatory muscle myositis are variable depending on how long it has been going on and what the dog’s particular disease course looks like. Sometimes there is a distinct acute phase, followed by a chronic phase. In other cases, the changes are more subtle at first. When that happens, dog parents may not be aware there is an issue until the MMM is more chronic.
Important note: If your dog is having difficulty opening his or her mouth, NEVER try to force the jaws apart. Not only is this incredibly painful, but too much force can dislocate or fracture the jaw. Any oral examination that your dog requires should be completed by your veterinarian, likely under anesthesia.
Signs of sudden (acute) MMM
Acute cases of masticatory myositis in dogs will typically have a rapid onset of clinical signs, including:
- Painful swollen muscles of mastication: As inflammation worsens, the muscles become painful and swollen. This interferes with a dog’s ability to open the mouth, eat, drink, and play with toys. Dogs may also be reluctant or unable to yawn.
- Drooling: Due to pain and difficulty moving the jaw, excessive drooling in dogs is possible.
- Exophthalmos (bulging appearance around the eyes): Swelling of the masseter muscles that sit behind and below the eye socket may make it look like the eyeball is pushed forward or the area around the eye is swollen.
- Fever: If you take your dog’s temperature, you may find it is higher than 103°F. This constitutes a fever, which may be a consequence of the inflammation.
- Lethargy: A lethargic dog may lie around or sleep more and seem to have less energy.
- Swollen lymph nodes: The dog’s lymph nodes near the affected muscle tissue may be larger. Most commonly, the submandibular lymph nodes under the dog’s chin are the ones that become swollen in MMM.
Symptoms of chronic MMM
Over time, prolonged inflammation and muscle damage lead to muscle scarring (fibrosis) and loss (atrophy). Dogs aren’t usually painful at this stage, but may show some of the following symptoms of chronic MMM:
- Inability to open the jaw: As the muscle scars down, the dog will have continued trouble opening the jaw.
- Drooling (ptyalism): Dogs with chronic MMM also have poor jaw function, so saliva may dribble out of the mouth.
- Difficulty eating (dysphagia): When the jaw muscles are weak and stiff, it makes it hard for the dog to pick up and chew his or her food.
- Weight loss: Over time, an inability to eat enough to meet caloric needs can lead to weight loss.
- Muscle atrophy: The muscles of mastication, which lie on the top of the head and by the jaw, will become smaller (atrophied). This makes the skull bones more prominent. Usually this happens on both sides, but it can sometimes happen on just one side.
Note: Interestingly, up to 44% of dogs with MMM will have symptoms involving their eyes. In addition to exophthalmos, as previously discussed, an elevated third eyelid in dogs, conjunctivitis, goopy eyes in dogs, enophthalmos (sunken eye) or even blindness are possible.
Seek veterinary care
If you notice some of the signs above, please make an appointment with your veterinarian. Getting a prompt diagnosis is one of the best ways to help your sweet dog on the road to recovery.
How will the vet diagnose a dog with MMM?
To reach an accurate diagnosis of MMM in dogs, the vet will rely on clinical signs and physical exam findings, blood tests (including an ELISA test), muscle biopsies, and sometimes imaging or EMG (electromyography).
History and physical exam
The vet will start by talking with you about the symptoms you are noticing at home. Then he or she will perform a complete physical exam. Some exam findings that point to masticatory muscle myositis include:
- Swollen or atrophied muscles of mastication
- Pain when palpating the muscles of mastication or putting gentle pressure on the eyeball
- Difficulty opening the jaw
- Swollen lymph nodes
- Fever
- Eye changes (enophthalmos, exophthalmos, discharge, redness, etc.)
However, there are also other conditions, such as jaw trauma or dislocation, retrobulbar abscess (infection behind the eye), TMJ disorders, tetanus, nerve sheath tumors, severe dental disease, etc., that may have similar signs. It’s worth noting, though, that dogs with MMM have a stiff jaw that will not open under anesthesia. This is in contrast to many of the other similar-appearing conditions.
Blood tests
The vet may perform blood tests for dogs and a urinalysis to gather more information. Potential blood and urine abnormalities may include:
- Elevated eosinophils: A complete blood count (CBC) may show elevated levels of specific white blood cells, including eosinophils, when inflammation is present. For this reason, some veterinarians also refer to MMM as eosinophilic myositis.
- Anemia: Sometimes anemia in dogs, a decrease in circulating red blood cells, is present.
- Elevated globulins: Globulins are proteins produced by the liver and the immune system. They may increase in dogs with infections or generalized inflammation.
- Elevated creatine kinase (CK): This enzyme helps with energy production for skeletal muscle cells to function efficiently. With muscle damage or inflammation, CK leaks out of muscle cells, leading to higher levels in the blood.
- Elevated AST: AST levels primarily increase in response to liver damage. But they may also be elevated in disorders affecting red blood cells and muscle tissue.
- Proteinuria: Dogs with MMM may have elevated levels of protein in the urine due to generalized inflammation.
ELISA test
If MMM is high on the differential list, the vet may also run an ELISA test to detect circulating 2M fiber antibodies. The presence of 2M antibodies indicates the immune system is targeting the muscles of mastication.
This is typically a reliable way to confirm or exclude MMM as a diagnosis. However, starting steroid therapy before running the 2M ELISA test, or allowing the condition to progress to a chronic stage with visible muscle wasting, can lead to a false negative result.
Muscle biopsy
In cases where the ELISA test is negative, but the symptoms still point to MMM, your veterinarian can collect a muscle biopsy (usually from the temporalis muscle) with your dog sedated. Then he or she will submit the sample to the diagnostic laboratory for evaluation by a veterinary pathologist.
Additional testing
Sometimes dogs may need other diagnostic tests including:
- Imaging: X-rays, CT, and ultrasound may help identify other conditions that could be responsible for your dog’s symptoms. Possibilities include old trauma to the temporomandibular joint or developmental disorders that affect the jaw.
- Electromyography (EMG): Measuring electrical activity in muscles helps differentiate acute-phase MMM from generalized myositis in dogs, which affects muscle tissues throughout the body. Plus, it may help the vet determine where to take the biopsy sample from.
What is the treatment for MMM in dogs?
While, like most immune-mediated diseases, MMM can’t be cured, there are steps you and your vet can take to manage the condition. Overall, treatment of masticatory muscle myositis focuses on keeping your dog comfortable and reducing inflammation as quickly as possible. That may involve the following medications and strategies:
Steroids
Steroids (prednisone for dogs or prednisolone) are the first line of defense in relieving inflammation and suppressing the immune response in acute cases of MMM. Often, dogs will need to stay on a high dose of steroids for at least four weeks (or until jaw function normalizes). Then the vet will slowly taper the steroids over six to eight months to decrease the risk of an MMM relapse.
It is important to keep in mind that higher doses of steroids can also cause muscle atrophy on the head. Thus, if your dog seems to be doing well but starts losing muscle on the head, it may be due to the steroids, not chronic MMM.
For dogs with chronic MMM, sometimes a lower dose of steroids can be beneficial to decrease the fibrosis. However, if there isn’t a change within a few weeks, the vet may taper and then discontinue the steroids. It is likely at that point that the changes to the jaw are not reversible.
Other immunosuppressants
Other immunosuppressants, including azathioprine or cyclosporine, may be added to the treatment protocol if your dog doesn’t respond well to the initial therapy or if the steroids cause too many unpleasant side effects.
Pain medications
Pain medication is also essential for keeping your dog comfortable during the early stages of recovery.
While NSAIDs (nonsteroidal anti-inflammatories) like carprofen for dogs or meloxicam are the first choice for pain management in many situations, dogs who are taking steroids for MMM should not receive NSAIDs. Concurrent steroid and NSAID use in dogs have been linked to an increased risk of digestive upset and stomach ulcers.
Thus, to manage jaw pain in conjunction with steroid therapy, the vet may prescribe something like gabapentin for dogs. This medication is generally safe to combine with steroids and controls pain by reducing the transmission of pain signals.
Offering soft food
Offering soft food (either a canned diet or kibble soaked in warm water) is a great way to help your dog eat with minimal pain. Your vet may also recommend a specific high-calorie diet so that your dog doesn’t have to eat as much to get the nutrients he or she needs.
Thankfully, dogs with MMM usually don’t have too much trouble drinking water. However, it is still important to keep an eye on your dog’s water consumption.
While it can be worrying to see your dog struggle to eat and drink, do not try to force-feed him or her food or water with a syringe unless instructed by your vet to do so. Syringe feeding can sometimes increase the risk of aspiration and make your dog’s discomfort worse.
If you have concerns about your dog’s ability to eat or drink, please reach out to the veterinary team. They would be happy to guide you on safe ways to support your dog’s nutrition and hydration.
Feeding tube
Occasionally, severely affected dogs may need an esophageal feeding tube or gastrotomy tube. This allows them to get the food and water they need while they are unable to take in sufficient amounts by mouth.
Encouraging chewing
Once your dog is less painful, chewing on safe chew toys for dogs can be a good way to rebuild masticatory muscle strength and function.
Apoquel
A 2024 Journal of Veterinary Dentistry study investigated the use of Apoquel in treating dogs with MMM. Apoquel (oclacitinib) is a prescription medication approved for use in dogs with allergy symptoms. While this study’s sample size was very small (three dogs), researchers concluded that Apoquel has the potential to alleviate symptoms of MMM without causing the side effects that typically occur with long-term steroid administration.
Antibody titers to the 2M myofibers did not return to normal. But clinically, the dogs showed improvement in their ability to open their mouths without pain. The potential benefit of this medication for dogs with masticatory muscle myositis seems promising.
Acupuncture for dogs with MMM
Acupuncture for dogs may potentially help relieve pain and inflammation in acute cases of MMM. However, it will not reverse the fibrosis and scar tissue that develop in dogs with chronic symptoms.
Natural treatments for MMM
Certain nutritional supplements may help alleviate inflammation associated with MMM. Ingredients like omega-3 fatty acids for dogs, turmeric, boswellia, and ashwagandha all have potential benefits. However, it’s crucial to speak with your veterinarian or seek guidance from a holistic or integrative veterinarian before adding any supplements to your dog’s daily routine.
Also, keep in mind that these natural treatments are generally additions to the treatment plan. Your dog may still need steroids or immunosuppressants to control the immune response.
What kind of monitoring do dogs with MMM need?
At home, it will be important that you keep a close eye on your dog. If you think that he or she isn’t eating or drinking enough or still seems painful, contact your vet.
Additionally, you should watch for any medication side effects. With high-dose steroids, it isn’t uncommon for dogs to drink more, urinate more, and act hungrier. Plus, being on steroids may also increase the risk of infections such as UTIs in dogs. If you have concerns, don’t modify your dog’s medication dose yourself as this could lead to a relapse. Instead, reach out to your vet for advice.
During the treatment process, the vet will likely want to recheck your dog periodically. He or she will weigh your dog to ensure caloric intake is sufficient, assess your dog’s clinical signs (and sometimes bloodwork), and ask about any medication side effects. Based on that information, the vet may make changes to the treatment plan.
What is the prognosis for dogs with MMM?
The long-term prognosis for MMM depends on the severity of the condition at the time of diagnosis and your dog’s response to therapy. When masticatory muscle myositis is caught in its acute stage and treated promptly, the prognosis is typically good, as long as the dog hasn’t yet developed jaw muscle wasting or scar tissue.
Generally, dogs with MMM regain jaw function within four weeks of starting treatment. But it may take up to 12 weeks in some cases. Across several studies, 57-91% of dogs responded to treatment, and somewhere between 22% and 55% were able to come off prednisone without a relapse.
However, in one study, 27.3% of dogs with MMM experienced a relapse of the condition either while still being treated or once off prednisone. Relapses can also be more challenging to manage if treatment is discontinued prematurely and symptoms reappear.
Sadly, dogs with chronic cases of MMM will lose muscle mass (muscle atrophy), leading to a sunken appearance to the eyes and facial muscles. Chronic MMM is not painful. But as the muscle bellies begin to shrink and develop scar tissue (fibrosis), dogs will still have trouble opening their jaws and eating due to those stiff and weak muscles.
Be proactive and partner with your vet
If you notice a change in your dog’s appetite or ability to eat, it’s a good idea to reach out to your veterinarian as soon as you can. Early diagnosis and timely treatment give your dog the best chance of recovering from MMM or whatever else is causing the discomfort.
In the case of MMM, staying in close communication with the veterinary team will also be important. Treatment of masticatory muscle myositis is a marathon, not a sprint. While the road may seem long, remember that every day that passes is one day closer to your dog getting back to scarfing down his or her food and catching balls again.
Has your dog suffered from MMM?
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