Myopathies, atrophic

In severe neonatal nemaline myopathy virtually every muscle fiber shows multiple rods and all muscle fiber types are affected. However in juvenile cases, two different patterns of fiber type involvement are seen. In one there is a clear size difference between type 1 fibers, which are abnormally small (hypotrophic or atrophic) and which contain numerous nemaline rods, and type 2 fibers, which are either of normal diameter or hypertrophic and contain few, if any, nemaline rods. Other patients show a gross predominance of type 1 muscle fibers, again with rods virtually confined to this fiber type. These findings may be explicable in terms of the involvement of isoforms of a-actinin specific to slow and fast muscle fiber types. 

Plate 15.11 HematoxYlin and eosin staining of biopsy of right quadriceps skeletal muscle showing rounded fibers with increased variabUity in fiber size due to atrophy and hypertrophy (diameter 5-160 pm) consistent with a moderately severe, chronic-active myopathy. Atrophic fibers contain one or more slit-like rimmed vacuoles. (Acknowledgment Dr Rabi Tawil.)... 

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