Macrophagic Myofasciitis (MMF)

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.

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.