Central and peripheral nervous system dysfunction in workers exposed to lead, zinc and copper. A follow-up study of visual and somatosensory evoked potentials.

To evaluate the effects of lead, zinc and copper in the causation of physiological changes in the central and peripheral nervous systems, we measured the latencies of pattern-reversal, visually-evoked potential (VEP) and of short-latency, somatosensory-evoked potential (SSEP) twice at a 12-month interval in nineteen gun metal founders exposed to lead, zinc and copper. The workers' initial blood-lead (BPb) concentrations ranged from 16 to 64 (mean 42) micrograms/dl. BPb and plasma zinc and copper concentrations decreased significantly during the study period owing to the improvement in working environments. In the first examinations, we found that the N2 latency of VEP (conduction time from the retina to the visual cortex) and the N9-N13 latency of SSEP (conduction from the brachial plexus to the medulla oblongata) were both significantly prolonged in exposed workers. In the second examination, the N2 latency had returned to the "normal" level. These changes were correlated positively with changes in the indicators of lead absorption and inversely with changes in the indices of zinc and copper absorption. In the first examination, the N9 latency of SSEP (median nerve conduction) was also found to be significantly prolonged. This change also returned to normal over the study period in proportion to the reduction in BPb and the increase in intra-erythrocytic enzyme delta-aminolevulinic acid dehydratase activity. On the other hand, no significant prolongation of the N13-N20 latency of SSEP (central nerve conduction) was found in either of the two examinations; this latency and alteration over the study period were inversely correlated with the indicators of zinc and copper absorption. These data suggest that lead interferes with both peripheral and central nerve conduction. zinc and copper appear to antagonize strongly the lead-induced conduction delay in the upper central nervous system, but only weakly in the lower central and peripheral nervous systems.