Inflammatory myopathy with abundant macrophage [IMAM]: Systemic analysis and pathological approach to distinguish it from dermatomyositis.

PMID 39973451 | PMCID PMC11949305 | DOI 10.1177/22143602251315376 · Journal of neuromuscular diseases · 2025 · View on PubMed ↗

BACKGROUND: Inflammatory myopathy with abundant macrophage [IMAM] is marked by macrophage infiltration and muscle fibers damage, resembling dermatomyositis [DM] but with unique pathology. Its mechanism remains unclear. Our study focused on exploring the clinicopathological characteristics, underlying pathogenic mechanisms, and the challenges in diagnosing and managing IMAM. METHODS: A systematic analysis of medical literature databases (Pub-med, Cochrane, Scopus, Google Scholar) was performed using the term "IMAM," excluding studies on other inflammatory myopathies [IMs]. Selected studies were independently assessed with the Newcastle-Ottawa Scale, and quantitative data underwent inter-statistical analysis, descriptive and odds ratio, to identify relevant findings. RESULTS: Eight studies, including 49 IMAM cases from 2003 to 2024, were analyzed. Five were case reports, and three were cross-sectional studies. IMAM showed no age or sex predilection. Common symptoms included proximal muscle weakness, pain, and fatigue, with atypical DM-like skin features in 65% of cases. Other association included hemophagocytosis, cutaneous panniculitis, and interstitial lung infiltration. Histologically, all cases showed myonecrosis infiltrated with CD68+ macrophages. Scattered CD3+ and CD4+ T-cells expressing IL-10 with no or rare CD8+ T-cells were identified. MAC deposition was limited to necrotic fibers, and perifascicular atrophy was absent in all cases. Anti-PL-7 and anti-U1 RNP antibodies were detected in 4% of cases. Elevated TNFα and IFN-γ levels, with low STAT1 and STAT6, were observed. Genetic analysis revealed MEFV polymorphisms in 7 cases and a TNFRSF1A mutation [C43R] in single case. Treatment involved steroids, with or without immunotherapy or chemotherapy, leading to remission and recovery in 43.7% of cases. CONCLUSION: IMAM is a distinct type of IMs that requires muscle biopsy for diagnosis as myositis antibody and cytokine tests are usually insensitive.