Dermatomyositis and microscopic polyangiitis overlap: a case-based review.

Idiopathic inflammatory myopathies (IIM) are rare autoimmune disorders primarily affecting skeletal muscles, with occasional involvement of other organs and an increased risk of malignancy. Dermatomyositis (DM), a major IIM subtype, is characterized by proximal muscle weakness, distinctive skin manifestations, and characteristic biopsy findings. Antineutrophil cytoplasmic antibody (ANCA)-associated vasculitides (AAV), including microscopic polyangiitis (MPA), involve small- to medium-sized vessels and can affect the kidneys and lungs. Although uncommon, overlap syndromes between IIM and AAV have been described. We report two cases of DM-MPA overlap. The first involved a 67-year-old male with interstitial lung disease (ILD), muscle weakness, and proteinuria. The second concerned a 53-year-old female presenting with characteristic skin rash, arthralgias, proteinuria, and mild pulmonary involvement. Both patients received high-dose corticosteroids and cyclophosphamide (CYC), resulting in clinical and laboratory improvement, including resolution of proteinuria and improved pulmonary function, maintained over a 6-month follow-up. The literature review identified five relevant case reports and one case series. Fifteen patients from these six studies were included, predominantly female (13/15), with ages ranging from 15 to 84 years. DM was the most common IIM subtype (7/15), followed by polymyositis (PM) (6/15), inclusion body myositis (IBM) (1/15), and clinically amyopathic dermatomyositis (CADM) (1/15). MPA accounted for 87% (13/15) of AAV cases, while GPA was reported in 2/15. Renal involvement was frequent, often presented as pauci-immune crescentic glomerulonephritis, whereas pulmonary involvement was less common. MPO-ANCA was positive in 12/15 patients. Initial therapy typically involved corticosteroids, with or without immunosuppressants, tailored according to the dominant organ involvement. Outcomes were variable; most patients achieved remission, although some experienced persistent organ dysfunction. IIM-AAV overlap is rare but potentially severe, frequently involving renal, pulmonary, muscular, and occasionally cardiac systems. Early recognition and individualized immunosuppressive therapy can yield favorable outcomes. Multicenter studies are required to clarify the epidemiology, clinical spectrum, and optimal management of this complex syndrome, and further research is needed to elucidate underlying pathogenic mechanisms.