Congenital Fiber Type Size Disproportion:
  Small type 1 Muscle fibers

• Muscular Dystrophy
  • Emery-Dreifuss
  • ? Myotonic Dystrophy 1
• Congenital myopathies
  • Centronuclear
  • Nemaline rod: May require ultrastructure for diagnosis
  • Fiber type size disproportion: Congenital
  • Rigid spine syndromes
    • Congenital muscular dystrophy (SEPN1)
    • LGMD 2A
    • Emery-Dreifuss variant
    • Reducing body
  • Multi-minicore: May require ultrastructure for diagnosis
  • Fingerprint body
• Exercise training: Type 2 hypertrophy
• Biopsy at too young an age to visualize other pathology

H&E stain

ATPase pH 9.4 stain

Congenital fiber type size disproportion in an infant. Type 1 muscle fibers (pale on ATPase) are much smaller than type 2 fibers. This change may also be found in more well defined congenital myopathies including centronuclear, nemaline rod and multicore. The large disparity in sizes of the 2 fiber types (> 50%) correlates with a clinical picture of hypotonia and nonprogressive weakness. See Fiber type disorders

ATPase pH 4.6 stain Type 1 muscle fibers (Dark) are small