Masticatory muscle myositis (MMM) is an immune-mediated inflammation of the jaw-closing muscles of dogs. Autoantibodies attack type 2M myofibers, a fiber type found only in the muscles supplied by the mandibular branch of the trigeminal nerve, so the temporalis, masseter and pterygoid muscles are hit while the limbs are spared. Caught early and treated with enough corticosteroid for long enough, most dogs recover useful jaw function; caught late, the muscle is replaced by scar and the jaw may stay locked.
Key facts
Masticatory muscle myositis at a glance- Species and system: dog (rare reports in cats); musculoskeletal, focal inflammatory myopathy.
- Cause: autoantibodies against type 2M fibers of the masticatory muscles; the trigger is unknown.
- Signalment: young adult, often large-breed dogs (average onset about 3 years); German Shepherd Dogs, Labrador and Golden Retrievers, Doberman Pinschers; Cavalier King Charles Spaniels can be affected as puppies.
- Classic presentation: a dog that cannot open its mouth, with painful swollen jaw muscles (acute) or a wasted, bony-looking head (chronic).
- Test of choice: serum 2M antibody, taken before corticosteroids.
- Treatment: immunosuppressive prednisone, tapered slowly over months.
- Not zoonotic or reportable.
What's inside
11 sections in the full guide- Key facts
- How the NAVLE tests this
- Cause and pathophysiology
- Clinical signs
- Diagnosis
- Differential diagnoses
- Treatment and management
- Prognosis
- Quick review
- Test yourself
- References
The full guide also has 4 tables, 3 figures, 3 self-test questions and 18 references, available to NAVLE members.
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