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In this single chapter, an attempt is made to provide an overview of the very large and difficult subject of peripheral nerve disease. Because the structure and function of the peripheral nervous system are relatively simple, one might suppose that our knowledge of its diseases would be fairly complete. Such is not the case. For example, when a group of patients with chronic polyneuropathy were investigated intensively in a highly specialized center for the study of peripheral nerve diseases several decades ago, a suitable explanation for their condition could not be found in 24 percent (Dyck et al, 1981) and equally discouraging figures prevail in our clinics today despite genetic testing. Moreover, the physiologic basis of many neuropathic symptoms continues to be elusive and in several of the neuropathies the pathologic changes have not been fully determined. However, rapidly advancing techniques in the fields of immunology and genetics are now clarifying entire categories of neuropathic disease. Also, effective forms of treatment for several peripheral neuropathies have been introduced, making accurate diagnosis imperative. For these reasons, clinicians now find the peripheral neuropathies among the most challenging and gratifying categories of neurologic disease.
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GENERAL CONSIDERATIONS
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It is important to have a clear concept of the extent of the peripheral nervous system (PNS) and the mechanisms by which it is affected by disease. The PNS includes all neural structures lying outside the pial membrane of the spinal cord and brainstem with the exception of the optic nerves and olfactory bulbs, which are special extensions of the brain. The nerves within the spinal canal and attached to the ventral and dorsal surfaces of the cord are the spinal roots, which continue to form the numbered spinal nerves; those attached to the ventrolateral surface of the brainstem are the cranial nerve roots, or cranial nerves.
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The dorsal, or posterior (afferent, or sensory), spinal roots consist of central axonal processes of the sensory and cranial ganglia. On reaching the spinal cord and brainstem, the roots extend for variable distances into the dorsal horns and posterior columns of the cord and into the spinal trigeminal and other tracts in the medulla and pons before synapsing with secondary sensory neurons, as described in Chaps. 7 and 8 that are devoted to the neurology of pain and sensation. The peripheral axons of the dorsal root ganglion cells are the sensory nerve fibers. They terminate as freely branching or specialized corpuscular endings—that is, the sensory receptors—in the skin, joints, and other tissues. The sensory nerve fibers vary greatly in size and in the thickness of their myelin covering; based on these dimensions, they are classified as type A, B, or C, as discussed in Chap. 7.
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The ventral, or anterior (efferent, or motor), roots are composed of the emerging axons of anterior and lateral horn cells and motor nuclei of the brainstem. Large, ...