Masticatory Muscle Myositis

(MMM)

• Occurs in dogs
• Tends to affect the masseter and temporalis muscles
• Autoantibodies selectively attack muscles of mastication (type IIM fibres)
• Acute stage = eosinophilic myositis
  • Extremely swollen, painful, hard masticatory muscles
• Chronic stage = atrophic myositis
  • Atrophy may occur without previous acute stage
• Grossly:
  • Bilateral but not necessarily symmetrical
• Histologically:
  • Acute lesions:
    • Mainly eosinophilic infiltrate, few lymphocytes, monocytes and plasma cells
    • In recurrent disease plasma cells predominate
    • Necrotic myofibres, atrophy and attempted regeneration
  • Chronic stage:
    • Atrophy predominates
    • Fasciculi are shrunken
    • Condensation of stroma -> enlarged endomysium
    • Focal plasma cell and lymphocyte infiltration