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Golden Retriever Volume 12 I Winter 2014 Early Diagnosis of Masticatory Muscle Myositis Is Needed for Treatment Success magine what it would be like if your are not present in any other muscle autoimmune disease apply to dogs. Golden Retriever could not open his group and antibodies against this Genes play a role in increasing sus - Imouth to eat and drink. That is often fiber type are not involved in any ceptibility to autoimmune diseases, what happens with an autoimmune other muscle disease. 1 but environmental triggers initiate disorder called masticatory muscle “Masticatory muscle myositis is the onset of clinical signs. myositis (MMM) that affects the jaw an inflammatory myopathy,” Shelton “In dogs that are predisposed to muscles, causing pain and dysfunction. explains. “It is a unique myopathy, or autoimmune reactions, suspected Swollen, painful masticatory muscle disease, in which dogs com - triggers include viral and bacterial (chewing) muscles and an inability to monly have jaw pain and inability to exposures, possibly vaccinations, open the mouth (trismus) are clinical open the jaw. The autoimmune process hormones, stress, allergens, medica - signs of the disorder. “These dogs are in this disease involves circulating tions, and environmental toxins . Most not able to pick up a ball or eat with - antibodies that specifically target the of the time, it is impossible to know out experiencing severe pain,” says with certainty what triggers the Brian E. Greenfield, D.V.M., who prac - The 2M Antibody Blood Test onset of an autoimmune disease.” tices at Animal Clinic Northview in Shelton agrees that MMM is not North Ridgeville, Ohio. “In the early asticatory muscle myositis is purely a genetic disease. “Hormonal, stages of the disease, the muscles that Man autoimmune disorder in environmental and other unknown are used for eating and chewing appear which antibodies attack the 2M factors come into play,” she says. The swollen. As the disease progresses, fibers in the masticatory (chewing) genes responsible for a dog’s suscepti - these muscles begin to atrophy, or muscle group. A blood test was bility to autoimmune diseases are part waste away. Sometimes the eyes developed in 2004 by researchers of the major histocompatibility complex. appear sunken, or, less commonly, at the University of California-San “Goldens seem susceptible to many they seem to protrude.” Diego to confirm the circulating autoimmune muscle diseases, including Although MMM can occur in any antibodies that attack 2M fibers. not only MMM but also polymyositis, breed, it occurs more commonly in The test is available to veterinari - extra ocular myositis and myasthenia large breeds, such as Golden Retrievers, ans through the Comparative gravis,” says Shelton. “Perhaps the most Doberman Pinschers, German Shepherd Neuromuscular Laboratory at the common autoimmune disease in Goldens Dogs, and Labrador Retrievers. Young University of California at San Diego. and many other breeds is autoimmune Cavalier King Charles Spaniels may be For information, visit http://medicine. thyroiditis, the underlying cause of severely affected and are believed to ucsd.edu/vet_neuromuscular . most canine hypothyroidism.” be genetically predisposed to devel - oping MMM. Although the disorder Determining an Accurate Diagnosis does not occur on a widespread basis masticatory muscles. We still don’t Bill Page of Wildwood, Mo., recalls in Golden Retrievers, anecdotal data know what causes the autoantibodies experiencing MMM with his first Golden suggest that the incidence may be to form or why they are directed Retriever many years ago. “’Buddy’ increasing. Thus, whenever a Golden specifically against the type 2M fibers.” was diagnosed when he was about is suddenly unable to open his mouth, One theory is that antibodies or 8 years old,” Page says. “It began as experts recommend immediate vet - T-cells generated in response to an soreness when he chewed, so we erinary care to determine the cause. infectious agent subsequently cross- thought perhaps he had a bad tooth. The condition occurs in males and react with self-antigens. Antibodies The veterinarian examined him and females, with an average age of onset directed against these bacterial anti - diagnosed MMM.” of 3 years, though puppies as young as gens potentially could cross-react with The veterinarian started Buddy right 4 months have been affected. Fortunately, the 2M fibers. In humans, autoantibodies away on prednisone and referred Page if MMM is diagnosed early, dogs can directed at Streptococcus pyogenes to a specialist, who performed a muscle be treated to increase the likelihood have been shown to attack cardiac biopsy that confirmed the diagnosis. of a full recovery. Research of this and skeletal muscle. Pericarditis and (Note: This was before the 2M antibody disease at the University of California- rheumatoid arthritis are examples of blood test was available.) The pred - San Diego led to the development of diseases in which autoantibodies are nisone was effective. One year later, a blood test in 2004 that detects the directed at specific myofibers. 1 the signs of MMM recurred, and Buddy presence of 2M antibodies and accu - Rhonda Hovan, research facilitator again began prednisone therapy, which rately identifies affected dogs. for the GRCA and a member of the effectively managed the condition. Pathologist Diane Shelton, D.V.M., club’s Health & Genetics Committee, “The quick diagnosis by our vet - Ph.D., DACVIM, and her team at the says, “The genetic components of erinarian in the beginning made all Comparative Neuromuscular Laboratory autoimmune diseases are very complex. the difference in Buddy’s outcome,” identified type 2M fibers in the mas - Although a great deal of research has Page says. ticatory muscle group and showed that been done to identify the causes of An accurate diagnosis is important in antibodies against type 2M fibers are human autoimmune diseases, much treating dogs with MMM. Early detection involved in the pathogenesis of MMM. less research has been done in dogs. and aggressive immunosuppressive They found that type 2M muscle fibers Still, the same overall principles of Continued on page 2 Masticatory Muscle Myositis help veterinarians differentiate this atrophy may not necessarily indicate continued from page 1 disease from other causes of trismus.” a worsening disease state. Masticatory muscle myositis is not Dogs that relapse may be harder to therapy can help reduce myofiber loss the only cause of a dog’s inability to treat during the subsequent course and muscle fibrosis, which can lead open the jaw. In MMM, the inability of treatment, Shelton says. Those that to irreversible jaw dysfunction and to open the jaw under anesthesia is are treated in the chronic phase of the severe muscle atrophy. a typical finding. However, this also disease have a more uncertain prognosis Complete physical and neurological can occur following trauma to the TM though they can do well if they do not examinations are necessary to determine joint or chronic arthritic changes in the experience persistent jaw dysfunction. that the clinical signs are restricted to TM joints that could restrict movement. “It is important that owners realize the jaw muscles. The most definitive Infrequent causes of restricted jaw that though jaw function should improve confirmation of MMM is the 2M anti - mobility include tetanus, muscular if treated in the chronic phase, there body test, which detects the antibodies dystrophy or other muscle diseases, may be residual fibrosis and muscle that attack and destroy the 2M muscle and even a foreign body in the mouth. atrophy that could be irreversible,” fibers. A blood chemistry profile deter - In these cases, the 2M antibody test explains Shelton. mines the creatine kinase (CK) level, is negative. Regarding breeding recommendations, which may be mildly elevated during the Golden Retriever Club of America the acute phase but becomes normal Treatment & Breeding Recommendations does not consider MMM to be a high- as the disease progresses. Although a As with Page’s Golden Retriever priority disease to target for reduction. blood test may produce a normal CK Buddy, an accurate diagnosis and “This is due to the genetic and environ - level, it does not rule out acute MMM. prompt treatment are vital to help a mental complexities, the low incidence “Clinical signs compatible with MMM dog regain the ability to open his mouth of the disease, the overall good out - and positive results from a 2M anti - without pain. Corticosteroids, partic - come with appropriate therapy, and ularly prednisone, are the cornerstone importantly, the concern for main - Recognizing Signs of MMM of therapy, Shelton says. During the taining as much genetic diversity in acute phase, corticosteroids help to the breed as possible,” Hovan says. achieve aggressive immunosuppression. “Instead, we suggest that breeders wners who recognize these “Immunosuppressive dosages of include MMM along with other auto - signs of masticatory muscle O prednisone should be continued until immune diseases, such as hypothyroidism, myositis (MMM) in their dogs should a dog has maximum jaw function and myasthenia gravis and immune-mediated promptly seek veterinary care. The his CK levels are normal,” says Shelton. hemolytic anemia, as one factor when sooner a dog is properly diagnosed “At that time, the dosage can be they evaluate the overall pros and cons and begins treatment, the greater tapered to the lowest every-other-day of each dog for breeding,” she says. the chance of recovery. dosage that prevents clinical signs. “A dog with a parent, littermate or • Inability to open the jaw (trismus) Once the lowest alternate dosage is offspring that has any autoimmune • Jaw pain reached that keeps the dog free of disease is at slightly increased risk of • Swelling or atrophy of the jaw clinical signs, the alternate-day therapy also having an autoimmune disease muscles should be continued for at least four — but not necessarily the same disease,” • Difficulty eating and drinking to six months. In most cases, this low Hovan says.

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