Peripheral nerve

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  • i. Neoplasms
    • 1. schwannomas – slow growing neoplasms of Schwann cells; usually found on cranial nerves and not spinal nerve roots (but is still the most common primary spinal tumor – accounts for 30% of tumors within the spinal canal); more common in women
      • a. has two histologic patterns: Antoni A areas with spindle shaped cells with rod shaped nuclei and dense pericellular reticulin and Antoni B areas with stellate or spindle shaped cells with smaller more hyperchromatic nuclei, tenuous cytoplasmic processes and scanty surrounding reticulin loosely arranged in a myxoid stroma
      • b. may also have verocay bodies and thickened blood vessels with myaline walls
      • c. small portion of schwannomas contain melanin
      • d. label positive for S-100 and vimentin
    • 2. neurofibromas
      • a. circumscribed but not encapsulated with wavy spindle shaped cells in mucoid matrix or between bundles of collagen; composed of fibroblasts and pericytes mixed with Schwann cells
      • b. reactive for vimentin but S-100 is patchy
    • 3. malignant nerve sheath tumors
      • a. heralded by rapid enlargement, increasing neurologic deficit and pain
      • b. fascicular and storiform patterns are typical with high nuclear:cytoplasmic ratio with lot of mitotic cells, foci of necrosis is often evident; often contain melanin
      • c. most do not label with S-100 antiboides
    • 4. peripheral nerve layers
      • a. outer – epineurium
      • b. outside of fascicles within a nerve – perineurium
      • c. around individual nerves – endoneurium
      • d. in between fascicles – mesoneurium
    • 5. injury to nerves
      • a. neuropraxia – disruption of neuronal transport, no Wallerian degeneration (distal degeneration of the axon and myelin to the most proximal site of axonal interruption and may occur in both the CNS and PNS); recovers in hours to months; conduction in the neuron ceases without without structural damage to the axon NOTE: compare with axonal degeneration which occurs in more generalized, metabolically determined polyneuropathies affecting distal parts of peripheral nerves and in some instances the terminal parts of central axons in the spinal gray matter and posterior columns of the spinal cord
      • b. axonotmesis – complete interruption of axon and myelin sheath with stroma intact; Wallerian degeneration occurs; endoneurium intact; stimulation of the nerve distal to the injury sight more than 4 days after the injury does not produce an action potential
        • i. damage to the axon provokes a series of morphologic and biochemical changes in the neuronal cell body including disruption and dispersion of Nissl bodies (chromatolysis) with marked accumulation of intermediate filaments
          • 1. chromatolysis is not visible on light microscopy but changes can be seen on electron microscopy and immunohistochemistry
          • 2. chromatolysis is associated with increased protein synthesis
          • 3. regeneration in peripheral nerves will occur at a rate of 3 mm/day
      • c. neurotmesis – nerve completely severed, regeneration is impossible; endoneurium, epineurium and perineurium disrupted; stimulation of the nerve distal to the sight of injury more than 4 days after the injury will not create an action potential
        • i. complete neurotmesis will cause both a retrograde and anterograde (Wallerian) degeneration; displacement of the nuclei to the periphery of the nerve will occur with accumulation of mitochondria at the nodes of Ranvier
        • ii. retraction bulbs form at both the proximal and distal ends of the cut nerve
        • iii. order of events after neurotmesis: axonal swelling within 12 hours, disintegration of neurofibrils, then fragmentation of myelin; myelin fragmentation usually occurs on fourth day after injury
      • d. clean sharp, fresh lacerating injuries should be repaired with tension free end to end anastomoses witin 72 hours
      • e. penetrating injuries with severe or complete deficit should be explored as soon as the primary wound heals; in gunshot wounds, surgery provides little benefit
      • f. traction injuries with incomplete postganglionic injuries tend to improve spontaneously; if not recovered after 3-4 months they should be explored; neuromas in continuity that conduct a SNAP are managed by neurolysis – those that are not require grafting
      • g. neurons in the CNS generally cannot regenerate their axons as well as the PNS because the oligodendroglia express higher levels of molecules that inhibit axonal growth on their surfaces than do Schwann cells of the PNS
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