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Congenital myotonic dystrophy (muscle biopsy) (1)

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This biopsy is from a 2-month-old term infant with congenital myotonic dystrophy. In this pattern, centronuclear fibers are few, but 2 populations of fibers are seen by size: large (normal sized) fibers and atrophic (more likely hypoplastic) rounded to angular fibers. Although the surface area is dominated by the larger fibers, histograms actually show a predominance of the small fibers. Histochemical stains reveal that the small fibers are nearly exclusively type I and the normal fibers are type II, corresponding to congenital muscle fiber-type disproportion. Modified Gomori trichrome stain. X250. (Contributed by Dr. Harvey Sarnat.)