In processes that affect axons, numbness and loss of pain and thermal sensations are the main features. When the neuropathy is primarily demyelinating rather than axonal, paresthesia may be the earliest feature. The sensory loss of polyneuropathy usually involves all modalities of sensation, and—although it is difficult to compare the degrees of impairment of pain, touch, temperature, vibration, and position senses—one modality may be impaired disproportionately to the others. This clinical feature is explained by the fact that particular diseases of the peripheral nerves selectively damage sensory fibers of different size. For example, degeneration or demyelination of the large fibers that subserve kinesthetic sense causes a loss of vibratory and position sense and relative sparing of pain, temperature, and, to some degree, tactile perception. When extreme, such a polyneuropathy results in pseudoathetoid movements of the outstretched fingers or toes, manifestations of searching movements of the limbs; it may also result in a sensory ataxia because of dysfunction of the large-diameter nerves destined for the spinocerebellar tracts.
By contrast, involvement of the small-caliber, thinly myelinated and unmyelinated axons affects pain, temperature, and autonomic function, with preservation of proprioceptive, vibration, and tactile sense—producing a syndrome of “pseudosyringomyelia,” simulating the dissociated pain from tactile sensory loss that is seen in this disease of the spinal cord (see further on, under “Sensory Spinal Cord Syndromes”). Prolonged analgesia may lead to trophic ulcers and Charcot joints. These patterns of sensory loss, as well as those produced by the plexopathies and mononeuritis multiplex, are discussed further in Chap. 43.
Involvement of Nerve Roots (Radiculopathy and Polyradiculopathy) (Figs. 8-1, 8-3, and Table 8-3)
The surface innervation of the sensory nerve roots serves as one of the most useful and dependable guides to localization in neurology and the main dermatomes are known to all physicians. As already noted, there is considerable overlap from adjacent roots so that division of a single sensory root does not produce complete loss of sensation in any area of skin. However, compression of a single sensory cervical or lumbar root (e.g., by a herniated intervertebral disc) can cause a segmental impairment of cutaneous sensation. When two or more contiguous roots have been completely divided, there may be a zone of sensory loss, which is surrounded by a narrow zone in which there is a raised threshold accompanied by excessive sensitivity (hyperpathia). For reasons not altogether clear, partial sensory loss from root lesions is easier to demonstrate by the use of a painful stimulus than by a tactile or pressure stimulus.
Disease of the nerve roots frequently gives rise to “shooting” (lancinating) pains and burning sensations that project down the course of their derivative sensory nerves. The common examples are sciatica, from lower lumbar or upper sacral root compression, and sharp pain radiating from the shoulder and down the upper arm, from cervical root compression.
When multiple roots are affected (polyradiculopathy) by an infiltrative, inflammatory, or compressive process, the syndrome is more complex and must be differentiated from polyneuropathy. The distinguishing features of a polyradiculopathy, typically aside from pain, are asymmetrical muscle weakness that involves proximal and distal parts differentially in each limb and a pattern of sensory loss that is consistent with the distribution of several roots, not necessarily contiguous ones. Viewed differently, the distribution of sensory and motor deficits defies the pattern expected of multiple single peripheral nerves and fails to display the length-dependent pattern of a generalized polyneuropathy. Further details can be found in Chap. 43.
Involvement of Dorsal Root Ganglia (Sensory Neuronopathy, Ganglionopathy) (See Table 8-3)