Masticatory Muscle Myositis (MMM)
The masticatory muscles, which include the temporalis and masseter
muscles, control movement of the mandible (lower jaw) to allow
chewing. Myositis is inflammation of these muscles, resulting
in swelling and pain making eating difficult.
The cause of this inflammation is unclear, however it is currently
suspected to be immune mediated (trigger by the immune system).
The immune system is responsible for defending the body by fighting
infection, but occasionally it is stimulated to over react and
also start attacking the body. There is no specific age, breed
or sex tendency for this condition.
The signs of MMM are pain and inability to open the jaw. As the
condition becomes more advanced it may become impossible to open
the jaw even under anesthesia due to fibrosis (scarring) of the
muscles.
Other conditions which effect the working of the jaw can produce
similar signs to MMM. These include inflammation or arthritis
of the mandibular joint (between jaw and the skull); swelling
behind the angle of the jaw due to abscess (infection) or tumors;
and damage to the bone, muscles, or nerves of the jaw secondary
to trauma. Radiographs (X-rays) will help rule out some of these
other problems. Biopsy and histopathology (microscopic examination
of a sample) of the masticulatory muscle is the most useful test
to identify MMM. Electromyographic Evaluation (EMG), which uses
small currents of electricity to evaluate how the muscle is functioning
may be used to distinguish MMM from some other unusual muscle
conditions.
Treatment involves giving the dog a minimum of 6 months of immunosuppressive
doses of corticosteroids. These doses will stop the immune system
attacking the body however they also carry the risk of reducing
the ability to fight infection. Whilst the dog is unable to chew
it is necessary to provide liquid feed and if the condition is
serious enough it may be necessary to supply food through a tube
inserted directly into the dog’s stomach. Surgery to release the
fibrosed muscles may be undertaken where necessary.
Where treatment is commenced early in the course of the disease
the prognosis is good however there is a risk of recurrence
Related sites
Uni California
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