Disuse: Type II muscle fiber atrophy

Type 2 Atrophy: Early

Congenital type II atrophy

Clinical features

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

Pathology

• Type II (dark at pH 9.4) muscle fibers are small
  • 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 are larger than type II