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Esthesiometer

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An esthesiometer (British spelling aesthesiometer) is a device for measuring the tactile sensitivity of the skin (or mouth, or eye, etc.). The measure of the degree of tactile sensitivity is called aesthesiometry.

There are different types of aesthesiometers depending on their particular function. The simplest is a manual tool with adjustable points similar to a caliper. It can determine how short a distance between two impressions on the skin can be distinguished. To differentiate between two points and one point of equal area (the sum of the areas of the two points equals the area of the third point), Dr. Sidney Weinstein created the three-point esthesiometer (re: Weinstein Aesthesiometer). A scale on the instrument gives readings in millimeter gradients.

Another type of manual aesthesiometer is used to test lower thresholds of touch or pain. The tool uses nylon monofilaments with varying calibrated diameters. The force needed to cause the monofilament to "buckle" determines the tactile reading. The filaments are calibrated by force applied, rather than by gram/mm2 pressure ratings, because sensation follows force (when the stimulated area is small).

The Semmes-Weinstein Aesthesiometer, and its variant the Weinstein Enhanced Sensory Test (WEST, e.g., WEST-hand), present nylon monofilaments of approximately the same length (38 mm) and of varying diameters. The diameter and length are used to control the force applied. Whereas Dr. Weinstein used 3-digit numbers to reflect the force of the Semmes-Weinsein Aesthesiometer (3 digit number equals the common log of the force measured in tenths of a milligram), the WEST esthesiometers (also created by Weinstein and group) use grams (e.g., 0.70 g) to describe the force.

For small-area stimulating instruments (e.g., WEST), force, rather than area, is the appropriate measure. This is because an approximately equal area of skin is indented for the heavy and light forces (see Weinstein et al., Evaluation of Sensory Methods in Neuropathy, in Tendon and Nerve Surgery in the Hand—a Third Decade, by Hunter et al.).

The area of stimulation of the Semmes-Weinstein Aestheiometer is not correctly described by the area of the stimulating nylon (the nylon twists on the skin, pushing a sharp edge into the skin). Therefore the unit gram/mm2 is descriptive of the geometry but not the function. The WEST esthesiometer has a bulb for a contacting tip, so when the tip bends it presents the same contacting face.

An non-intrusive device called a corneal aesthesiometer is used to test cornea nerve sensitivity by using a controlled pulse of air as stimulation. The device gives readouts in millibars. Also, a thermal aesthesiometer is used to determine sensitivity of thermal stimuli.

Weinstein and group created an air-based corneal esthesiometer using gram-force (tens of micrograms force). They also created an air-based oral esthesiometer. For example, smokers' upper throats are much less sensitive than nonsmokers.

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