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

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.