The Sensory System

To evaluate the sensory system, you will test several kinds of sensation:

■ Pain and temperature (spinothalamic tracts)

■ Position and vibration (posterior columns)

■ Light touch (both of these pathways)

■ Discriminative sensations, which depend on some of the above sensations but also involve the cortex

Familiarize yourself with each kind of test so that you can use it as indicated. When you detect abnormal findings, correlate them with motor and reflex activity. Is the underlying lesion central or peripheral?

See Table 16-12, Disorders of the Central and Peripheral Nervous Systems (pp. 615-617).

Patterns of Testing. Because sensory testing quickly fatigues many patients and then produces unreliable results, conduct the examination as efficiently as possible. Pay special attention to those areas (1) where there are symptoms such as numbness or pain, (2) where there are motor or reflex abnormalities that suggest a lesion of the spinal cord or peripheral nervous system, and (3) where there are trophic changes, such as absent or excessive sweating, atrophic skin, or cutaneous ulceration). Repeated testing at another time is often required to confirm abnormalities.

Meticulous sensory mapping helps to establish the level of a spinal cord lesion and to determine if a more peripheral lesion is in a nerve root, a major peripheral nerve, or one of its branches.

The following patterns of testing help you to identify sensory deficits accurately and efficiently.

■ Compare symmetric areas on the two sides of the body, including the arms, legs, and trunk.

Hemisensory loss due to a lesion in the spinal cord or higher pathways

■ When testing pain, temperature, and touch sensation, also compare the distal with the proximal areas of the extremities. Further, scatter the stimuli so as to sample most of the dermatomes and major peripheral nerves (see pp. 542-546). One suggested pattern includes both shoulders (C4), the inner and outer aspects of the forearms (C6 and T1), the thumbs and little fingers (C6 and C8), the fronts of both thighs (L2), the medial and lateral aspects of both calves (L4 and L5), the little toes (S1), and the medial aspect of each buttock (S3).

Symmetric distal sensory loss suggests a polyneuropathy, as described in the example on the next page. You may miss this finding unless you compare distal and proximal areas.

■ When testing vibration and position sensation, first test the fingers and toes. If these are normal, you may safely assume that more proximal areas will also be normal.

■ Vary the pace of your testing. This is important so that the patient does not merely respond to your repetitive rhythm.

■ When you detect an area of sensory loss or hypersensitivity, map out its boundaries in detail. Stimulate first at a point of reduced sensation, and move by progressive steps until the patient detects the change. An example is shown at right.

Vibration. Use a relatively low-pitched tuning fork of 128 Hz . Tap it on the heel of your hand and place it firmly over a distal interphalangeal joint of the patient's finger, then over the interphalangeal joint of the big toe. Ask what the patient feels. If you are uncertain whether it is pressure or vibration, ask the patient to tell you when the vibration stops, and then touch the fork to stop it. If

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