Clinical findings, biochemical results and EMG investigation are important in the diagnosis of myopathy, and muscle biopsy is unavoidable for its further differentiation. In an older man with muscle pain and rapid development of muscle weakness with head-drop and dysphagia, polymyositis with focal endomysial CD8+ lymphocytic infiltrates was diagnosed on muscle biopsy.
Once the patient had been hospitalized, an advanced carcinoma of the prostate and anti-Ri paraneoplastic antibodies were found. A paraneoplastic polymyositis was diagnosed, which is a rare diagnosis in patients with prostate cancer.