DEFINITION/OVERVIEW

• Focal inflammatory myopathy affecting the muscles of mastication (temporalis and masseter muscles) and sparing the limb muscles

ETIOLOGY/PATHOPHYSIOLOGY

• Suspected immune-mediated cause owing to autoantibodies against type 2M fibers and a positive clinical response to immunosuppressive doses of corticosteroids
  
• Unknown genetic cause
  
• As with autoimmune diseases in general, the appropriate genetic background must exist
  
• Cavalier King Charles spaniels (CKCS) have a familial form and may be affected at less than 6 months of age
  
• Possible previous bacterial or viral infection
  
• Vaccination may exacerbate active disease

SIGNALMENT/HISTORY

• Dogs: various breed predilections
  
  
  
  • Rottweilers, Dobermans, Samoyeds, and CKCS develop severe forms
  
  
• No obvious age or sex predilections
  
• Usually related to abnormalities of jaw movement, jaw pain, and masticatory muscle atrophy; not a “tabletop” diagnosis; usually requires laboratory testing to confirm diagnosis
  
• Acute or chronic pain when opening the jaw; inability to pick up a ball or get food into the mouth; acutely swollen muscles; progressive muscle atrophy

CLINICAL FEATURES

• Marked jaw pain with manipulation and/or trismus (the inability to open the jaw), even under general anesthesia
  
• Muscle swelling with exophthalmos
  
• Inability to open the jaw under anesthesia

DIFFERENTIAL DIAGNOSIS

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