Masticatory myositis is quite an unusual disease. I normally see only about one or two cases a year! It is basically an autoimmune disease, where the body's defence system starts attacking itself. The immune system starts to react and produce antibodies against the fibres in the muscles that control chewing (masticatory muscles) and occasionally, the muscles around the eyes (extraocular). The cause of the immune assault is not completely understood.
It tends to be found in larger dogs like GSD, golden retrievers, weimeraners and rottweilers.
Normally, the dog ends up with painful and swollen muscles making the animal reluctant to open the mouth (leading to less eating). After some time, once the inflammation subsides, the damaged muscle fibres are replaced with fibrous tissue (stiff and rigid tissue). This leads to severe jaw muscle wasting (atrophy) and an inability to open the mouth.
There are some tests to check for antibodies against the muscle fibres. Most of the time though, a muscle biopsy of the jaw muscles may be performed to confirm the diagnosis. Beware though, these dogs can be sometimes impossible to intubate (put airway tube in for anaesthetic) if the jaw doesn't open!
Treatment normally involves immunosuppressive drugs long term to minimise pain, fibrosis (fibrous tissue build up) and decreased range of jaw motion. Drugs used include steroids, azathioprine and/or cyclosporin. Liquid/soft food may need to be given. The jaw can sometimes be gently opened manually on a daily basis although this can sometimes cause a joint dislocation or jaw bone fracture.
If the disease is caught early before severe muscle wasting/destruction and fibrosis, the prognosis to normal muscle movement is good. But once severe damage is caused, the likelihood of reversing it is very low.
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