Atypical masticatory muscle myositis in three cavalier King Charles spaniel littermates

This case report describes a novel manifestation of the immune‐mediated disease, masticatory muscle myositis. Clinical signs, including difficulty in opening the mouth (trismus), were seen in three of four 12‐week‐old cavalier King Charles spaniel littermates. Diagnosis was established by 2M immunohistochemistry, supported by characteristic histopathological changes in affected temporal muscle. Treatment using corticosteroids at immune‐modifying doses resulted in resolution of clinical signs in all the affected animals. Masticatory muscle myositis should be considered as a differential diagnosis in groups of young dogs with clinical signs of myositis localised to the head.