Trismus, masticatory myositis and antibodies against type 2M fibers in a mixed breed cat

A 1-year-old male neutered mixed breed cat presented with a 2 month history of inability to fully open the mouth when yawning and decreased ability to prehend food. Physical examination revealed severe bilaterally symmetrical masticatory muscle atrophy, a restricted vertical mandibular range of motion of 11-12 mm, and a normal body condition score. Skull radiography was normal. A canine ELISA system against unique masticatory muscle fibers (2M antibody titer), was positive at 1:1000 (reference interval <1:100 in dogs, and was <1:100 using serum from five archived normal cats), indicating the presence of cross-reacting antibodies. Owing to the chronicity and clinical severity, corticosteroid treatment did not result in improved jaw mobility, consistent with end-stage masticatory myositis. Masticatory muscle biopsy was declined at initial presentation. However, 1 year later at elective euthanasia, CT ruled out temporomandibular joint osseous restrictions, and masticatory and biceps femoral muscle histopathology evaluation confirmed end stage feline masticatory myositis with normal limb muscle.

Masticatory myositis should be included in the differential diagnosis of trismus in cats. A canine ELISA can be used to indicate the presence of feline 2M cross-reacting antibodies. More cases are needed to fully elucidate the clinical presentation and best course of treatment.