Because of their hereditary nature, human muscular dystrophies are assumed to be inborn errors of metabolism. The ultimate classification of these diseases will depend upon recognition of the specific metabolic fault in each. At the present time, however, McArdle's disease (deficiency of muscle phosphorylase) is the only inherited myopathy in which the enzymatic abnormality has been identified. Classification, therefore, depends upon clinical grounds, and these are sometimes imprecise.The myopathy of myotonic dystrophy is different from most other dystrophies for three reasons: (1) Limb weakness is primarily distal; (2) cranial muscles (especially facial, masticatory, and oropharyngeal) are frequently affected; and (3) myotonia is present. If myotonia were not present, the syndrome would resemble "distal myopathy." Furthermore, the ocular muscles are said to be involved in some cases of myotonic dystrophy, and in some cases of "ocular myopathy," other cranial muscles and limb muscles may be involved. Transitional forms of
SCHOTLAND DL, ROWLAND LP. Muscular Dystrophy: Features of Ocular Myopathy, Distal Myopathy, and Myotonic Dystrophy. Arch Neurol. 1964;10(5):433–445. doi:10.1001/archneur.1964.00460170003001
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