Abstract
Background: Canine thymomas are associated with multiple paraneoplastic syndromes, among which myasthenia gravis is the most common. Acquired myasthenia gravis is an autoimmune disease characterized by the presence of antibodies against acetylcholine receptors. Acetylcholine receptors antibodies are the most commonly formed, but the production of antistriational antibodies binding to skeletal and cardiac muscle proteins has also been recorded both in humans and dogs. An association between the occurrence of antistriational antibodies and a severe form of myocarditis, giant cell myocarditis, has been described in humans.
Case description: A 4-year-old mixed-breed dog was referred because of one month history of exercise-induced weakness, hypersalivation and regurgitation. The neurologic examination was indicative of a neuromuscular junction disease and myasthenia gravis was suspected. A Computed Tomographic scan examination showed the presence of megaoesophagus and a thymic mass. Serum antibodies against acetylcholine receptors confirmed the diagnosis of myasthenia gravis. Treatment with pyridostigmine was started and the thymic mass was surgically excised and a diagnosis of thymoma was confirmed by histology. 24 hours after surgery the dog developed a third-degree atrioventricular block. Severe arrhythmia and increased troponin serum levels suggested myocarditis which rapidly led to cardiopulmonary arrest. Histopathologic examination of heart, esophagus and diaphragm revealed a lymphocytic and macrophagic infiltration, consistent with myocarditis and polymyositis. Scattered rare giant multinucleated cells were also detected in the myocardium.
Conclusions: To the author’s knowledge this is the first report of thymoma-associated myasthenia gravis with concurrent polymyositis and giant cell-like myocarditis in a dog.

Key words: dog, myasthenia gravis, myocarditis, polymyositis, thymoma