Sensory nerve roots supply cutaneous innervation to specific dermatomes. The dermatome innervation of the extremities is complex, in part due to the migration of the limb buds during embryonic development. As a result, the C4-C5 dermatomes abut T1-T2 on the upper chest, and the L1-L2 dermatomes are close to the sacral dermatomes on the inner aspect of the thigh near the genitalia. The generally available dermatomal charts are primarily derived from three sources: Head and Campbell, Foerster, and Keegan and Garrett, who all used very different approaches. Figure 26.5 shows the dermatome distributions as depicted by Keegan and Garrett.

Sensory function is divided clinically into primary modalities and secondary or cortical modalities. The primary modalities include touch, pressure, pain, temperature, joint position sense, and vibration. The cortical or secondary modalities are those that require synthesis and interpretation of primary modalities by the sensory association area in the parietal lobe. These include two-point discrimination, stereognosis, graphesthesia, tactile localization, and others. When the primary modalities are normal in a particular body region, but the cortical modalities are impaired, a parietal lobe lesion may be responsible. Itch and tickle sensations are closely allied to pain; they are probably perceived by the same nerve endings and are absent following procedures used for the relief of pain.

Many terms have been used, not always consistently, to describe sensory abnormalities. The definition of esthesia is perception, feeling, or sensation (Gr. aesthesis “sensation”). Algesia refers to the sense of pain (Gr. algos “pain”). Hypalgesia is a decrease, and analgesia (or analgesthesia) an absence, of pain sensation. The combining form “algia” refers to any painful condition. Hypesthesia is a decrease, and anesthesia an absence, of all sensation. Paresthesia is an abnormal sensation; dysesthesia (Gr. dys “bad”) is an abnormal, unpleasant, or painful sensation. Table 22.1 summarizes some of the definitions.

Sensory abnormalities may be characterized by an increase, decrease, absence, or perversion of sensation. An example of increased sensation is pain—an unpleasant or disagreeable feeling that results from excessive stimulation of certain sense organs, fibers, or tracts. Perversions of sensation take the form of paresthesias, dysesthesias, and phantom sensations. Impairment and loss of sensation result from decreased acuity of the sensory organs or receptors, impaired conduction in sensory fibers or tracts, or dysfunction of higher centers causing impairment in the powers of perception or recognition.

The sensory examination is performed to discover whether areas of absent, decreased, exaggerated, or perverted sensation are present, and to determine the type of sensation affected, the degree of abnormality, and the distribution of the abnormality. Findings may include loss, decrease, or increase of one or more types of sensation; dissociation of sensation with loss of one modality type but not of others; loss of ability to recognize differences in degrees of sensation; misinterpretations (perversions) of sensation; or areas of localized hyperesthesia. More than one of these may occur simultaneously.

The sensory examination is arguably the most difficult and tedious part of the neurologic examination. Some examiners prefer to assess sensory functions early in the course of the examination, when the patient is most likely to be alert and attentive. Fatigue causes faulty attention and slowing of the reaction time, and the findings are less reliable when the patient has become weary during the examination. Others argue the routine sensory examination is the most subjective and least useful part of the neurologic examination, and prefer to leave it until the end. Since the results depend largely on subjective responses, the full cooperation of the patient is necessary if conclusions are to be accurate. Occasionally, objective evidence, such as withdrawal of the part
stimulated, wincing, blinking, and changes in countenance, may aid in the delineation of areas of sensory change. Pupillary dilation, tachycardia, and perspiration may accompany painful stimulation. Keenness of perception and interpretation of stimuli differ in individuals, in various parts of the body, and in the same individual under different circumstances.