Nephrotic syndrome as an atypical presentation of polymyositis

Case report.

Abstract

Introduction: Polymyositis is an autoimmune idiopathic myopathy characterized by collagen disorders and mainly affects the proximal muscles and lungs. Its onset is subacute and evolves over weeks. It is more prevalent in women than in men. Renal manifestations are uncommon; when present, the most common include acute tubular necrosis secondary to hemoglobinuria and myoglobinuria associated with acute rhabdomyolysis. In addition, symptoms such as dysphagia and dyspnea may occur.

Case report: We report the case of a 40-year-old female patient with a history of dermatitis herpetiformis and hyperprolactinemia, who consulted for a one-month clinical picture, characterized by fever, anasarca, muscle weakness, significant proteinuria and pericardial effusion. Electromyography showed findings compatible with inflammatory myopathy and muscle biopsy confirmed the diagnosis of polymyositis. Renal biopsy showed an increase in the mesangial matrix with immunofluorescence IgM deposits, compatible with mesangial glomerulopathy. The patient was treated with corticosteroid pulses and presented a favorable clinical evolution.

Discussion: Renal involvement in patients with polymyositis (PM) is uncommon and is usually limited to tubular damage secondary to rhabdomyolysis or glomerulopathies, such as focal segmental glomerulosclerosis or tubulo-interstitial nephritis. The occurrence of a nephrotic syndrome in this setting requires a detailed etiologic evaluation. In the case presented, the renal biopsy showed IgM-associated mesangial nephropathy, a rare and still controversial entity that has been discussed as a variant of disease characterized by minimal changes or FSGS. The literature reports few cases of PM-associated glomerulopathies, with a predominance of membranous glomerulonephritis or immunocomplexes. The pathophysiology may involve common immune mechanisms, including CD8+ T-cell activation and cytokine-mediated glomerular damage.

Conclusions: Complete remission of nephrotic syndrome after immunosuppressive treatment targeting PM suggests a possible shared immune-mediated origin.