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Fiber Type Size Disproportion

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Type 1 Fiber Smallness: Differential Diagnosis

• Muscular Dystrophy
  • Emery-Dreifuss: Emerin; LMNA
  • ? Myotonic Dystrophy 1
• Congenital myopathies
  • Centronuclear
  • Nemaline rod: May require ultrastructure for diagnosis
  • Carey-Fineman-Ziter: TMEM8C
  • CMND: SPTBN4
  • Fiber type size disproportion: Congenital
  • Lamin A/C
  • Rigid spine syndromes
    • Congenital muscular dystrophy (SEPN1)
    • LGMD 2A
    • Emery-Dreifuss variant
    • Reducing body
  • Multi-minicore: May require ultrastructure for diagnosis
  • Fingerprint body
  • Native American myopathy
• Exercise training: Type 2 hypertrophy
• Biopsy at too young an age to visualize other pathology

Type 2 Fiber Smallness

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INFANT

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. Type 1 smallness may also occur 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 ACTA1 mutation: CFTD with type 1 muscle fibers small

CHILD (5 years)

ATPase H&E stain Fiber size: Bi-modal variation

ATPase pH 9.4 stain Type 1 muscle fibers (Light) are small Type 2 muscle fibers (Dark): Largest, & Some small, fibers

ATPase pH 4.3 stain Type 1 muscle fibers (Dark) are small Type 2 muscle fibers (Light) are the largest Type 2C fibers (Intermediate): Small or Large

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

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CHILD (12 years)

ATPase H&E stain Fiber size: Bi-modal variation

ATPase pH 9.4 stain Type 1 muscle fibers (Light) are small Type 2 muscle fibers (Dark): Largest, & Some small, fibers

ATPase pH 4.3 stain Type 1 muscle fibers (Dark) are small Type 2 muscle fibers (Light) are mostly large and occasionally small

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Co-Expression of Fast & Slow Myosin in Same Larger Muscle Fibers

Small Muscle fibers: Stain only for Slow Myosin (Type I)

Fast myosin in muscle fibers

Slow myosin in muscle fibers

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Fiber Predominance: Type 2

ATPase pH 9.4 stain

ATPase pH 4.3 stain

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See
  Fiber type disorders
  Fiber type properties

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10/1/2022