Are faster or slower large myelinated nerve fibers more sensitive to chronic lead exposure? A study of the distribution of conduction velocities

Neurotoxicity Assessment of Chemicals on Exposed Workers — A Review of Neurobehavioral and Neurophysilogical Tests

With the wide utilization of neurotoxic substances, more and more people are exposed to them occupationally or environmentally. Neurotoxicity has been defined as any adverse effect on the structure or function of the central and /or peripheral nervous system by biological, chemical and physical agent. Neurotoxic effect may be permanent or reversible, caused by neuropharmacological or neurodegenerative properties of a neurotoxicant. The nervous system is very sensitive and fragile to chemicals. The early adverse effects should be detected as early as possible because they are reversible, functional and chemical, not structural. A multidisciplinary approach is necessary to assess neurotoxicity because of the complexity and diverse functions of the nervous system. Many of the relevant effects can be measured directly by neurochemical, neurophysiological, and neuropathological techniques, whereas, others must be inferred from observed behavior and psychic performance. Neurotoxicity in humans is most commonly measured by relatively noninvasive neurophysiological and neurobehavioral methods that assess cognitive, affective, sensory, and motor function. The biomarker assay can be complement. In general, due to the speciality and difficult accessibility, the biomarkers that manifest the neurotoxicity of chemicals in nervous system are difficult to obtain, but a number of biochemical and molecular parameters similar to those involved as toxicity targets in the nervous system are also present in more easily accessible tissues, such as cerebrospinal fluid (CSF), blood, plasma and peripheral blood cells. These tissues can be surrogate indicators. With the multidisciplinary approach, the neurotoxicity of chemicals can be assessed or screened sensitively and practically.

Vibration perception thresholds in workers with long term exposure to lead

OBJECTIVES

To evaluate the impact of long term occupational exposure to lead on function of the peripheral nervous system as reflected by vibration perception threshold (VPT), measured with a portable vibrameter.

METHODS

217 Workers in a lead battery factory were required to have an annual blood lead measurement during each of the 5 years preceding this study. All were invited to take the VPT test. A total of 206 workers were studied. The associations were analysed between VPTs and current blood lead concentration, mean concentration of blood lead over the past 5 years, maximum blood lead concentration during the past 5 years, index of cumulative blood lead (ICL), time weighted index of cumulative blood lead (TWICL), and percentage of lifespan spent at work in the plant, as well as the other potential confounders. Ordinary multiple regressions, generalised additive models, and hockey stick regression analyses were used to explore the potential existence of a threshold effect of blood lead variables on VPT.

RESULTS

VPT at a frequency of 220 Hz ranged from 6 to 100 (10(-2) g, or 0.098 m/s(2)) with a mean (SD) of 19.8 (14.2) for the feet and from 4 to 43 with a mean (SD) of 10.2 (6.1) for the hands. The five variables of exposure to lead were all significantly correlated with VPT of the feet but not the hands. In multiple linear regression analyses, the mean of the blood lead concentrations and the TWICL were significantly associated with VPT of the feet. The relation between VPT of the feet and mean blood lead was shown to be a J shaped curve with a generalised additive model and local smoothing technique. In the hockey stick regression, evidence was found of a threshold effect at a mean blood lead concentration of 31 microgram/dl. Above this threshold it was estimated that each increase of 1 microgram/dl mean blood lead over 5 years would increase VPT of the feet by 0.29 (10(-2) g) or 0.028 m/s(2) (at a frequency of 220 Hz) with other potential confounders held constant.

CONCLUSION

This study suggests that measurement of vibration sensory threshold is a relatively effective tool for detecting lead neuropathy in field studies, and that lead might cause sensory neuropathy with an effect threshold corresponding to a 5 year mean blood lead concentration of 31 microgram/dl.

Subclinical neurophysiological effects of lead: A review on peripheral, central, and autonomic nervous system effects in lead workers

BACKGROUND

The peripheral nerve conduction velocity (NCV), distribution of nerve conduction velocities (DCV), somatosensory, visual, and brainstem auditory evoked potentials (SEP, VEP, and BAEP, respectively), event-related potential (P300), computerized static posturography with spectral analysis (postural balance), and electrocardiographic R-R interval variability (CV(RR)) with spectral analysis appear to be promising techniques for assessing subclinical effects of lead on the peripheral, central and autonomic nervous systems. This article presents an overview of research addressing subclinical neurophysiological effects of lead in workers exposed to lead.

METHODS

We reviewed 102 articles to examine the effects and dose-effects relationships of lead on peripheral, central, and autonomic nervous system function together with reversibility of the effects, interaction between lead and other metals, and relative sensitivity and specificity of each technique. Background and methodology were also reviewed.

RESULTS AND CONCLUSION

Available data suggest that, on a group basis, the reduction in the NCV, together with the effects on the P300 latency, postural balance, and CV(RR), occurs at a mean blood lead concentration (BPb) as low as 30-40 microg/dL; the effects on the latencies of the short-latency SEP, VEP, and BAEP, as well as the DCV, start at a BPb as low as 40-50 microg/dL. Further cross-sectional and preferably prospective studies by using each of those methods are needed to establish more precise dose-effects (and response) relationships of lead.

Effects of low level exposure to lead on neurophysiological functions among lead battery workers

OBJECTIVES

Assessment of neurophysiological functions in workers with low level exposure to lead and evaluation of the efficacy of bone lead measurements in the prediction of effects of lead.

METHODS

Exposure to lead of 60 workers from a lead battery battery factory was estimated from historical blood lead measurements and analysis of lead in the tibial and calcaneal bones with x ray fluorescence. Peripheral and central nervous system functions were assessed by measuring conduction velocities, sensory distal latencies, sensory amplitudes, and vibration thresholds as well as by quantitative measurement of the absolute and relative powers and mean frequencies of different electroencephalograph (EEG) channels.

RESULTS

Sensory amplitudes, and to a smaller degree sensory or motor conduction velocities, showed a negative correlation with long term exposure to lead, most clearly with integrated blood lead concentration and exposure time. Vibration thresholds measured in the arm were related to recent exposure to lead, those measured in the leg to long term exposure. The alpha and beta activities of the EEG were more abundant in subjects with higher long term exposure to lead. Calcaneal lead content reflected short term exposure, tibial lead content reflected long term exposure. Blood lead history showed a closer relation with effects of lead than the tibial or calcaneal lead concentrations.

CONCLUSIONS

Vibratory thresholds, quantitative EEG, and to a smaller extent the sensory amplitude, provide sensitive measures of effects of lead in occupationally exposed adults. Most accurate estimates of health risks induced by lead can be obtained from a good history of blood lead measurements. If such a history of blood lead concentrations is not available, analysis of bone lead may be used for the assessment of health risks.

Changes in the distribution of nerve conduction velocities in diabetics

To examine which of the faster or slower large myelinated nerve fibers (alpha fiber group) are affected earlier by diabetes mellitus (DM), the distribution of nerve conduction velocities (DCV) and conventional sensory nerve conduction velocities (SCV) in the median nerve were measured in 10 male DM patients aged 35-58 (mean 50) years. The patients had suffered from DM for 0-30 (mean 11) years; their fasting blood sugar (FBS) levels were 105-363 (mean 219) mg/100 ml. The DCV was expressed by the conduction velocities below which 10, 20, ......, 80 and 90% of active fibers lie (V10, V20, ......, V80 and V90 velocities, respectively). In DM patients, the V30 to V90 velocities of the DCV as well as the SCV were significantly smaller than those of age-matched male controls. The results of multiple regression analysis indicated that FBS was inversely related to V50 to V90 velocities; duration of the disease was also significantly related to V80 and V90 velocities. These data suggest that faster nerve fibers are more susceptible to DM than slower fibers are; changes in conduction velocities of the faster fibers are closely associated with FBS and duration of DM.

Autonomic and central nervous system effects of lead in female glass workers in China

To clarify the effects of lead on autonomic and central nervous system functions, electrocardiographic R-R interval variability (CVRR) as well as visual and brainstem auditory evoked potentials (VEP and BAEP) were measured in 36 female workers exposed to lead (exposed group) and in 15 female textile workers (unexposed group). The C-CVLF, C-CVHF (two component CVs of the CVRR reflecting sympathetic and parasympathetic activities, respectively), and LF/HF ratio (indicator of sympatho-vagal balance) were also computed from component spectral powers by means of autoregressive spectral and component analyses. The exposed group had engaged in glass work for 2-17 (mean 7.8) years. Blood lead (BPb) concentrations were 25.8-79.3 (mean 55.6) micrograms/dl in the exposed group and 4.7-8.6 (mean 6.3) micrograms/dl in the unexposed group. The CVRR, C-CVLF, C-CVHF, and LF/HF ratio in the exposed group were significantly lower than those in the unexposed group. Also, the exposed group had more complaints of subjective symptoms and signs than did the unexposed group. On the other hand, no significant differences in either VEP or BAEP latencies were found between the two groups. It is suggested that autonomic nervous function is more susceptible to lead than visual and auditory nervous functions; lead affects sympathetic activity more strongly than parasympathetic activity.