Muscle biopsy specimens from 179 cases of muscular dystrophies and from 140 cases of anterior horn cell disorders (from a total of 1,348 biopsied patients) were examined histologically. There were 72 cases of Duchenne type muscular dystrophy (DMD), five of Becker type MD, four girls with myopathy resembling DMD, 40 with limb-girdle, 10 with facioscapulohumeral, seven with late onset, 13 with congenital, and 28 with unclassifiable muscular dystrophies. Groups of small atrophied muscle fibres were encountered in 42 (23%) of the cases in this group, most frequently in patients with limb-girdle, facioscapulohumeral, and least frequently with DM dystrophy. In the second group there were 25 cases of infantile, 38 of juvenile, and 39 of adult spinal muscular atrophy (SMA); there were 21 patients with motor neurone disease (MND), six with poliomyelitis, and 11 with an unclassifiable type of anterior horn cell disorder. Pseudomyopathic changes were encountered in 43 (30%) of all cases in this group. They were most frequently present among patients with juvenile and adult SMA and in those with MND. The presence of group atrophy in muscular dystrophy is considered significant myopathological evidence of a denervation process. On the other hand, pseudomyopathic changes, variation in fibre size, rounding, central nuclei, and increase in connective tissue occurring in various anterior horn cell disorders are seen not to be specific `myopathic' changes. Thus there was an overlap of pathological reactions in muscles from the dystrophies and the neurogenic atrophies. Comparably atrophied fibres (much less than 2 SDs below the normal mean diameter) and hypertrophied fibres (much more than 2 SDs above the normal mean diameter) were encountered in both dystrophy and neurogenic atrophy, considering the large muscles of the limb. Likewise, the mean fibre diameters were comparable in DMD and in juvenile SMA. The fourth evidence of a neurogenic factor in muscular dystrophy was derived from an examination of SDH preparations of muscle. There was a preponderance of type I muscle fibres in dystrophic muscles compared with specimens from controls, suggesting depletion of type II fibres. It appears that the concept of muscular dystrophy as a primary muscle disease needs to be re-examined.
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