The detail with which sensation is tested is determined by the clinical situation. If the patient has no sensory complaints, it is sufficient to test vibration and position sense in the fingers and toes and the perception of pinprick over the extremities, and to determine whether the findings are the same in symmetrical parts of the body. A rough survey of this sort occasionally detects sensory defects of which the patient was unaware. More thorough testing is in order if the patient has complaints referable to the sensory system or if one finds localized atrophy or weakness, ataxia, trophic changes of joints, or painless ulcers.
A few other general rules are useful. It is easier for a patient to perceive the boundary of an abnormal area of sensation if the examiner proceeds from an area of reduced sensation toward the normal area. One should not press the sensory examination in the presence of fatigue, for an inattentive patient is a poor witness. Also, the examiner must avoid suggesting symptoms to the patient. After explaining in the simplest terms what is required, the examiner interposes as few questions and remarks as possible. Patients should generally not be asked, “Do you feel that?” each time they are touched; they are simply told to say “yes” or “sharp” every time they are touched or feel pain. Repetitive pinpricks within a small area of skin should be avoided, as this will make inapparent a subtle hypalgesia, because of the phenomenon of temporal summation as discussed earlier. In patients who may be overinterpreting slight changes in pinprick, differentiating between warm and cold is often more informative than differentiating between “sharp” and “dull.” The patient should not directly observe the part under examination. A cooperative patient may, if asked to use a pin or his fingertips, outline an analgesic or anesthetic area or determine whether there is a graduated loss of sensation in the distal parts of a leg or arm. Finally, the findings of the sensory examination should be accurately recorded in narrative or on a chart by shading affected regions on a preprinted figure of the body or a sketch of a hand, foot, face, or limb.
Described below are the usual bedside methods of testing sensory function. These tests are sufficient for most clinical purposes. For clinical investigation and research into the physiology of pain, which require the detection of threshold values and quantification of sensory impairment, a wide range of instruments are available. Their use has been described by Dyck and colleagues.
Testing of Tactile Sensation
This is usually done with a wisp of cotton, a tissue, or light touch of a finger. The patient is first acquainted with the stimulus by applying it to a normal part of the body. Then, with eyes closed, he is asked to indicate if the sensation feels “natural” or to say “yes” each time various other parts are touched. A patient with factitious sensory loss may say “no” in response to a tactile stimulus. Cornified areas of skin, such as the soles and palms, will require a heavier stimulus than other areas and the hair-clad parts a lighter one because of the numerous nerve endings around the follicles. Patients will be more sensitive to a moving stimulus of any kind than to a stationary one. The deft application of the examiner’s or the patient’s roving fingertips is a useful refinement and aids in demarcating an area of tactile loss, as Trotter and Davies originally showed.
More precise testing is possible by using a von Frey hair. By this method, a stimulus of constant strength can be applied and the threshold for tactile sensation determined by measuring the force required to bend a hair of known length. Spurious areas of hypesthesia or hyperesthesia may arise when a series of contactual stimuli lead to a decrement of sensation, either through adaptation of the end organ or because the initial sensation outlasts the stimulus and seems to spread.
Testing of Pain Perception