"Myopathic" Changes in Chronically Denervated Muscle

TLDR

Neuromuscular disorders are classified as neuropathies, which damage nerves and cause grouped muscle atrophy, or myopathies, which directly affect muscle fibers and produce scattered architectural changes, with biopsy interpretation relying on these distinct patterns. Muscle biopsy interpretation relies on two principles: denervation causes grouped atrophy of fibers within a motor unit, whereas myopathies produce random, scattered changes in individual fibers.

Abstract

NEUROMUSCULAR diseases are generally divided into two categories: the neuropathies and the myopathies. Neuropathic disorders are those in which motor nerve cells or their processes are first affected, resulting secondarily in atrophy of muscle. In the primary myopathies the disease process is assumed to involve the muscle directly, without damaging its nerve supply. Clinical, laboratory, and electromyographic findings are often useful in distinguishing between these two classes of disorders, but the muscle biopsy may be the deciding factor. In general, interpretation of the muscle biopsy has been based on two principles: (1) Following denervation, muscle fibers belonging to the same motor unit undergo atrophy together and are located in groups. By contrast, myopathies attack individual muscle fibers at random, giving rise to scattered changes throughout the muscle. (2) In myopathic disorders architectural changes are found in medium-sized and larger muscle fibers. On the other hand, denervation leads to