Masticatory Muscle Myositis

Overview

Masticatory muscle myositis (MMM) occurs in dogs and tends to affect the masseter and temporalis muscles. Autoantibodies selectively attack the muscle of mastication, which contain type IIM fibres. MMM can be classified as either acute or chronic:

• Acute stage: Eosinophilic myositis with extremely swollen, painful, hard masticatory muscles.
• Chronic stage: Atrophic myositis where atrophy may occur without previous acute stage.

Grossly MMM is normally bilateral, but not necessarily symmetrical.

Histology

Acute lesions:

• Mainly eosinophilic infiltrate, few lymphocytes, monocytes and plasma cells.
• In recurrent disease plasma cells predominate.
• Necrotic myofibres, atrophy and attempted regeneration.

Chronic lesions:

• Atrophy predominates.
• Fasciculi are shrunken.
• Condensation of stroma leading to an enlarged endomysium.
• Focal plasma cell and lymphocyte infiltration.