Overflow proteinuria as a manifestation of unrecognized polymyositis

Hyun Ho Kim,¹ Jae Young Kim,¹ Sung Jun Kim,^1,2 Eun Su Park,³ Seok Joon Shin,^1,2 Kwi Young Kang,^1,4 Yeon Sik Hong,^1,4 Hye Eun Yoon<sup>1,2

1</sup>Department of Internal Medicine, College of Medicine, The Catholic University of Korea, ²Division of Nephrology, Department of Internal Medicine, Incheon St Mary's Hospital, ³Department of Pathology, Incheon St Mary's Hospital, ⁴Division of Rheumatology, Department of Internal Medicine, Incheon St Mary's Hospital, Incheon, Republic of Korea

Abstract: Polymyositis is a rare and gradually progressive autoimmune disease of skeletal muscle. Two main types of renal involvement have been described: acute tubular necrosis related to rhabdomyolysis and glomerulonephritis. However, cases of overflow proteinuria related to polymyositis have rarely been reported. Herein, we report a case of a 41-year-old male who presented with edema of both lower extremities. Laboratory studies revealed elevated creatine phosphokinase level, hypoalbuminemia, and a moderate amount of proteinuria, although albuminuria was not dominant. Urine electrophoresis showed an abnormally restricted zone in the β-fraction, which suggested overflow proteinuria of non-glomerular origin. Despite intravenous hydration, his serum creatine phosphokinase level did not decrease and his symptoms did not improve. Electromyography showed myopathy, and muscle biopsy revealed findings consistent with polymyositis. After corticosteroid therapy, his creatine phosphokinase level and proteinuria decreased and his clinical symptoms improved. This case demonstrates an atypical presentation of polymyositis manifested by overflow proteinuria.

Keywords: polymyositis, proteinuria, rhabdomyolysis