This is most efficiently assessed by pinprick, although it may be evoked by a variety of noxious stimuli. Patients must understand that they are to report the feeling of sharpness, not simply the feeling of contact or pressure of the pinpoint. If pinpricks are applied rapidly in one area, their effect may be summated and a heightened perception of pain may result; therefore, they should be delivered about one per second and not over the same spot. Small differences in intensity can be discounted. An effective approach is to ask the patient to compare the pin sensation in two areas on a scale of 1 to 10; a report of “8 or 9” as compared to “10” is usually insignificant.
It is almost impossible, using an ordinary pin, to apply each stimulus with equal intensity. A pinwheel (Wartenberg wheel) is sometimes more effective because it allows the application of a more constant pressure, but risk of transmission of blood-borne infection from patient to patient has made this method outdated. For research purposes, this difficulty can be overcome by the use of an algesimeter, which delivers stimuli of constant intensity or with devices using lasers to produce graduated spots of heat intensity on the skin. Quantification of small-fiber sensation can be better assessed in the clinical laboratory by the use of thermal stimuli delivered by a computerized device as described below.
If an area of diminished or absent touch or pain sensation is encountered, its boundaries should be demarcated to determine whether it has a segmental or peripheral nerve distribution, or is lost below a certain level on the trunk. As mentioned, such areas are best delineated by proceeding from the region of impaired sensation toward the normal. The changes may be confirmed by dragging a pin lightly over the parts in question. Areas of reduced pinprick sensation can be corroborated by the thermal sense examination, as below.
Testing of Thermal Sense
A quick but rough way to assess thermal loss (or to corroborate a previously found zone of hypalgesia) is to warm one side of a tuning fork by rubbing it briskly against the palm and apply its alternate sides to the patient’s skin and asking the patient which side is colder (or warmer). One advantage of this mode of testing is that it offers the patient a binary choice of “warm” or “cold” response. This suffices for most bedside examinations.
If more careful examination is required, the skin should first be exposed to room air for a brief time. The test objects should be large, ideally two stoppered test tubes containing hot (45°C/113°F) and cold (20°C/68°F) tap water. The use of warm and cool water from a tap is adequate and more practical for this purpose. The side of each tube is applied successively to the skin for a few seconds and the patient is asked to report whether the flask feels “less hot” or “less cold” in comparison to a normal part. It is of interest that patients are able to recognize a difference of 1°C (1.8°F) or even less in the range of 28°C (82.4°F) to 32°C (89.6°F); in the warm range, differences between 35°C (95°F) and 40°C (104°F) can be recognized, and in the cold range, between 10°C (50°F) and 20°C (68°F). If the temperature of the test object is below 10°C (50°F) or above 50°C (122°F), sensations of cold or heat become confused with pain. This technique has been largely supplanted by commercial devices that can present a series of slightly differing thermal stimuli in sequence to a probe placed on the finger or toe. Special algorithms are used to vary the order and magnitude of temperature change and to determine whether the patient’s reports are consistent and valid. The results are reported in the form of a “just noticeable difference” (JND) between temperatures or intensities of pain.
Testing of Deep-Pressure Pain