Dysfunction of somatosensory axons in nerves or posterior spinal nerve roots may cause the symptom of numbness or the sign of diminished somatosensation, usually of all modalities, of the innervated areas of the body.
Dysfunction of peripheral somatosensory axons may also cause: spontaneous or evoked positive somatosensory symptoms that are not unpleasant, called paresthesias; spontaneous positive somatosensory symptoms that are unpleasant or painful, called dysesthesias; or the perception of pain to stimuli that are not normally noxious, called allodynia. These can be called positive somatosensory abnormalities, as opposed to the negative somatosensory abnormalities of numbness or somatosensory loss. Negative somatosensory abnormalities may occur with central or peripheral lesions, but positive somatosensory abnormalities occur more often with peripheral lesions.
Somatosensory abnormalities may include any or all of the symptoms of numbness, paresthesias, dysesthesias, and allodynia, or the sign of somatosensory loss. Symptoms from dysfunction of somatosensory pathways may radiate (spread) beyond the somatosensory territory of the involved structure, but the sign of somatosensory loss is usually restricted to only that territory. Some modalities of somatosensory loss can be somewhat quantified on examination by applying the same stimulus to normal and abnormal areas of skin, and asking the patient to estimate a percent they feel the stimulus compared to 100% on the normal skin.
Somatosensation can be quickly screened with touch, or examined in more detail for the other primary modalities. Touch sense can be gross or fine: gross touch perception is poorly localized to a large area, while fine touch sense localizes to a small area, such as distinguishing two points applied to the skin very close together. Fine and gross touch sense usually travel in different tracts in the central nervous system. Pain sense is usually examined with a pin, or nailbed pressure for patients with decreased arousal. Temperature sense is usually examined with the cool metal of a tuning fork, although any cool or warm object can be used. Vibration sense is examined with a vibrating tuning fork held against a bony prominence. Position sense is examined by passively moving a joint with the patient’s eyes closed; passive movement refers to the examiner moving a body part rather than the patient moving it.