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CANINE MASTICATORY MYOPATHY

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Masticatory Myopathy
What is Masticatory Muscle Myosotis

THE PET HEALTH LIBRARY

By Wendy C. Brooks, DVM, DipABVP
Educational Director, VeterinaryPartner.com
http://www.veterinarypartner.com

MASTICATORY MYOPATHY (EOSINOPHILIC MYOSITIS)

It may start suddenly one day or come on gradually. The dog seems to be painful when his mouth opens or he attempts to chew. Perhaps he will not open his mouth at all. In time, the muscles around the head (particularly the temple region) hollow out giving the dog a thin faced look. What is happening?  There are plenty of possibilities but here is one that must be ruled out.

What is “Trismus?”
In short, trismus is the inability to open the mouth. Regardless of whether or not the act is painful to attempt, the mouth simply cannot be opened. Of course, a dog with a painful mouth may be unwilling to open his jaws and there is no way to ask the dog if simply hurts to open the mouth or if it is not possible to open the mouth.

Here are the possible explanations:
A foreign body may be stuck in the soft tissues of the mouth
 
Tetanus infection
 
An abscess behind the eye (which almost always stems from an injury inside the mouth)
 
Dislocation of the jaw (the jaw can actually be fused closed if there is enough arthritis)
 
Polymyositis (a general muscle inflammation)
 
Muscular Dystrophy
 
Craniomandibular osteopathy (a jaw bone growth abnormality)
 
Masticatory Myositis
Sorting these out will require general anesthesia to get the mouth open and check for painful oral conditions (broken teeth, oral foreign body, growths inside the mouth), and radiographs to assess the tempero-mandibular (jaw) joints and jaw bones themselves.  General blood tests are also drawn including a special test for Masticatory Myositis (the “2M antibody” test). In more advanced cases of Masticatory myositis the patient’s jaws will not open even under general anesthesia.

Polymyositis is difficult to distinguish from Masticatory myositis. Poly myositis is a more generalized muscle inflammation involving other muscles beyond those of mastication. Polymyositis patients will be negative on the 2M antibody test but so are up to 15% of patients with Masticatory myositis. A muscle biopsy may be necessary to distinguish these conditions. Electromyography (which measures electrical activity in muscle) may also be helpful.

What are “Masticatory” Muscles?
The masticatory muscles are the muscles used in mastication (chewing). They include the powerful jaw muscles and muscles of the temples (the temporalis muscles, the masseter muscles, the pterigoid muscles and the rostral digastricus muscles).  The word “myositis” literally means “muscle inflammation.” No other muscles are affected in Masticatory myositis.

The masticatory muscles are all innervated by the Mandibular branch of the Trigeminal nerve. Any disease that affects the Trigeminal nerve will lead to marked atrophy of the muscles of mastication. These patients, however, have dropped jaw that cannot stay closed rather than trismus.

What makes these muscles so unique that a disease process would affect only them?
Embryologically, the chewing muscles have a special molecular structure because of the unique motor nerve branches that serve them. Chewing muscles contain what are called type 2M muscle fibers, which occur no where else in the body. Masticatory myositis arises when the immune system inappropriately attacks these 2M muscles fibers. What causes the immune system to do this is still unknown.

Profile of the Masticatory Myositis Patient
The average patient age is 3 years. The most common breeds are German shepherds, Labrador retrievers, Doberman pinschers, Golden Retrievers, and Cavalier King Charles spaniels.  Patients can be of either gender. In the acute phase of the disease, the masticatory muscles are swollen and the eyes appear to bulge due to the swollen pterigoid muscles behind them. There may be a fever and local lymph node swelling at this stage. Results are best if therapy is initiated at this point but unfortunately many owners do not notice the problem until the muscles begin to atrophy and the jaws are rigidly closed making eating difficult.

One would expect symmetrical atrophy and pain but this is often not the case. Lack of symmetry certainly is not evidence against Masticatory Myositis.

The 2M Antibody Blood test
Thanks to Dr. G. Dian Shelton at the University of California at San Diego, there is a blood test to make the diagnosis of Masticatory Myositis. Positive results are felt to be accurate in all cases (no false positives have been seen with this test) though up to 15% of patients will falsely test negative. In these patients further testing is needed to reach the correct diagnosis.

Specimens for submission are generally sent directly to Dr. Shelton’s lab. For details on preparing samples plus the appropriate forms, one may refer one’s veterinarian to:

http://medicine.ucsd.edu/vet_neuromuscular/

Treatment
In short, treatment is suppression of the immune system usually through high doses (rather than the more commonly used lower “anti-inflammatory” doses) of prednisone or dexamethasone. High doses should be maintained until the jaw seems to open normally. After that, the dose may be gradually tapered over 6 months. In many cases the drug cannot ever be completely stopped.

Patients on long term prednisone will drink and urinate excessively. Screening for latent bladder infection is important.

If prednisone therapy is problematic, azathioprine can be used to spare the amount of prednisone necessary to achieve remission. Azathioprine is an agent of chemotherapy as well as an immune suppressive agent and is not used lightly. Monitoring blood tests are recommended with long term use.
If therapy is discontinued prematurely, relapse is common.

Semi liquid diets may be needed to feed the patient with trismus.  It is important not to try to force the jaws open but encouraging chewing of toys can be helpful physical therapy.

In a study of 18 dogs with Masticatory Myositis: Short term follow up was available in 14/18 dogs. Complete response, i.e., full range of jaw motion regained, was seen in 8/14 with 8/8 treated with immunosuppressive doses of prednisone. Partial response i.e., improved but not full range of jaw movement, was seen in 5/14 dogs--immunosuppressive doses of prednisone were given in 4/5 and an antiinflammatory dose of prednisone given in 1/5. No response was seen in 1/14 who was treated with low dose dexamethasone. Recurrence following initial treatment was seen in 3/13 with partial or complete response initially.

Long term (5 mos to 7 years) follow up was available in 9/14. Eight had no recurrence and good jaw mobility and 5/8 were off all medication, 2/8 died of unrelated causes while still on prednisone, and 1 was still on prednisone 1 year postdiagnosis. The remaining dog was the one who had shown no response--no improvement was seen.

Masticatory myopathy in the dog: A retrospective study of 18 cases
J Am Anim Hosp Assoc 28[4]:300-306 Jul/Aug'92 26 Refs
Margi A. Gilmour, DVM, DipACVO; *Rhea V. Morgan, DVM, DipACVO; Frances M. Moore, DVM,DipACVP
Dept. of Urban Practice; College of Veterinary Medicine; University of Tennessee; Knoxville, TN 37901
Copyright 2006 - 2007 by Wendy Brooks, DVM. Used with permission. All rights reserved
This work was originally published by Veterinary Information
Network, Inc. (VIN) and is republished with VIN's permission.
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What is masticatory muscle myositis?

http://www.upei.ca/cidd

related term: eosinophilic myositis
This disease affects the masticatory muscles - the large muscles on the top and sides of the head which act to close the jaw. These muscles have a unique muscle protein composition. Masticatory muscle myositis is an immune-mediated disease in which the body’s immune system attacks the unique muscle protein found in the masticatory muscles. This causes the muscles to become painful and swollen initially, and the muscles will atrophy as the disease progresses. Opening the mouth is particularly painful.

How is masticatory muscle myositis inherited?
Unknown.

What breeds are affected by masticatory muscle myositis?
The German shepherd dog is most frequently affected, but this disease is also reported in other large breed dogs, including the English pointer and springer spaniel.

For many breeds and many disorders, the studies to determine the mode of inheritance or the frequency in the breed have not been carried out, or are inconclusive. We have listed breeds for which there is a consensus among those investigating in this field and among veterinary practitioners, that the condition is significant in this breed.

What does masticatory muscle myositis mean to your dog & you?
Many dogs affected with masticatory muscle myositis will improve with drug therapy.

How is masticatory muscle myositis diagnosed?
Your veterinarian will suspect this disease if your dog has painful, swollen masticatory muscles, with pain on opening the jaw. The diagnosis can be confirmed with a muscle biopsy to examine the cells with a microscope. Special techniques may be used to test for specific antibodies.

How is masticatory muscle myositis treated?
Affected dogs are usually treated medically, with glucocorticoids such as prednisone. Most dogs improve with drug therapy, although the treatment may be long-term.

Breeding advice
Affected animals should not be bred.
FOR MORE INFORMATION ABOUT THIS DISORDER, PLEASE SEE YOUR VETERINARIAN.

Resources
Kornegay JN. 1995. Disorders of the skeletal muscles. In EJ Ettinger and EC Feldman (eds) Textbook of Veterinary Internal Medicine, p.727-736. WB Saunders Co., Toronto.
Ackerman L. 1999. The Genetic Connection: A Guide to Health Problems in Purebred Dogs, p. 121. AAHA Press,Lakewood, Colorado.
Copyright © 1998 Canine Inherited Disorders Database. All rights reserved.Revised: May 16, 2002.
This database is a joint initiative of the Sir James Dunn Animal Welfare Centre at the Atlantic Veterinary College, University of Prince Edward Island, and the Canadian Veterinary Medical Association.
reprinted with kind permission from:-
 Alice Crook, DVM,Coordinator, Sir James Dunn Animal Welfare Centre, Atlantic Veterinary College,University of Prince Edward Island, 550 University Ave.Charlottetown, PEI C1A 4P3
http://www.upei.ca/cidd

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Masticatory Muscle Myositis
 Masicatory Muscle Myositis


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The above information is simply informational. It's intent is not to replace the advice of a veterinarian nor to assist you in making a diagnosis of your pet. Please consult with your own veterinarian for confirmation of any diagnosis. Your pets life may depend on it.