Disuse: Type II muscle fiber atrophy

Type 2 Atrophy: Early

Congenital type II atrophy

Type II atrophy: Clinical features

• Weakness: Proximal > Distal; Symmetric
• Muscle size: Wasting > Weakness
• Associated disorders
  • Weight loss: > 15%
  • Disuse
  • Aging
  • Systemic disease
    • Paraneoplastic
    • Corticosteroid myopathy
  • Childhood
    • Congenital hypotonia
    • Myasthenia gravis

Type II atrophy: Pathology

• Type II (dark at pH 9.4) Muscle fibers
  • Small: Reduced cross-sectional area\
  • Shape: Angular
  • Early changes
    • Atrophy in type IIB (Intermediate staining at pH 4.6) fibers
    • Narrow elongated muscle fibers
  • Disuse & weight loss atrophy: Small fibers are angular
  • Congenital: Small fibers are polygonal or rounded
• Type I (lighter at pH 9.4) Muscle fibers
  • Larger than type II
  • Often atrophic compared to normal type 1 fibers
• NADH stain: Small fibers usually pale
• Nuclei
  • Number: Unchanged
  • Myonuclear domain size: Reduced
• Recovery of fiber size after Disuse atrophy ¹
  • Probably doesn't require satelllite (Pax7) cells