Use a tuning fork that vibrates at 128 per second. A normal adult can feel a vigorously vibrating tuning fork for 12-15 s at the ankle and for 15-20 s on the distal phalanx of the index finger.
Instruct the patient as you place the handle of the vibrating fork on his sternum or jaw. Tell him you want to know when the vibration stops. Demonstrate this by grasping the vibrating tines of the fork with your other hand while the handle is firmly against his sternum. This will stop the vibration.
Make the tuning fork vibrate by hitting it close to the base of the tines with the heel of your hand. Put the handle on a distal bony prominence (eg, the big toe or either malleolus at the ankle; if there is no sense of vibration, you must move to a more proximal bony point). Ask the patient to tell you when the vibration stops. Let the tuning fork "run down" on its own.
When the patient says that the vibrating has stopped at the right medial malleolus, if you then quickly put the handle on the left medial malleolus (without striking the fork again) the vibration is felt again for a few seconds. In spite of what is stated in some textbooks, this is normal.
The decrease in vibratory sense may be gradual, being absent at the big toe and ankle and felt for 2-3 s at the knee, 5 s at the iliac spine, and 15 s over the spinous process of the first lumbar vertebra. These findings are con sistent with a peripheral nerve lesion or a degenerative disease of the posterior columns of the spinal cord.
If vibratory sense is absent at the ankle, knee, and pelvis and normal at some spinous process, this is consistent with a transverse, compressive, or destructive lesion of the spinal cord.
Vibratory sense and passive joint movements are each served by different portions of the posterior columns of the spinal cord and may not be equally abnormal. Apparently, otherwise normal older people have decreased or absent vibration sense at the ankles because of a segmental peripheral neuropathy of unknown cause. Subacute combined degeneration of the spinal cord is marked by a greater vibratory loss than passive joint movement loss. Tabes dorsalis is the reverse. Vibratory sense is not impaired in cerebral lesions above the thalamus, while defective sense of passive movement in one big toe may be a critical physical sign of the parasagittal, parietal meningioma (ie, it is a cortical sensation).
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