This previously healthy pediatric patient presented with peculiar facial muscle hypertrophy, and genetic testing revealed one pathologic variant and two variants of uncertain significance (VUSs). A muscle biopsy of the thigh was performed to further evaluate this patient. Frozen sections and ultrastructural examination images of the inflammatory infiltrate are shown below. What is your diagnosis?
Answer: Macrophagic Myofasciitis (MMF)
The muscle biopsy demonstrates a few endomysial and perimysial foci of mild chronic inflammation and associated variably frequent collections of CD68-positive histiocytic cells with moderate to abundant amounts of pale basophilic cytoplasm. No significant associated muscle fiber injury is noted. The chronic lymphocytic inflammatory cells are predominantly comprised of CD8-positive T-cells. The overall morphologic features are consistent with Macrophagic Myofasciitis (MMF), a relatively uncommon localized inflammatory process related to prior alum (aluminum) containing vaccines. See references.
Chkheidze R, et al. Morin stain detects aluminum-containing macrophages in macrophagic myofasciitis and vaccination granuloma with high sensitivity and specificity. J Neuropathol Exp Neurol. 2017;76:323-331.
Gherardi RK, Authier FJ. Macrophagic myofasciitis: characterization and pathophysiology. Lupus. 2012 Feb;21(2):184-189.
Israeli E, Agmon-Levin N, Blank M, Shoenfeld Y. Macrophagic myofaciitis a vaccine (alum) autoimmune-related disease. Clin Rev Allergy Immunol. 2011;41(2):163-168.
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