These relatively few pathologic reactions do not, in themselves, differentiate the many dozens of diseases of the peripheral nerves, but when they are considered in relation to the selective effects on various types and sizes of fibers, the topography of the lesions, and the time course of the process, they furnish criteria for fairly accurate diagnosis. Moreover, the identification of these basic reactions is of great value in the inspection of pathologic material obtained from biopsy or autopsy.
There are additional special pathologic changes, not specifically neural in nature that characterize certain diseases of the peripheral nervous system. These involve inflammatory or vascular changes or deposition of material in the interstitium of the nerve. For example, acute demyelinative polyneuritis of the Guillain-Barré type is characterized by endoneurial infiltrations of lymphocytes and other mononuclear cells in the nerves, roots, and sensory and sympathetic ganglia. Deposition of amyloid in the endoneurial connective tissue and walls of vessels affecting the nerve fibers is the distinctive feature of inherited and acquired amyloid polyneuropathy. Diphtheritic polyneuropathy is typified by the demyelinative character of the nerve fiber change, the location of this change in and around the roots and sensory ganglia, the subacute course, and the lack of inflammatory reaction. A number of neuropathies are characterized by the deposition of antibodies and complement on the myelin sheath or on elements of the axon. These changes can be demonstrated by immunohistopathologic techniques. Many other polyneuropathies (paraneoplastic, nutritional, porphyric, arsenical, and uremic) are topographically symmetrical and represent forms of axonal degeneration but cannot be easily distinguished from one another on histopathologic grounds.
Concerning the pathology of the mononeuropathies, our knowledge is somewhat more complete. Compression of nerve or nerve roots, local or segmental ischemia, stretch, and laceration of nerves are understandable mechanisms and their pathologic changes can be reproduced experimentally. Tumor infiltration and importantly, vasculitis with ischemic infarction of nerve account for a proportion of cases. Of infections and granulomas localized to single nerves, leprosy, sarcoid, and herpes zoster represent identifiable disease states. For most of the acute mononeuropathies that are a result of transient compression, the pathologic changes have yet to be fully defined, as they are usually reversible states that provide no opportunity for complete pathologic examination. Experimental models of nerve compression indicate disruption of tubular transport and local demyelination. The common symptoms of compression such as paresthesias are explained, as discussed further on, by exposure of sodium channels along denuded axons and spontaneous and ectopic electrical discharges.
There are a number of motor, sensory, reflex, autonomic, and trophic symptoms and signs that are typical of peripheral nerve disease. Grouping them into syndromes based on their temporal and topographic features has proved to be of great value in clinical diagnosis. Although motor, sensory, reflex, and trophic changes are taken together to determine specific diagnosis, each element of the neuropathic diseases is given in the following pages.
Impairment of Motor Function