Myopathic grouping
  Necrosis & Rhabdomyolysis
  Regional Ischemic Immune
  Statin toxicity

General features

Muscle Fiber Necrosis (Rhabdomyolysis episode): Morphology

Segmental necrosis: Scattered necrotic & hypercontracted muscle fibers

H & E Stain
Normal muscle fiber
Necrotic muscle fiber: Pale; Little phagocytosis
Necrotic muscle fiber: Phagocytosis by macrophages
Hypercontracted necrotic muscle fiber

H&E stain
Necrotic muscle fibers: Pale; Little phagocytosis (Arrows)

Gomori trichrome stain

Necrotic muscle fiber, Early
  • Fiber is pale & enlarged.
  • Internal nuclei may reflect
    invasion by macrophages.

Necrotic muscle fibers: Internal architecture has lost NADH staining (Arrow)

NADH stain

H&E stain
Necrotic muscle fibers: Phagocytosis (Arrows)

Gomori trichrome stain

Esterase stain
Necrotic muscle fiber, Late
Muscle fiber is invaded and
  replaced by phagocytes.

NADH stain
Phagocytic cells within pale necrotic muscle fibers: Mild NADH staining

Acid phosphatase stain
Necrosis: Stages
  Early: Pale stained muscle fibers
  Later: muscle fibers replaced by acid phosphatase + cells

MHC Class I stain
MHC Class I: Stains capillaries & cells phagocytsing muscle fibers; No staining of most muscle fibers

Alkaline phosphatase stain
Necrosis: Alkaline phosphatase stains around the rim of muscle fibers

Toluidine blue stain

Necrotic muscle fiber (Longitudinal section)
  Invaded by cells

Toluidine blue stain

Necrotic muscle fiber (Cross-section)
  Invaded by cells

Toluidine blue stain

Necrotic muscle fiber
  Invaded by cells

Toluidine blue stain

Cells in necrotic fiber

Toluidine blue stain

Non-necrotic, abnormal muscle fiber (Left): Dilated SR

Necrotic muscle fibers: Complement C5b-9 (MAC) deposition

Stain: C5b-9 components of complement (Membrane attack complex (MAC))
Necrosis: C5b-9 deposition diffusely within muscle fiber cytoplasm

Rhabdomyolysis: PAS stain is pale in all fibers

Also see: Regional Ischemic Immune Myopathy

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