INFLAMMATORY MYOPATHY WITH ABUNDANT HISTIOCYTIC CELLS

• Cell features
  • Size: Large
  • Cytoplasm: Abundant
  • Acid phosphatase staining
    • Confluent
    • Different from granulomas which have multifocal staining around each cell nucleus
  • Some patients also have other foci composed of mononuclear cells
• Locations of foci: Perimysial & Endomysial

• No necrosis or regeneration
• MHC Class I upregulation: Common; Diffuse or near cell foci

H&E stain Cellular Foci: Large cells with abundant cytoplasm located in perimysium or endomysium

GT stain

Acid phosphatase Acid phosphatase positive cells: Confluent staining

Sudan black Perimysial cells contain lipid PAS stain PAS staining of perimysial cells