Generalised idiopathic polymyositis mimicking masticatory myositis in a dog

Masticatory myositis (MM) and generalised idiopathic polymyositis (gIPM) are the most common immune-mediated myositis in dogs. In humans, the diagnosis and classification of myositis have continuously evolved over the past few decades with the combination of histopathological and immunohistochemical evaluation alongside the identification of myositis-specific autoantibodies (MSAs). In dogs, however, MSAs have only been described in MM and breeds such as Newfoundland and Boxer. We report a case of a dog that presented with sub-acute pain when chewing and exophthalmos, followed by chronic progressive masticatory muscle atrophy and enophthalmos. Clinical signs and electromyography were primarily consistent with MM, but serological autoantibodies against type 2M myofibres were negative. Creatine kinase activity, histological and immunohistochemical examination of the temporal and triceps brachii muscles and negative infectious disease tests were consistent with gIPM. This atypical case highlights the requirement for multiple muscle biopsies and appeals for extended immunohistochemical studies in canine myositis.