Vibration sense is tested by placing a tuning fork with a low frequency and long duration of vibration (generally 128 Hz) over the bony prominences, making sure that the patient responds to the vibration, not simply to the pressure of the fork, and that he is not trying to listen to the generated sound. Quantitative tuning forks with a 0 to 8 scale are available but it is sufficient for clinical purposes to compare the point tested with a normal part of the patient or the examiner. The examiner may detect the vibration after it ceases for the patient by holding a finger under the distal interphalangeal joint, the handle of the tuning fork being placed on the dorsal aspect of the joint. Or the tuning fork is allowed to run down until the moment that vibration is no longer perceived, at which point the fork is transferred quickly to the corresponding part of the examiner and the time to extinction is noted. There is a small degree of accommodation to the vibration stimulus, so that slight asymmetries detected by rapid shifting from a body part on one side to the other or to the examiner should be interpreted accordingly. The perception of vibration at the patella after it has disappeared at the ankle or at the anterior iliac spine after it has disappeared at the knee is indicative of a length-dependent peripheral neuropathy. The approximate level of pinprick loss from a spinal cord lesion can be corroborated by testing vibratory sensation over the iliac crests and successive dorsal vertebral spines. As with thermal and pain testing, there are mechanical devices that are able to quantitate vibration sense.
Testing of Discriminative (Parietal Lobe Cortical) Sensation
Damage to the parietal lobe sensory cortex or to the thalamocortical projections results in a particular type of disturbance—namely, an inability to make sensory discriminations and to integrate spatial and temporal sensory information (see further under “Sensory Loss Caused by Lesions of the Parietal Lobe” and Chap. 21). Lesions in these structures usually disturb complex sensory perception but leave the primary modalities (touch, pain, temperature, and vibration sense) relatively less affected. The integrity of discriminative sensory functions can be assessed only if it is first established that the primary sensory modalities on which they depend (mainly touch) are largely normal. If a cerebral lesion is suspected, discriminative function can be tested further in the following ways.
The ability to distinguish two points from one is tested by using a compass or similar device, the points of which should be blunt and applied simultaneously and painlessly. The distance at which such stimuli can be recognized as a distinct pair varies but is roughly 1 mm at the tip of the tongue, 2 to 3 mm on the lips, 3 to 5 mm at the fingertips, 8 to 15 mm on the palm, 20 to 30 mm on the dorsal hands and feet, and 4 to 7 cm on the trunk. It is characteristic of the patient with a lesion of the sensory cortex to mistake two points for one, although occasionally the opposite occurs.
Localization of cutaneous tactile or painful stimuli is tested by touching various points on the body and asking the patient to place the tip of his index finger on the point stimulated or on the corresponding point of the examiner’s limb. Recognition of numbers or letters traced on the skin (these should be larger than 4 cm on the palm) with a pencil or similar object or the direction of a line drawn across the skin also depends on localization of tactile stimuli but the two may be dissociated. Normally, traced numbers as small as 1 cm can be detected on the pulp of the finger if drawn with a pencil. These are also useful tests of posterior column function.
Appreciation of Texture, Size, and Shape