What is MMM?
Masticatory muscle myositis (MMM) is an autoimmune disease in which the dog's immune system selectively attacks type 2M muscle fibres in the masticatory muscles (temporalis, masseter, pterygoids). Unlike generalised polymyositis, MMM affects only the jaw muscles. It is the most common immune-mediated myopathy in dogs.
Clinical phases
Acute phase
- Swelling and pain of the temporal and masseter muscles.
- Difficulty or inability to open the mouth (trismus).
- The dog refuses to eat, chew or pick up toys.
- Eyes pushed outward (exophthalmos) by inflamed temporal muscle.
- Fever, lethargy.
Chronic phase
- Marked temporal muscle atrophy: the head looks bony or narrow as the temporal fossa becomes visible.
- Permanently restricted jaw opening (irreversible fibrosis if untreated).
Diagnosis
- Anti-2M antibody test (serum ELISA): highly sensitive and specific for MMM. The reference test.
- CK (creatine kinase): often elevated in the acute phase.
- Muscle biopsy: histological confirmation in doubtful cases.
- MRI or CT: assesses degree of muscle involvement and rules out orbital masses.
Treatment
MMM responds well to glucocorticoids when given early and for long enough:
- Prednisolone 1–2 mg/kg/day for 4–8 weeks, then very slow tapering over months.
- Goal: full, free jaw opening before any dose reduction.
- Relapses are common if tapered too quickly.
- Refractory cases: azathioprine, cyclosporine or mycophenolate as steroid-sparing agents.
The prognosis is good when treatment starts before permanent fibrosis develops. Dogs treated early regain normal jaw opening within weeks. Temporal muscle atrophy can partially reverse over time.