Prospective study on the reversibility of neurobehavioual effects in workers exposed to manganese dioxide

In 1987, a cross-sectional study in a dry-alkaline battery plant in Belgium revealed subclinical neurobehavioral dysfunctions associated with inhalati on exposure to manganese dioxide (MnO2) particulate. The overall geometric mean of the time-weighted average concentration of manganese (Mn) in "total " dust (MnT) amounted, at that time, to 1 mg Mn/m(3) and the duration of ex posure was 5.5 years on average. An 8-year longitudinal investigation was c onducted in this cohort (n = 92) in order to iind out whether early effects on eye-hand coordination (EHC), hand steadiness (HST), and simple visual r eaction time (VRT) were reversible when the airborne manganese concentratio n at the workplace was abated. During the observation period from 1988 to 1 995, MnT monitoring was implemented on a monthly basis producing more than 1300 personal air samples, EHC tests were given yearly to assess the precis ion of the hand-forearm movement (PN1), and HST and VRT tests were carried out yearly since 1991. By the end of the study, the cohort size had dropped to 34 subjects. The model of unbalanced repeated measurements with unstruc tured covariance matrix and a time-varying covariate (log MnT) was the most appropriate to analyze the data. Wald chi(2) statistic was used for testin g time-trends. The reduction of MnT over time was significantly associated with an improvement of the PN1 values (total cohort: Wald chi(2) = 8.5, p=0 .004; beta(log MnT) = -6.098 +/- 2.096). Like in the total cohort, time-tre nds were also found in the three exposure subgroups which could be identifi ed in the cohort (average MnT over 1987-1992 were about 400, 600, and 2000 mu g Mn/m(3) for the low, medium, and high exposure subgroups, respectively ). Only in the low exposure subgroup the PN 1 value normalized when MnT(pro visional estimates) decreased from about 400 to 130 mu g Mn/m3 by the end o f the study. Solely the reduction in MnT explained these findings on PN1, w hile a "healthy-worker-effect" mechanism was unlikely to have operated. The prognosis for the medium and high exposure subgroups remains uncertain as the improvement of their EHC performance may have been affected by past MnO 2 exposure to such an extent that the persistence of a partial loss of EHC ability is suggested. The time courses of the HST and VRT test results, how ever, indicated the absence of any improvement, suggesting irreversible imp airment of hand stability (postural tremor) and simple visual reaction time . A separate examination in a group of 39 control subjects, re-tested 10 ye ars alter the first test in 1987, virtually precluded age as confounding fa ctor in this prospective study. The findings of the longitudinal study are corroborated by the outcome of a separate follow-up study in a group of 24 ex-Mn employees, who showed in 1996 a significant improvement of eye-hand c oordination alter at least three years with no MnO2 exposure; as to HST and VRT; there was no significant change in the deficit of these two neurobeha vioral markers. (C) 1999 Inter Press, Inc.