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Masticatory Muscle Myositis

17ckcssorejaw1Ginger had been a completely normal puppy, playing 
enthusiastically, and eating hungrily. Over a twelve-hour period, everything changed. She became dull and quiet, no longer wanting to play, and she stopped eating. When her owner reached out his hand to pet her head and reassure her, she backed away, and yelped as if in pain.

Ginger will be able to lead a relatively normal life
Ginger was brought in to see me at once. She sat on the consulting table, looking miserable. She wouldn’t let me open her mouth at all. Was there something stuck inside her mouth? I admitted her to investigate and to my surprise, even after I’d given her a general anaesthetic, I was unable to open her mouth more than one inch wide. I took x-ray pictures of her skull to see if there was some type of abnormality or injury but everything was normal.
I processed blood samples in our laboratory, but again everything was normal. I telephoned a local referral clinic to discuss the case, wondering if specialised tests such as “electromyography” (EMG) might be available. These tests are common in the human world, using electrical recording apparatus to assess the function of different muscle groups but they’re rarely used in the veterinary world. The referral clinic was very helpful: they were not running EMGs, but they had come across a case report that could be relevant.

The report described three Cavalier King Charles Spaniel puppies that had developed severe jaw pain, just like Ginger. Full investigations – including electrical tests and muscle biopsies – had proven that the puppies were suffering from a condition known as Masticatory Muscle Myositis. This rare condition happens when the body develops an immune reaction to its own muscle fibres. The muscle fibres around the jaws are “rejected”, in the same way as a new kidney can be rejected after a kidney transplant. As a result, the muscles become swollen and painful. Once the condition is diagnosed, it can be effectively treated by giving drugs to suppress the immune system.

I sent an email to a specialised veterinary neuromuscular centre in San Diego at once, explaining Ginger’s background. Their reply was encouraging. They regularly saw small numbers of Cavalier puppies with this problem, and they had developed a special antibody test that could be used to confirm the diagnosis without the need for complicated workups or muscle biopsies. If I could send them a small blood sample from Ginger, they’d be able to confirm the diagnosis. I did this at once.
While we waited for results, I gave Ginger strong pain relief, and whilst this did not solve the problem, it did ease her signs so that she started to eat

Two weeks later, the results from the Californian blood tests arrived, and they confirmed the diagnosis. Ginger was definitely suffering from Masticatory Muscle Myositis. I started her onto a high level of immunosuppressive drugs to stop her body from rejecting her own jaw muscles. She responded well immediately, and she has been more-or-less pain-free since the drugs were started.
Ginger still has problems: her head muscles have been scarred, and she is unable to open her mouth as fully as she should. Her tongue lolls out of her mouth in a strange way, and she is smaller than other puppies of her age. She’ll never be the same as her brothers and sisters, but thanks to the help of modern medicine, she’s well enough to live a reasonably normal life.

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