Several additional techniques test the ability of the sensory cortex to correlate, analyze, and interpret sensations. Because discriminative sensations depend on touch and position sense, they are useful only when these sensations are either intact or only slightly impaired.
Screen a patient with stereognosis, and proceed to other methods, if indicated. The patient's eyes should be closed during all these tests.
■ Stereognosis.
Stereognosis refers to the ability to identify an object by feeling it. Place a familiar object such as a coin, paper clip, key, pencil, or cotton ball, in the patient's hand and ask the patient to tell you what it is. Normally a patient will manipulate it skillfully and identify it correctly within 5 seconds. Asking the patient to distinguish “heads” from “tails” on a coin is a sensitive test of stereognosis.
■ Number identification (graphesthesia).
If arthritis or other conditions prevent the patient from manipulating an object well enough to identify it, test the ability to identify numbers. With the blunt end of a pen or pencil, draw a large number in the patient's palm (Fig. 1). A normally abled person can identify most such numbers.
FIGURE 1. Test stereognosis.
■ Two-point discrimination.
Using the two ends of an opened paper clip, or two pins, touch a finger pad in two places simultaneously (Fig. 2). Alternate the double stimulus irregularly with a one-point touch. Be careful not to cause pain.
FIGURE 2. Test two-point discrimination.
Find the minimal distance at which the patient can discriminate one from two points (normally <5 mm on the finger pads). This test may be used on other parts of the body, but normal distances vary widely from one body region to another.
■ Point localization.
Briefly touch a point on the patient's skin. Then ask the patient to open both eyes and point to the place touched. Normally a person can do so accurately.
■ Extinction.
Stimulate one side or simultaneously stimulate corresponding areas on both sides of the body. Ask where the patient feels your touch. Normally both stimuli are felt.
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