Motor unit territory and fiber density in myopathies

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Abstract
By means of multilead recordings, the territory and the number of muscle fibers per cross-sectional area of a motor unit (fiber density) were determined in myopathies. In the pseudohypertrophic type of progressive muscular dystrophy (10 patients), both motor unit territory and fiber density were significantly lower than in normal muscle. In the facioscapulohumeral and the limb-girdle types of muscular dystrophy (8 patients), average motor unit territory and fiber density were the same as in normal muscle. In myotonic dystrophy (4 patients), the average motor unit territory was reduced to the same degree as in the generalized form of muscular dystrophy but the fiber density was the same as in normal muscle. In polymyositis (6 patients), the motor unit territory was reduced to the same degree as in the generalized form of muscular dystrophy but the fiber density was the same as in normal muscle. The changes in territory and fiber density are related to other electro-myographic findings in myopathies, such as the decrease in average action potential duration and the increased incidence of polyphasic potentials. In terms of histologic features, these changes are related to the average diameter of muscle fibers, the proliferation of connective tissue, and the increase in subunit size by peripheral sprouting.