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Normal somatic sensation reflects a continuous monitoring process, little of which reaches consciousness under ordinary conditions. By contrast, disordered sensation, particularly when experienced as painful, is alarming and dominates the patient's attention. Physicians should be able to recognize abnormal sensations by how they are described, know their type and likely site of origin, and understand their implications. Pain is considered separately in Chap. 11.

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Positive and Negative Symptoms

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Abnormal sensory symptoms can be divided into two categories: positive and negative. The prototypical positive symptom is tingling (pins and needles); other positive sensory phenomena include altered sensations that are described as pricking, bandlike, lightning-like shooting feelings (lancinations), aching, knifelike, twisting, drawing, pulling, tightening, burning, searing, electrical, or raw feelings. Such symptoms are often painful.

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Positive phenomena usually result from trains of impulses generated at sites of lowered threshold or heightened excitability along a peripheral or central sensory pathway. The nature and severity of the abnormal sensation depend on the number, rate, timing, and distribution of ectopic impulses and the type and function of nervous tissue in which they arise. Because positive phenomena represent excessive activity in sensory pathways, they are not necessarily associated with a sensory deficit (loss) on examination.

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Negative phenomena represent loss of sensory function and are characterized by diminished or absent feeling that often is experienced as numbness and by abnormal findings on sensory examination. In disorders affecting peripheral sensation, it is estimated that at least one-half the afferent axons innervating a particular site are lost or functionless before a sensory deficit can be demonstrated by clinical examination. This threshold varies in accordance with how rapidly function is lost in sensory nerve fibers. If the rate of loss is slow, lack of cutaneous feeling may be unnoticed by the patient and difficult to demonstrate on examination, even though few sensory fibers are functioning; if it is rapid, both positive and negative phenomena are usually conspicuous. Subclinical degrees of sensory dysfunction may be revealed by sensory nerve conduction studies or somatosensory evoked potentials (Chap. e45).

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Whereas sensory symptoms may be either positive or negative, sensory signs on examination are always a measure of negative phenomena.

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Terminology

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Words used to characterize sensory disturbance are descriptive and based on convention. Paresthesias and dysesthesias are general terms used to denote positive sensory symptoms. The term paresthesias typically refers to tingling or pins-and-needles sensations but may include a wide variety of other abnormal sensations, except pain; it sometimes implies that the abnormal sensations are perceived spontaneously. The more general term dysesthesias denotes all types of abnormal sensations, including painful ones, regardless of whether a stimulus is evident.

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Another set of terms refers to sensory abnormalities found on examination. Hypesthesia or hypoesthesia refers to a reduction of cutaneous sensation to a specific type of testing such as pressure, light touch, and ...

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