Masticatory muscle myositis

Causes

MMM is caused by the presence of 2M fibers in the muscles of the jaw, 2M fibers are unique to the masticatory muscles and are made up of myosin. In affected animals, the immune system recognizes these proteins as foreign to the body and attacks them, resulting in inflammation.

Symptoms

The inflammation from MMM typically results in anorexia, loss of body weight, pyrexia, and torpidity. Optic neuritis and blindness can occur in some cases. Acute cases last for one to three weeks without treatment. Chronic or repeat cases can result in fibrosis, atrophy, and enophthalmos. The fibrosis and atrophy can result in an inability to open the mouth in severe chronic cases.

Diagnosis

Diagnostic results of acute myositis are eosinophilia, heightened serum levels of creatine phosphokinase, a positive 2M antibody test, and histopathology and biopsy of the temporalis or masseter muscles.

Biopsy reveals an orange colour to affected muscles and histopathology results are degeneration of the muscle fibre and neutrophil and eosinophil infiltration.

Differential diagnoses

• Abscess
• Orbital cellulitis
• Polymyositis
• Temporomandibular joint disease

Treatment

Treatment is usually with corticosteroids such as prednisone, often with decreasing doses for up to 4–6 months, and in the case of trismus, manual opening of the mouth under anesthesia. Feeding very soft or liquid food during this time is usually necessary. The ultimate degree of recovery of jaw function and muscle mass will depend upon the extent of damage to the muscle tissue. Recurrence of MMM may occur.

References

References

1. (2006). "Evidence for MHC I-restricted CD8+ T-cell-mediated immunopathology in canine masticatory muscle myositis and polymyositis". Muscle Nerve.
2. (1995). "Textbook of Veterinary Internal Medicine". W.B. Saunders Company.
3. (2022-08-12). "Essentials of Veterinary Ophthalmology". Wiley.
4. Gelatt, Kirk. (2002). "Treatment of Orbital Diseases in Small Animals". Proceedings of the 27th World Congress of the World Small Animal Veterinary Association.
5. (2003). "Neurology for the Small Animal Practitioner". Teton New Media.
6. (2003). "Myasthenia gravis and masticatory muscle myositis in a dog". Can Vet J.