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George E. Carvell, PhD, PT   University of Pittsburgh
 
PERIPHERAL NERVE DYSFUNCTION IV: AXONAL DEGENERATION
Ben.  fig 13-31  p 421
Peripheral Neuropathies are diseases of the peripheral nervous system axons anywhere from the roots to distal axonal branches in the periphery.  Distal axonal degeneration results in denervation of muscle fibers and sensory receptors in the periphery. Both sensory and motor axons may be affected but some neuropathies have a predilection for one type vs. another. Axonal degeneration is accompanied by a reactive change in the motoneuron soma: chromatolysis. The soma swells and the RNA machinery gears up for axonal regeneration. Motor Nerve Conduction Studies may be normal until many axons are involved. Sensory Nerve Conduction may show reduced amplitudes of evoked potentials early, or no evoked response if many
axons are involved.This process is usually seen in the distal most segments and may progress proximally (dying-back phenomenon). A dying-back is thought to be related to poor health of the distal segments due to poor axonal transport and/or poor perfusion in the distal limb. Sensory changes associated with large fiber conduction (e.g., discriminative touch, proprioception) or small fiber (pain & temperature) may be found, as well as, reduced or absent deep tendon reflexes for involved musculature. Electrical signs of muscle denervation are typically found in the clinical EMG needle exam (fibrillation potentials and positive sharp waves). Abnormal Motor Unit Potentials may be seen especially if denervated fibers are "rescued" by surviving axon branches. Weakness may be variable depending on the stage and severity of nerve involvement. Early muscle fatigue may be the most significant functional problem that may or may not be revealed by a routine manual muscle exam. Polyneuropathies are often distal and symmetric. Examples include diabetic, alcoholic, or toxic polyneuropathies that show distal muscle weakness and atrophy, and a distal "stocking-glove" pattern of sensory loss. There are many variations to these patterns of involvement, e.g., small fiber vs. large fiber disease. Some investigators have suggested that the nerves are sick and therefore more susceptible to a second insult such as pressure or ischemia.
GMOMM  2001
Recruit motor units more often/
Unit of force