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Inflammatory Myopathy with Abundant Histiocytic Cells (IMAM)

Cell Foci

• 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
• Ultrastructure: Aluminum crystals in histiocytes
• Also see: Macrophagic myofasciitis

Muscle Fibers

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

Differential Diagnosis

• Granuloma
• Fasciitis
• IMPP
• MMF

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H&E stain

IMAM: Lesions

  Locations: Perimysium & Endomysium
  Cells: Mostly large & Histiocytic; May contain sub-foci of non-histiocytic cells (Arrow)

H&E stain

H&E stain

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H&E stain

IMAM: Clusters of Histiocytic cells

VvG stain

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IMAM: Smaller lesion confined to perimysium

VvG stain

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IMAM: Vascularization

Cell foci often contain smaller vessels, some with thick basal lamina

VvG stain

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Vessels in perimysial cell foci stain for Ulex

UEAI stain

Alkaline phosphatase stain

Vessels in cell foci stain for Alkaline phosphatase & ATPase

ATPase pH 9.4 stain

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IMAM: Cell Foci   Large cells with abundant cytoplasm   Location: Perimysium > Endomysium H&E stain

H&E stain

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H&E stain IMAM Cell Foci   Large cells with abundant cytoplasm   Location: Perimysium > Endomysium

Gomori trichrome stain

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Congo red stain

IMAM cells
  Cells: Large
  Nuclei: Large; Irregular shapes

VvG stain

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Histiocytic cells   Acid phosphatase positive   Confluent staining Acid phosphatase stain

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Histiocytic cells:
  May extend from perimysium into endomysium

Acid phosphatase stain

Esterase stain

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IMAM cells in perimysium: Contain lipid   Dark on Sudan   Gray on VvG Sudan black

VvG

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IMAM perimysial cells: May be PAS positive PAS stain

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IMAM: MHC Class I

Cells in foci: Strong staining
Muscle fibers: Upregulation by morphologically normal fibers

MHC Class I stain

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IMAM: Cell Types

CD68 stain

Cells in foci: Strong staining for CD68

CD68 stain

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CD3 stain

Cells in foci: Little staining for CD3 or CD20

CD20 stain

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IMAM: Ultrastructure

From: R Schmidt

From: R Schmidt

From: R Schmidt

From: R Schmidt

From: R Schmidt

From: R Schmidt

From: R Schmidt

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Return to Inflammatory myopathies
Return to IMAM

11/30/2021