Masticatory Muscle Myositis

What It Is

Masticatory muscle myositis (MMM) is a neuromuscular disease in which the muscles used by the dog to chew – the jaw and temporal muscles, which are called masticatory muscles – are inflamed, and it is painful and difficult or impossible for the dog to open its mouth.

MMM is an immune-mediated disease which is reportedly well known in the Cavalier King Charles spaniel as being hereditary. Other breeds in which MMM is most common are German shepherds, Labrador retrievers, Doberman pinschers, and Golden Retrievers.

“Myositis” is a general term for inflammation of the muscles. “Immune-mediated” diseases are conditions in which the immune system may over-react or start attacking the body. MMM has also been known as “eosinophilic myositis” or “atrophic myositis”. The inability to open the mouth is called “trismus”.

This condition is limited to the masticatory muscles because they have a molecular structure, called 2M muscle fibers, which are found nowhere else in the dog’s body. Masticatory myositis results when the immune system’s antibodies specifically target these 2M muscle fibers.

The disorder is known to have an early onset in Cavalier King Charles spaniels, having occurred in Cavalier litter mates as young as twelve weeks of age, and several other CKCSs before six months. The reported typical age of onset is about three years

In a 2005 study, Dr. Diane Shelton, board certified in veterinary internal medicine, concluded:

“In a search of the database of the Comparative Neuromuscular Laboratory over the years 2001-2005, masticatory muscle myositis was confirmed in 11 young Cavalier King Charles Spaniels with onset at less than 6 months of age. In several cases, onset followed within 10 days of vaccinations. Although long-term information is not yet complete, there was resolution of clinical signs of masticatory muscle myositis in most cases. Of interest, other complications such as early onset hypothyroidism and allergies were reported. Additional information will be provided as more is learned about this early-onset masticatory muscle myositis in this breed.”


Symptoms

Masticatory muscle myositis may start suddenly or appear gradually. The main clinical sign is difficulty in opening the mouth (called “trismus”). In the initial (acute) stage, there may also be swelling of the jaw and temporal muscles, pain in the jaw, and bulging eyes due to the swollen muscles behind them. There may be a fever and a swollen local lymph node. The dog will seem to in pain when he tries to open his mouth or attempts to chew and will be reluctant to eat.

If MMM is not noticed early enough, it develops into the chronic stage, in which damaged muscle fibers are replaced with fibrotic connective tissue which results in further restriction of opening the mouth. In the chronic stage, there is noticeable atrophy of the masticatory muscles and inability to open the mouth due to the fibrosis. Instead of appearing swollen, the dog’s jaw and temple muscles will hollow. The fibrosis can become so severe that the mouth can not be opened even under general anesthesia.– a very serious form of lockjaw. Unfortunately, many owners do not recognize that their dogs are having a problem until they reach the chronic phase.


Diagnosis

You may expect your Cavalier to endure a battery of diagnostic tests, including complete physical and neurologic examinations, general and specialized blood tests – there is a unique blood test for masticatory muscle myositis called the “2M antibody” test – as well as a muscle biopsy, x-rays, urinalysis, and electromyography.

The Most Accurate Blood Test for MMM

The 2M antibody blood test detects circulating antibodies against masticatory muscles’ 2M fibers. Only the masticatory muscles have the molecular structure called 2M fibers. Masticatory myositis results when the immune system’s antibodies specifically target these 2M fibers.

The 2M antibody test is an “immunocytochemical” test, in which positives for MMM reportedly have been proven to be 100% accurate, and false negatives for MMM have occurred only 10% to 15% of the time.

The test is conducted by the Comparative Neuromuscular Laboratory at the University of California, San Diego. Serum samples should be sent to the laboratory; its turn around time for reporting results is five to seven week days. The laboratory’s director is Dr. G. Diane Shelton, ACVIM board certified in internal medicine. Contact information is on this website: Comparative Neuromuscular Laboratory

The purpose of some of the tests is to rule out other possible causes of the symptoms. For example, the physical and neurologic examinations are to confirm that clinical signs are limited to the masticatory muscles. Polymyositis is a more generalized muscle inflammation of the masticatory muscles and other muscles but otherwise is difficult to distinguish from masticatory myositis. Other possible causes of such symptoms include temporomandibular joint disorders, and endocrine disorders, such as Cushing’s syndrome and hypothyroidism.

General anaesthesia may be called for to get the dog to open its mouth to check for other possible causes of the pain, such as broken teeth or bones or a dislocated jaw. In advanced chronic cases, the dog’s jaws may not open even under anaesthesia.

If the dog’s jaw or temporal muscles have started to atrophy (a symptom in the chronic stage of MMM), the examining veterinarian should find out if the dog has been treated with lengthy (seven days or longer) corticosteroid therapy for another disorder, since corticosteroids have been known to cause these muscles to atrophy.

The owner should tell the veterinarian about any recent vaccinations of the dog. Dr. Diane Shelton, board certified in veterinary internal medicine, recently reported that several young Cavaliers developed MMM within ten days of vaccinations.


Treatment

It is important to begin therapy of the MMM-affected dog as early as possible, so that the acute stage does not progress to the chronic stage.  However, treatment should not begin before thorough testing and diagnosis.

The standard course of treatment consists of heavy, lengthy doses of corticosteroids, such as prednisone, until the dog’s jaw seems to open normally. Then, the doses may be lowered gradually over six to nine months. If the medication is discontinued too early, the dog may be expected to relapse. So, sufficient dosages of corticosteroids need to be given and for a sufficient period of time. MMM is known to respond well initially to corticosteroid therapy, but a relapse usually will occur if the treatment is discontinued prematurely. In some cases, the drug cannot ever be completely stopped.

Prednisone is a synthetic corticosteroid which serves as an “immunosuppressant” intended to suppress the dog’s immune system. Prednisone may have some harmful side effects. Patients on long term prednisone will drink and urinate excessively. For dogs not able to tolerate the side effects of corticosteroids, another immunosuppressive drug, azathioprine (Azasan, Imuran, Azamun, Imurel) may be prescribed in addition to corticosteroid treatment.

Other drugs which have been considered for treatment of MMM include dexamethasone, which is approximately ten times stronger than prednisone, and cyclosporine, which is another immunosuppressive drug, and colchicine, which is used for its anti-fibrotic properties in liver disease.

The dog may require a semi-liquid diet during initial recovery. Its mouth should not be forced open, but the dog should be encouraged to chew its toys, as a form of physical therapy.


Prognosis

Prognosis of recovery from masticatory muscle myositis is determined by the degree of fibrosis present and the dog’s response to treatment. The prognosis is good if: (a) MMM is detected early, in the initial “acute” stage; and (b) the treatment is appropriate, meaning mainly aggressive immunosuppressive therapy comprised of high doses of medication given long enough to avoid a relapse. “Good” prognosis means that a full or significant partial range of jaw motion is regained.

Dogs which are not treated until they are in the “chronic” stage of masticatory muscle myositis may be expected to have a less favourable prognosis.


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33 Responses to Masticatory Muscle Myositis

  1. Kim says:

    Our 8 year old Brittany, Thursday, was given MMM as a likely diagnosis today. The blood test will go out tonight and we are worried sick. The vet did not want to start steroid therapy before the blood was drawn. Thursday’s only symptom has been a sudden inability to fully open her mouth. She does not seem to be in pain but we giving softer foods just in case. Thank you for all the info provided on this site.

    • Elaine says:

      Hope she is ok now.. Just remember sometimes MMM does not always show up in bloodwork.
      What did they end up finding??

  2. Merran Hamilton says:

    Hi..
    I just stumbled across this info. I have a 4 year old German Sheoherd who was tested for MMM at the end of last year. He visited two specialist vets one who said he had it and made the diagnosis from the dogs presenting clinical signs. Which were muscle atrophy of his left temporal muscle. He didn’t suggest any treatment other than for me to observe. I was concerned about that given what my own research told me about the disease. My dog had no jaw pain. I took him elsewhere and he had blood tests sent to the US, CT scans, and a muscle biopsy. The blood tests came back negative. And the other tests showed nothing remarkable other than the atrophy of his muscle. He was diagnosed with Idiopathic Trigeminal Nerve Neuritis. He seems well but on occasions he pants profusely even when it’s really cold, his skin breaks out, so he’s treated for that and it appears to be immune related, and I feel that his head shape is continuing to change. He had a beautiful large expressive male head, even the GSD people refer to him as a female if they don’t know him. That never happened before because it was so obvious that he was a male because if his head size/shape. I was wondering if anyone else can relate to this and if so did you consider a second round of blood tests because the first came back negative? I don’t know what else to do. He was not put on any treatment for his muscle atrophy because the second specialist believed it was just a once off. I don’t know what to do. Thanks to anyone who can help…. :)?

    • admin says:

      Thanks for your input. I have not heard of MMM affecting GS but then who is to say it doesn’t. The change of head shape is a good indicator something is going on. Do you have before and after photos you can show your vet? Can your dog chew? Does your dog seem to be in pain or discomfort? I’m not a vet or trained in any way I just did a lot of research when my own dog got MMM and I found it so hard to find information I thought I would try to compile something based on both what I found and my out experience. To me the mis shapen head of your dog is a clear sign there is something serious going on. Maybe not MMM but something. I hope you find a solution. We had several blood tests done on Max to prove it was MMM xx

  3. Jenna says:

    Our boxer was diagnosed with MMM at about 6 months of age. It was very scary, and I felt so helpless. Our vet originally thought she had broken her jaw, but we didn’t know how that would have been possible. We saw a specialist who diagnosed the MMM. She was on steroids for many months, but she got better. Months after starting treatment she was able to pick up her tennis ball again to play. I cried tears of joy the day that happened. :) Anyway, she is 4 years old now and I always watch her closely for any signs of relapse. Everyone going through this should stay positive because your babies can get better!

    • Elaine says:

      So happy she’s on the mend, continue to watch her(for another outbreak) be sure she is keeping her jaw active( tennis balls, sticks, marrow bones etc. :)

  4. Sue says:

    My dog has just been admitted to the vets with suspected mmm! He has been crying in pain trying to open his mouth for only the last 12 hours with no symptoms before that.It is something I have never heard of before so am desperately reading about other peoples experiences

  5. Elaine says:

    Great article. Our 2yr old golden retriever has been diagnosed with MMM, unfortunately. We are able to recognize the early stage when an outbreak is ready to happen, and than start a steroid treatment which usually lasts for about a month. He responds well to them but hate to leave him on them since they can effect the body and temperament negatively. We recently started a liver cleanse and probiotics (both are in a powder form) given by a holistic vet. We are hoping to see positive results within the next few months.
    Any information on how these can help and aid with any outbreaks??

    • admin says:

      Elaine my dog was on the Steroids for months and months at a time and never did we see a change in his temperament. He was always the Same. He was sad though as he couldn’t eat though he would try. So we would soften his food and leave it over night to get really soft so he could almost drink it. He was a good old soul and was always wagging his tail and licking you. I’ve never heard of a holistic vet. They weren’t about in poor Max’s day. I hope it works for you. X

      • Elaine says:

        Thank you for the follow up.. :) we are trying a new treatment or a “remedy” it is in a 100% natural ingredient in form of powder. The vet wants to try it to see if they can keep his immune system up to prevent frequent outbreaks of. MMM. with this.. Here’s hoping!! And keep up the good work/info on this :)

      • Elaine says:

        Also, I look forward to any info you have for sharing. I have a lot of info on this as well, let me know if I can help at all. I am not a vet! but at this point! I have seen a lot and gathered a lot of info on this rare and sad disease. Xo

        • admin says:

          Thank you if you would like to share here about the holistic side I’m sure people would love to read it. thank you for your offer of sharing, that is very kind. x

    • Sue says:

      Can you tell me the signs to look for before an outbreak?

      • admin says:

        Hello Elaine, I am not qualified just a dog owner who has done a lot of research which i have posted here, my own dog Max had this and the first signs we noticed was he was pawing at his mouth and wouldn’t eat he would try but would drop the kibble right out of his mouth, this would be the defining moment i think. He couldnt open his mouth even when you would try to help. another symptom is a stiff neck although our Max didn’t have this. A blood test taken by your vet can confirm or rule out this illness. but it will have to be sent to the states for diagnosis. (or at least in my day this was the case) So there is a lengthly wait for the results sadly. Oh dry eyes also, Max had dry eyes. hope this helps you in some small way.

        • Elaine says:

          Often a blood test will not diagnose this. In fact, Bentley’s came back negative. He has not had an outbreak for 3 months since we started giving him a probiotic powder and a liver cleanse!!
          You can detect an outbreak if your dog likes to hold a tennis ball or stuck, how he eats his food, or even barks. We watch him daily during his activities just to be sure.
          I have never believed that these holistic powders would work, and it could be a strange coincidence but I urge you to look into this if you can
          :) let me know any questions..

  6. Sonya says:

    Our Boxer was diagnosed with MMM and is in the chronic stage. We are trying pred. now but we do not have high hopes unfortunately. We have been going back and forth to the vet for nearly a year but our boy didn’t show the normal signs for the disease. This article helped me a lot with better understanding it. Thank you!

  7. michael says:

    i have a doberman male 3 years old, unfortunately in the country i live in we have no labratory where we can do i biopsy test for mmm.
    all i noticed is that my dog is loosing muscle in his for head at the side and also in his back, there is some sort of hollowness in his back.
    i have been advice to start a course of predisone and anitibiotics but i have no idea on the dosage and length of treatment.
    any help will be grateful.
    regards.
    michael

    • admin says:

      The tests for this are or were sent to the states for confirmation of the condition. And it can take some time. I am not a vet and couldn’t say about dosage your vet would have to do that I’m afraid. My web site is from my own experience and research. I hope it helped you a little. Please soften your dogs food with water as it will be painfull for him to eat. X

  8. emma says:

    We have a Jack Russell terrier who has just been diagnosed with MMM and have been searching to find out more about this. Your article is clearly written and easy to understand – from diagnosis to prognosis I now feel better informed and more at ease. Thank you

    • admin says:

      Thanks Emma, Im Glad you found it of help, i know when our Max had it we couldn’t find any help and it was scary. Hope your pet isn’t too bad. give soft food, like if he is used to kibble soften it with water for an hour before feeding make it easier to chew. wishing you well x

  9. Mick Buldger says:

    Hi there, I check your new stuff on a regular basis.
    Your writing style is witty, keep doing what you’re doing!

  10. Brian McGuinness says:

    Thanks for finally talking about > Masticatory Muscle Myositis | Canine Creche < Liked it!

  11. Max Burns says:

    Howdy! This blog post could not be written much better!
    Looking through this article reminds me of my previous roommate!
    He continually kept preaching about this. I most certainly will
    forward this article to him. Pretty sure he will have a very good
    read. Thank you for sharing!

  12. Ayarza says:

    Perfect work you have done, this website is really cool with great information.

  13. Gerard Roma says:

    It’s really a nice and helpful piece of information.I’m glad that you shared this useful info with us.Please keep us up to date like this.Thanks for sharing

  14. admin says:

    Just thinking about your question, you would think as a lay person that if you are giving treatment for something you are alleviating symptoms and therefore when testing comes along the results will be tainted… but i will look into it …

  15. Gail Noren says:

    If dog is started on prednisone prior to diagnostic testing 2m antibody test), is there a greater likelihood of a false negative?

    • admin says:

      Hi Gail, I will have to try and fond out an answer to that for you as i am not qualified to answer, but i will try to find out for you.

      • Tabitha says:

        I would like to know this as well…..we are considering having our dog checked for this but don’t want meds to disrupt diagnosis….

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