Muscle biopsy: A fresh frozen biopsy from the cranial tibial muscle showed multifocal areas of mononuclear and eosinophilic cellular infiltration consistent with a diagnosis of inflammatory myopathy/polymyositis. (H&E). Since infectious agents including T. gondii and N. caninum were not identified, immune mediated or paraneoplastic etiologies were likely.

Treatments and Clinical Outcome
Prednisone therapy was initiated at 20 mg twice daily for 3 days with gradual tapering over an approximately 2 month period. Sucrafate was also included as a stomach protectant. Clinical evaluations over the following 2 months showed the dog was doing well with the CK concentration returning to the normal range. The dog could not be tapered off of the prednisone without return of clinical weakness. Approximately 6 months after the original diagnosis the dog presented with weakness(Fig.A) and a moderate size mass in the muscle of the left rear leg (Fig. B). Repeat thoracic radiographs showed a diffuse nodular pulmonary pattern suspected to be infectious or neoplastic (Fig. C). Cytology of a tracheal wash and lung aspirate supported a probable pyogranulomatous pneumonitis. Aerobic, anaerobic, and fungal cultures of blood, the tracheal wash, and the muscle mass were negative. A biopsy of the muscle mass showed marked cellular infiltration composed predominantly of histiocytes with accumulations of degenerating neutrophils in necrotic areas (Fig. D). Most of the histiocytes contained mitotic figures. A diagnosis of diffuse, aggressive histiocytic lymphoma was made with a poor prognosis. The dog was euthanitized. Additional tissues evaluated at necropsy including lymph node (not shown) and lung (Fig. E) showed marked histiocytic infiltration with almost complete obliteration of the normal architecture.

Conclusion
In humans, the incidence of associated malignancy and polymyositis may be up to 28% and increases with age (1). The associated neoplasia in humans is most frequently a carcinoma (2). The detection of a neoplasm may precede or follow the onset of polymyositis. In most cases, the muscle disease and the malignancy develop within a year of each other (3). In the dog of this report, neoplasia was identified 6 months following the diagnosis of polymyositis. When possible, the removal of the tumor sometimes results in improvement of the muscle weakness; on occasion, the reappearance of muscle weakness coincides with tumor regrowth. The incidence of malignancy associated polymyositis in dogs is not known. We suspect that the original diagnosis of polymyositis in this dog was a paraneoplastic syndrome. This association of polymyositis and malignancy should alert clinicians to the possibility of an underlying neoplasm in dogs with polymyositis and warrant the initial search for and continued monitoring for neoplasia in these dogs.