Academic journal article Environmental Health Perspectives

Effect of Lead Exposure and Ergonomic Stressors on Peripheral Nerve Function

Academic journal article Environmental Health Perspectives

Effect of Lead Exposure and Ergonomic Stressors on Peripheral Nerve Function

Article excerpt

In this study we investigated the effect of recent and chronic lead exposure, and its interaction with ergonomic stressors, on peripheral nerve function. In a cross-sectional design, we used retrospective exposure data on 74 primary lead smelter workers. We measured blood and bone lead levels and, from historical records, calculated lead dose metrics reflecting cumulative lead exposure: working-lifetime integrated blood lead (IBL) and working-lifetime weighted-average blood lead (TWA). We additionally created five metrics related to IBL that cumulated exposure only above increasing blood lead levels ranging from 20 to 60 [micro]g/dL (IBL20-IBL60). Current perception threshold (CPT) assessed large myelinated (CP[T.sub.2000]), small myelinated (CP[T.sub.250]), and unmyelinated (CP[T.sub.5]) sensory nerve fibers. Using multiple linear regression, we modeled CPT on the different measures of lead dose after adjusting for relevant covariates. CPT had a curvilinear relationship with TWA, with a minimum at a TWA of 28 [micro]g/dL. Both TWA and IBL accounted for a significant percentage of the variance of CP[T.sub.2000] ([DELTA][R.sup.2] = 8.7% and 3.9%, respectively). As the criterion blood lead level increased from IBL20 through IBL60, so did the percentage of CP[T.sub.2000] variance explained, with [DELTA][R.sup.2] ranging from 5.8% (p < 0.03) for IBL20 to 23.3% (p < 0.00) for IBL60. IBL60 also significantly contributed to the explanation of variance of CP[T.sub.250] and significantly interacted with ergonomic stressors. Measures of chronic blood lead exposure are associated with impairment of large and small myelinated sensory nerve fibers. This effect is enhanced at the highest doses by ergonomic stressors. Key words: bone lead, cumulative lead dose, ergonomic stressors, lead dose thresholds for peripheral nerve, peripheral nerve fiber size. Environ Health Perspect 113:1730-1734 (2005). doi:10.1289/ehp.8106 available via http://dx.doi.org/[Online 8 August 2005]

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The classic description of lead neuropathy is that of a motor neuropathy that typically presents as wrist drop. More recently, investigators demonstrated that, in the development of lead neuropathy, sensory nerve fibers are affected earlier than motor nerve fibers (Chuang et al. 2000; Kovala et al. 1997; Rubens et al. 2001; Schwartz et al. 2001; Singer et al. 1983), and nerve conduction studies showed mild slowing of both sensory and motor conduction velocities as well as diminished amplitude of the sensory potential (Araki et al. 1986; Ashby 1980; Baker et al. 1984; Bordo et al. 1982; Buchthal and Behse 1979; Catton et al. 1970; Chen et al. 1985; Chia et al. 1996a, 1996b; Jeyaramam et al. 1985; Kovala et al. 1997; Pasternak et al. 1989; Rubens et al. 2001; Seppalainen and Hernberg 1980; Seppalainen et al. 1979; Singer et al. 1983; Yeh et al. 1995). After reviewing the lead neuropathy literature from 1974 to 1984, Ehle (1986) concluded that sensory nerve conduction is more likely to be affected than is motor nerve conduction, that the upper extremities are more likely to be involved than the lower extremities, and that these effects usually occur after a year of lead exposure, with a continuous linear relationship between blood lead and nerve conduction velocity only when blood lead exceeded 70 [micro]g/dL.

In evaluating peripheral nerve function, electrodiagnostic testing examines the integrity of only large myelinated nerve fibers with the fastest conduction velocities. Current perception threshold (CPT), a neuroselective test, measures sensory nerve conduction threshold in three nerve fiber populations--large myelinated (A[beta]), small myelinated (AS), and unmyelinated (C) nerve fibers. In peripheral neuropathies associated with a variety of medical conditions, CPT abnormalities have demonstrated good agreement with nerve conduction studies (Katims et al. 1989; Rendell et al, 1989; Weseley et al. 1989). Additionally, pathology in the small myelinated and unmyelinated nerve fibers shown with CPT but not detected by routine nerve conduction studies occur in Fabry's disease (Ro et al. …

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