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Vetdle Archive

Archive Case #166: Canine Masticatory Muscle Myositis

Dog veterinary case - Hard - August 16, 2026

Clinical Clues

  1. A dog is brought in because it suddenly cannot open its mouth properly.
  2. The owner reports pain when the dog tries to chew and a marked drop in food intake.
  3. The temporal and masseter muscles are swollen and tender on examination.
  4. Later in the disease course, these muscles can become visibly wasted.
  5. Testing supports an immune-mediated inflammatory disease targeting type 2M muscle fibres.
  6. This disorder selectively affects the muscles of mastication in dogs.

Diagnosis

Canine Masticatory Muscle Myositis

This case is most consistent with Canine Masticatory Muscle Myositis because the dog has painful trismus with selective involvement of the jaw muscles and supportive immune testing. The most important clues are the inability to open the mouth and masticatory muscle changes, which make a temporomandibular fracture less likely.

Learning review

Review the case evidence

Use the paired clues to explain how the evidence in this dog case supports the provided diagnosis, Canine Masticatory Muscle Myositis, then compare your reasoning with the case explanation.

Evidence checkpoints

Clues 1-2

  1. Clue 1: A dog is brought in because it suddenly cannot open its mouth properly.
  2. Clue 2: The owner reports pain when the dog tries to chew and a marked drop in food intake.

Clues 3-4

  1. Clue 3: The temporal and masseter muscles are swollen and tender on examination.
  2. Clue 4: Later in the disease course, these muscles can become visibly wasted.

Clues 5-6

  1. Clue 5: Testing supports an immune-mediated inflammatory disease targeting type 2M muscle fibres.
  2. Clue 6: This disorder selectively affects the muscles of mastication in dogs.

Self-review prompts

  1. After clues 1-2, what evidence from this dog case shaped your initial interpretation?
  2. When you compare clues 3-4 with clues 1-2, what changed in your reasoning about Canine Masticatory Muscle Myositis?
  3. How do clues 5-6 align with the case explanation for Canine Masticatory Muscle Myositis?

Educational Use

Vetdle archive cases are educational veterinary games for diagnostic reasoning practice. They do not provide veterinary advice, diagnosis, treatment, or professional medical guidance.