An electroneurographic assessment of subclinical lead neurotoxicity

Clinical description of toxic neuropathies

Toxic neuropathy, although rare, is an important consideration in the setting of a known or suspected toxic exposure in the workplace or other environment. This chapter discusses the clinical and electrodiagnostic evaluation of peripheral neuropathies, highlighting findings that direct further workup and may point to specific toxins as etiology. The difficulty of establishing causality of a toxin in relation to peripheral neuropathy is discussed; guidelines for establishing causality are presented. Examples of common industrial toxins are listed, including their typical industrial uses and their mechanisms of action in producing neuropathy. Characteristic clinical presentations of specific toxic neuropathies are highlighted with selected case studies.

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.

Influence of bone-lead stores on the observed effectiveness of lead hazard intervention

Lead hazard interventions have reduced children's blood-lead concentrations, but do not eliminate lead altogether from the bloodstream. Several studies suggest that blood-lead concentrations, measured 6 to 12 months after such interventions, decline by approximately 25%. The Environmental Protection Agency is preparing to promulgate a rule prescribing residential lead levels in paint, dust, and soil that constitute a lead-based paint hazard. Such a rule will prompt interventions of primary prevention character (i.e., precluding exposure before it occurs) rather than the secondary prevention character interventions (i.e., alleviating exposure after it has adversely affected the resident child) documented in the literature. It is important to attempt to estimate the efficacy achieved from the primary prevention interventions prompted by the rule's promulgation. As bone-lead stores represent the principal confounding factor to relating secondary prevention results to primary prevention, this paper addresses the impact of lead stored in bone, which may later be released to the blood and other parts of the child's body. A simple, but thoroughly documented, modeling exercise is presented to estimate the maximum length of time for which bone-lead stores alone could account for continuing elevated blood-lead levels observed in children following an intervention. The approach is based on a two-compartment model for the transfer of lead between blood and bone tissues within the body and the elimination of lead from the body. Modeling results suggest that bone-lead mobilization can impact blood-lead levels of young children for considerably long periods following an intervention. These results may explain the seemingly contradictory fact that low declines in blood-lead concentrations are observed despite the significant reduction in residential dust-, paint-, and soil-lead levels observed following lead hazard interventions. An intervention which reduces a 5-year-old child's total lead exposure by 50% might, due to mobilized bone-lead stores, produce only a 25% decline in the child's blood-lead concentrations measured 12 months following the intervention. The results also suggest, however, that those intervention strategies for which less than 25% declines were observed 12 months following the intervention likely eliminated less than 50% of the children's total lead exposure.

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.

[Possible role of the immune system in lead peripheral polyneuropathy. Case report]

This is a case report of a twenty-five years old man who developed, due to lead intoxication, a severe axonal peripheral predominantly motor neuropathy, after a shotgun injury. The projectile was retained in the right hip. Before this diagnosis had been done he was treated with corticosteroids in immunosuppressive doses and showed an improvement, but he had worsened at each attempt to interrupt the drug. Because he had also other signs of lead intoxication, such as abdominal cramps, severe anemia and seizures it was search for the blood levels of lead that was 101.2 micrograms/dl. The patient was treated with calcium disodium edetate and surgical removal of lead fragments. After that he had a good outcome with no need of corticosteroids. It is emphasized the possible relevance of the immune system on the mechanism of plumbic intoxication and the importance of the withdrawal of the lead material retained in joints.

[Neurological disorders in workers with low levels of lead in the blood. I: Peripheral neuropathy]

This is a cross-sectional study with a randomized choice of individuals aiming at studying the validity of the Brazilian biological exposure limits applied to lead level in the blood (PbB) and delta-aminolevulinic acid in the urine (ALAU), which are 60 micrograms/dl and 10 mg/g.creat., respectively. For this purpose, twenty workers, whose PbB and ALAU values have been below these limits over the past two years, were selected at random at a battery production plant in the State of S. Paulo, Brazil. The workers were submitted to a peripheral nerve conduction study. The results were compared with those obtained for workers of a control group also chosen at random. The lead workers showed a decrease in the velocity conduction of the radial nerves. Comparing this group with a randomized control group, a significant difference was observed (p-value = 0.0067). The results suggest that the Brazilian biological exposure limits above should be rearranged.

Combined electroneurographic and electromyographic studies in lead workers

OBJECTIVES

To evaluate the effects of chronic exposure to lead on the peripheral nervous system in lead workers.

METHODS

Nerve conduction velocity and electromyographic studies were performed on 31 lead workers of a battery recycling factory and 31 sex and age matched controls. 25 cases with mild distal extensor weakness of the upper limbs were classified as the lead neuropathy subgroup and the rest of the lead workers as the lead exposure subgroup. Blood lead concentrations and haematological and biochemical data were recorded. An index of cumulative exposure to lead was calculated by the summation of multiplying the average blood concentration of lead with the duration of exposure at various jobs.

RESULTS

Compared with the control group, the distal motor latency of the median nerve was significantly prolonged in the lead neuropathy subgroup, but not in the exposure subgroup. Only six of 31 workers had nerve conduction abnormalities, whereas electromyographic evidence of denervation was found in 93.5% of the lead neuropathy subgroup and 83.5% in the lead exposure subgroup. The electromyographic abnormalities found were neurogenic polyphasic waves in all 29 workers with abnormal electromyographic findings (grade in seven cases and grade ++ in the rest). Spontaneous activity was only recorded in seven workers, with grade + in four and grade ++ in three. There was a positive linear correlation between the index of cumulative exposure to lead and the distal motor latencies of the tibial nerve as well as a negative correlation with conduction velocities of the sural nerve after multivariate analysis and control of potential confounding by age and sex. No correlation could be found between the electrophysiological values and a simple duration of exposure or concentration of blood lead. A non-parametric analysis showed that there was a trend of higher index of cumulative exposure to lead with more severe electromyographic changes. Electromyographic abnormality also occurred in workers with blood lead concentrations between 17.4 and 58 micrograms/dl.

CONCLUSION

Electromyographic study in the distal extensors of the upper limbs may be used as a tool for biological monitoring of effect in lead workers.

Latency of the Achilles tendon reflex for detection of reduced functions of the peripheral nervous system in workers exposed to lead

The latency of the soleus muscle potential, evoked by a tap of the Achilles tendon, was used in the mass assessment of healthy male workers exposed to lead. Three groups were studied: a control, a low exposure, and a high exposure group. Mean blood lead concentrations were 9.3, 19.2, and 53.1 micrograms/100 ml respectively. Latencies were adjusted for age and height and then compared among the three groups. The mean corrected latency of the high exposure group showed a 4% increase compared with the other two groups (p < 0.01). The latencies of the other two groups showed no between group differences. These results were consistent with previous neurophysiological studies done by many researchers on workers exposed to lead. They suggested that nerve conduction velocities decreased from 3 to 13% among workers whose mean blood lead concentrations were more than 30 micrograms/100 ml. The method was simple, time conserving, non-invasive, and non-aversive, and provided a quantitative measure of the nerve conduction velocities of peripheral nerves. Thus it may be useful for early detection of occupationally related impairment of peripheral nerves.

Assessment of thyroid, testes, kidney and autonomic nervous system function in lead-exposed workers

The objective of the study was to assess whether moderate occupational exposure to lead may be associated with early changes in potential target organs (thyroid, testes, kidney, autonomic nervous system). Workers exposed to lead in a lead acid battery factory (n = 98; mean blood lead 51 micrograms/dl, range 40-75 micrograms/dl) and 85 control workers were examined. None of the indicators of kidney function (in urine: retinol-binding protein, beta 2-microglobulin, albumin, N-acetyl-beta-D-glucosaminidase; in serum: creatinine, beta 2-microglobulin), endocrine function (follicle-stimulating hormone, luteinizing hormone, thyroid-stimulating hormone, thyroxine, triiodothyronine) and autonomic nervous system (R-R interval variations on the electrocardiogram) were correlated with lead exposure (blood lead or duration of exposure) or showed significantly different mean values between the exposed group and controls. These results and an assessment of the published data suggest that compliance with the Directive of the Council of the European Communities on lead exposure (health surveillance in workers whose lead in blood exceeds 40 micrograms/dl and removal from exposure when blood lead exceeds 70-80 micrograms/dl) would prevent the occurrence of significant biological changes in the majority of lead-exposed workers.