Method for detecting small changes in vibrotactile perception threshold related to tactile acuity

Two metrics, expressing the change in mechanoreceptor-specific vibrotactile thresholds at a fingertip over a time interval of months or years, and the shift in threshold from the mean values recorded from the fingertips of healthy persons, have been constructed for thresholds measured from individual fingers. The metrics assume the applicability of the acute adaptation property of mechanoreceptors, which has been confirmed by thresholds obtained from 18 forest workers on two occasions, separated by 5 years. Hence, when expressed in decibels, both threshold changes and threshold shifts may be averaged at frequencies mediated by the same receptor population to improve precision. Differences between threshold changes at frequencies mediated by the same receptor population may be used to identify inconsistent subject performance, and hence potentially erroneous results. For this group of subjects, the threshold changes and threshold shifts at frequencies believed mediated by the slowly adapting type I (SAI) (4 and 6.3 Hz) and rapidly adapting type I (FAI) (20 and 32 Hz) receptors within each finger were correlated. In these circumstances, which may be expected to occur for some work-induced and systemic peripheral neuropathies, both threshold changes and threshold shifts may be summed over SAI and FAI receptors to improve precision, and hence the potential for interpretation.

Association between a quantitative measure of tactile acuity and hand symptoms reported by operators of power tools

An association between a quantitative measure of tactile acuity at the fingertips and symptoms of reduced manipulative function, as established by responses to a questionnaire, was demonstrated in a population of 81 manual workers from the mining industry (62 power-tool operators and 19 nonusers). Mechanoreceptor-specific vibrotactile thresholds were determined for the slowly adapting type I (SAI) and fast-adapting types I and II (FAI and FAII) receptors at the fingertip of the third digit of each hand. Statistically significant threshold shifts in SAI and/or FAII acuity were found in persons responding affirmatively to questions concerning finger/hand numbness, blanching, and difficulty buttoning clothing. The best predictors of a quantitative change in tactile acuity were questions relating to difficulty manipulating small objects and buttoning clothing, yielding positive predictive values of from 90% to 100% and false positive rates of from 0% to 2.8%. The demonstration of an association between a quantitative measure of tactile acuity at the fingertips and some symptom reports, obtained by means of a questionnaire, provides the basis for the development of a screening procedure for persons at risk of such disturbances in hand function.

Tactile perception in hands occupationally exposed to vibration

The sensory changes that occur in hands occupationally exposed to vibration have been assessed clinically by conventional neurologic tests and, independently, by improved techniques for the determination of tactile spatial resolution (gap detection) and vibrotactile perception thresholds at frequencies from 2 to 400 Hz. Data from 10 forest workers who were exposed to chain saw vibration and seven laboratory workers of similar age, all of whom were screened to exclude confounding factors, revealed, for the first time, three patterns of response, two of which are associated with a vibration-induced neuropathy. The first appears to be characterized by normal or better than normal thresholds in SAI, FAI, and FAII mechanoreceptor types, while a second, extreme response involves elevated thresholds in all three receptor systems (and abnormal two-point discrimination). The third pattern appears to be characterized by elevated thresholds in SAI and/or FAII receptor types.

Assessment of impaired tactile sensation. A pilot study

Three methods are compared for assessing impaired tactile sensation in vibration-exposed workers: a medical examination including traditional neurological tests and refined measures of vibrotactile perception and gap detection. Of 18 subjects only 12 were judged free of confounding factors--five forestry workers exposed to chain-saw vibration, aged 28 (SD 5) years, and seven laboratory workers not exposed to vibration, aged 36 (SD 7) years. Each method identified the same subject as suffering the most from tactile impairment, but they differed in their ranking of the severity of sensory changes. The ranking by gap detection and vibrotactile perception at low frequencies was the most consistent [Spearman rank correlation coefficient (r) = 0.90]. The clinical results, when staged according to the neurological component of vibration-induced white finger, ranked the thresholds for gap detection and low-frequency vibrotactile perception equally well (r = 0.70). In contrast, the Taylor-Pelmear staging of the clinical results poorly represented the ranking of tactile thresholds recorded for these workers (r = 0.00 and -0.20, respectively). It also appeared that improved techniques for measuring vibrotactile and gap perception thresholds can detect sensory changes in the fingers not consistently found by conventional clinical tests.

Physiological noise and its influence on vibrotactile perception thresholds

Vibration of physiological origin (physiological noise) was studied when a small diameter probe, attached to a vibrator and accelerometer for the determination of vibrotactile perception thresholds, was held in contact with a fingertip. The acceleration spectrum consisted of a broad plateau between 0.1 and 10 Hz, where the power spectral density was about -20 dB re 1 (m/s2)2/Hz, and it fell rapidly with increasing frequency above 10 Hz. Substantial contributions from respiration (0.2-1 Hz), blood circulation (1-5 Hz), and hand tremor (6-8 Hz) could be identified. The physiological noise was largely independent of subject, contact force, and probe diameter, and it was approximately equal in amplitude to the threshold of the vibrotactile perception. Current knowledge of the masking and adaptation of vibrotactile signals indicates that vibrotactile thresholds in the frequency range 2-250 Hz may be influenced by the magnitude of this physiological noise under some conditions of flesh stimulation.

Assessing the severity of the neurological component of the hand-arm vibration syndrome

In view of the observation in epidemiologic studies that the neurological component of the hand-arm vibration syndrome develops first, a conservative measure of the presence and progression of the syndrome may be obtained from the degree of numbness and loss of fine touch. Degraded tactile spatial resolution, as measured by step and gap detection with an esthesiometer, has been found, on the average, in stages 2 and 3 of the Taylor-Pelmear classification for vibration-induced white finger (VWF). Present evidence suggests that this occurrence is due to the degeneration of slowly adapting (type I) and, possibly, fast adapting (type I) mechanoreceptors, and/or their nerve fibers. The apparatus commonly employed for measuring vibrotactile perception thresholds in clinical studies records the sensitivity of fast adapting type II mechanoreceptors (pacinian corpuscles), which play no role in step or gap detection. They may, however, provide early evidence of neurological changes occurring in the syndrome, as a significant degradation in the threshold of fast adapting type II receptors appears to develop, on the average, before stage 2 of VWF.