Polymyositis is an immune-mediated disease of skeletal muscle.
This disease can affect all skeletal muscles including the masticatory muscles, limb muscles and esophageal, laryngeal and pharyngeal muscles. In rare cases, secondary myocarditis and right atrial dilatation has also been reported.
It can occur alone or in association with other immune-mediated diseases such as systemic lupus erythematosus.
Polymyositis has also been reported subsequent to Vogt-Koyanagi-Harada (VKH)-like syndrome in a dog, an autoimmune disease that targets melanocyte antigens.
The initiating immune-mediated process associated with polymyositis presumably involves the initial auto-antibody attack of myosin by auto-expression of MHC class I and CD8+ T-cell infiltration, followed by subsequent release of serotonin (5-HT), leading to fibrosis and cicatrization of skeletal muscles.
In dogs, a number of reports have linked polymyositis with lymphoma, thymoma, myeloid leukemia, bronchogenic carcinoma, tonsillary carcinoma, anaplastic round cell tumor and plasmacytoma. These diseases may well result in secondary infiltration of muscle fibres with inflammatory byproducts.
Clinically affected dogs are often middle-aged, and present with generalized weakness, dysbasia (stiff stilted gait), variable myalgia, intermittent vomiting and dysphagia, dysphonia, weight loss and muscle atrophy and contractures to many skeletal muscles, and facial and tongue atrophy.
Hematology and biochemical analysis of blood samples is often unrewarding, although there may be a mild eosinophilia and increases in creatinine kinase and aspartate aminotransferase levels.
Diagnosis is usually confirmed by multiple muscle biopsies, such as the biceps femoris or vastus lateralis.
A differential diagnosis would include Toxoplasma gondii, Hepatozoon americanum, Sarcocystis neurona, Neospora spp, Hammondia hammondi, masticatory muscle myositis, tetanus, myasthenia gravis, Duchenne muscular dystrophy, Golden Retriever Muscular Dystrophy, thymoma and dermatomyositis.
Polymyositis is usually responsive to prednisolone at immunosuppressive doses, and full recovery is not uncommon, as evidenced by histological evidence of muscle regeneration provided the inflammation and fibrosis can be stopped or reduced.
- Veterinary Neurologist
- Uni of California, San Diego
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