Anesthesia refers to a complete loss and hypesthesia to a partial loss of all forms of sensation. Loss or impairment of specific cutaneous sensations may be indicated by an appropriate prefix or suffix, for example, thermoanesthesia or thermohypesthesia (loss and reduction in temperature sense), analgesia (loss of pain sense), hypalgesia (reduction in pain sensibility), tactile anesthesia, and pallanesthesia, or apallesthesia (loss of vibratory sense). The term hyperesthesia, as explained in Chap. 7, refers to an increased sensitivity to various stimuli and is usually used with respect to cutaneous sensation. It implies a heightened activity of the sensory apparatus. Under certain conditions (e.g., sunburn), there does appear to be an enhanced sensitivity of cutaneous receptors, but usually the presence of hyperesthesia betrays an underlying sensory defect. Careful testing will demonstrate an elevated threshold to tactile, painful, or thermal stimuli; but once the stimulus is perceived, it may have a severely painful or unpleasant quality (hyperpathia). Some clinicians use this last term to denote an exaggerated response to a painful stimulus (hyperalgesia which is subtly different from hyperpathia, denotes an abnormally painful reaction to a painful stimulus; see Table 8-2). In alloesthesia, or allesthesia, a tactile or painful stimulus delivered on the side of hemisensory loss is experienced in a corresponding area of the opposite side or at a distant site on the same side. This phenomenon is observed most frequently with right-sided putaminal lesions (usually hemorrhage) and with anterolateral lesions of the cervical spinal cord; it presumably depends on the existence of an uncrossed ipsilateral spinothalamic pathway (see the original studies of Ray and Wolff).
Most neurologists would agree that sensory testing is the most challenging part of the neurologic examination. For one thing, test procedures are relatively crude and are unlike the natural modes of stimulation with which the patient is familiar. It is also fair to say that few diagnoses are made solely on the basis of the sensory examination; more often the exercise serves to complement the remainder of the examination. There is often difficulty in evaluating the response to sensory stimuli, as it depends on the patient’s interpretation of sensory experiences. This, in turn, will depend on the patient’s general awareness and responsiveness and ability to cooperate, as well as degree of suggestibility. Children, by virtue of their simple and direct responses, are often better witnesses than more sophisticated individuals who are likely to analyze their experiences minutely and report small and insignificant differences in stimulus intensity.
Quite often, no objective sensory loss can be demonstrated despite symptoms that suggest the presence of such an abnormality. Only rarely does the opposite occur, in which one discovers a sensory deficit when there has been no complaint of sensory symptoms. Sensory symptoms such as paresthesias or dysesthesias may be generated from nerves not sufficiently diseased to reduce sensory function. Furthermore, loss of sensory function may be so mild and gradual as to pass unnoticed.
Before proceeding to sensory testing, the physician should question patients about changes in sensation; this poses particular problems. Patients may be confronted with derangements of sensation that are unlike anything they have previously experienced, and they have limited terms to describe what they feel. They may say that a limb feels “numb” and “dead” when in fact they mean that it is weak. Occasionally a loss of sensation is discovered almost accidentally, for example, by a lack of pain on touching an object hot enough to blister the skin or unawareness of articles of clothing and other objects in contact with the skin. Often, disease induces new and unnatural sensory experiences such as a band of tightness, a feeling of the feet being encased in cement, lancinating pains, an unnatural feeling when stroking the skin, a sensation as if walking on pebbles, and so on. Even adventitious muscle movements may be reported as a sensory experience akin to paresthesias or other sensations.