Muscle Histology
Weakness Myopathy
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At an early stage, denervation causes atrophy of isolated myofibers, which are scattered in a random fashion. In chronic denervating processes such as chronic neuropathy and motor neuron disease, remaining healthy axons sprout and synapse with denervated fibers (collateral reinnervation). As a result of the combined denervation and reinnervation, motor units enlarge, and their fibers, instead of being scattered, come to lie adjacent to one another. In histochemical stains, such motor units appear as groups of myofibers of the same histochemical type (fiber type grouping). When ultimately these motor units lose their innervation and there are no healthy axons left to connect with them, all their fibers shrink together (group atrophy).
Reinnervation often causes a change in myofibers, the target fiber, characterized by absence of oxidative enzyme activity in the center, surrounded by a rim of more intense than normal activity. Target fibers may be confused with central cores, which occur in a congenital myopathy-central core myopathy.
In the EMG, large motor units appear as polyphasic or giant motor unit potentials. Denervated muscle is overexcitable. Spontaneous discharges from individual myofibers are picked up by the EMG as fibrillations. Spontaneous firing of an entire motor unit causes cotraction of a small group of myofibers that appears as a ripple on the surface of the muscle (fasciculation). Fasciculations of the tongue are an important sign of motor neuron disease. Since denervation does not cause myonecrosis, there is no elevation of CK.
Denervation atrophy is caused by peripheral
neuropathies and motor neuron diseases. The
most common motor neuron disease in adults is
amyotrophic lateral sclerosis. In children, it is
the autosomal recessive spinal muscular atrophy
and its variants . Lower motor neuron damage may also be
caused by enteroviruses which include the poliomyelitis
virus and some arthropod borne viruses, especially West
Nile Virus.