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                                  Weakness Myopathy-Muscle histology

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WEAKNESS; Myopathy, Anterior horn cell disease, Neuropathies, Neuromuscular transmission disease

 
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Muscle histology 

Muscle is not too smart and can only react in a limited number of ways to insult.  Thus most primary muscle diseases have non-specific features in common, such as muscle fiber necrosis, evidence for muscle fiber regeneration, structural abnormalities such as centrally located muscle fiber nuclei and an increase in muscle connective tissue (figure 6).  Some primary muscle diseases do show diagnostic changes such as nemaline rod formations or central cores.  Inflammation in muscle is important as it may indicate a treatable disease

 

Figure 6  Typical, non-specific pathological findings in a primary myopathy.  A necrotic fiber (asterix), and a hypercontracted muscle fiber (star), are shown. The entire muscle is shortened and thus, the hypercontracted fiber is thicker.  The connective tissue between the muscle fibers is increased.  

Muscle denervation 

Anterior horn cell disease or a peripheral neuropathy result in exactly the same histological findings in the muscle!  The poor muscle can only interpret these events as "I am denervated."  The pathological hallmarks of denervation are type grouping and group atrophy (figure 7).  Because one anterior horn cell/motor axon innervates a number of muscle fibers, it follows that disease of an anterior horn cell or its axon results in denervation of a number of muscle fibers.  These muscle fibers that have lost their innervation may now be innervated by healthy axons that normally innervate adjacent muscle fibers.  The end result is that now one axon innervates more muscle fibers than normal, (a giant motor unit) and also the normal checkerboard pattern of innervation is lost.  That is, a whole group of type 1 or 2 fibers can now be seen adjacent to one another (type grouping).  With progression of the disease, the axon that sprouted to innervate previously denervated muscle fibers may now also become diseased, resulting in an entire group of adjacent muscle fibers becoming atrophic (group atrophy).  

Figure 7

Denervation: Loss of nerve supply. There are many causes of denervation. Denervation may be due to a disease as, for example, in polio where the death of motor neurons causes the denervation of muscle fibers. Denervation may be due to a chemical (such as botox) or physical injury or interruption of a nerve (as by accident or to relieve pain).

Diseases that affect the lower NERVES at any point cause muscle fiber atrophy. The motor nerve exerts a influence on muscle. This influence is mediated by electrical impulses and by chemical substances (trophic factors) which, acting through the synapse, influence protein synthesis in muscle. Myofibers that lose their innervation become angular and shrink. They lose 80% to 90% of their mass within a few months. At an extreme stage of atrophy, virtually all sarcoplasm is lost and the myofiber is reduced to a cluster of nuclei. In the process of denervation, there is loss and disarray of myofilaments but no myonecrosis occurs. Myofiber atrophy can be best appreciated in cross sections

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.

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