Behavioral Measures of Neurotoxicity (1990) / Chapter Skim
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Neurobehavioral Toxicity of Selected Environmental Chemicals: Clinical and Subclinical Aspects
Neurobehavioral Toxicity of Selected Environmental Chemicals: Clinical and Subclinical Aspects
Pages 226-242
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From page 226... ...
This is true for some metals (e.g., aluminum, lead, and mercury) , as well as for some organic compounds such as specific solvents and solvent mixtures as well as polychlorinated biphenyls (PCBs)
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From page 227... ...
Such full-blown clinical syndromes are rare in occupational and environmental exposure today, due to increased hygienic awareness, improved preventive countermeasures, and improved early diagnosis. It is recognized however that subtle, insidious, and rather nonspecific alterations of NS functioning, often classified as subclinical effects indicative of asymptomatic neurotoxic disease, do occur, may even be widespread under certain circumstances (Schaumberg and Spencer, 1987)
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From page 228... ...
After these more general remarks, specific examples illustrate how neurotoxic effects of some environmental chemicals are being characterized in terms of neurobehavioral deterioration and the extent to which psychological test methods can be used to detect early, subclinical signs of NS involvement before irreversible damage occurs. Although clinical conditions are described to some extent, the majority of findings deal with subclinical neurobehavioral effects in asymptomatic subjects.
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From page 229... ...
Blood lead concentration (PbB) is an representative marker of current lead exposure, whereas tooth lead concentrations have been used as markers of past exposure (Needleman et al., 1979~.
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Such clinical findings have led to the hypothesis that long-term low-level childhood exposure to lead might be associated with subclinical neurobehavioral deficit in asymptomatic children as well, which due to its subtlety may often go undetected. Since the early 1970s this hypothesis has been tested in about 30 cross-sectional studies using different psychological tests as well as behavior ratings to assess the degree of CNS involvement, and PbB or tooth lead levels as markers of current or past exposure.
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From page 231... ...
These effects include impaired neurobehavioral development, reduced gestational age, lowered birth weight, and other possible effects on early development and growth." In view of some divergent findings between studies, in the present author's opinion, this must be qualified as a bold statement, although it is certainly true that some of these results do raise concern about persistent neurobehavioral effects of low-level lead exposure at early stages of brain maturation. Such concern is supported by animal studies showing long-lasting neurobehavioral deficit in different species after perinatal lead exposure associated with blood levels below 30 ,ug/dL (Winneke, 1986~.
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From page 232... ...
and an outbreak in Iraq 20 years later (Bakir et al., 1973~. Population exposure in the Minamata incident was through contaminated fish from Minamata Bay, which had been polluted for years by metallic mercury from industrial sources; this was then methylated by marine microorganisms and thus introduced into the food chain.
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From page 233... ...
The likelihood of mental retardation increased with increasing maternal MeHg hair levels. In the Minamata case, follow-up studies revealed strong associations between cord blood MeHg levels and mental retardation in 20-year-old victims of prenatal exposure (Harada et al., 1977~.
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From page 234... ...
The PCB level was measured in cord and maternal serum at term. Multiple regression analysis revealed significant PCB associations only for hypotonicity and hyporeflexia; birth weight and head circumference were not related to PCB serum levels.
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In addition, cognitive performance in the Visual Recognition Memory Test (Fagan and McGrath, 1981) at 7 months of age exhibited significant association with cord serum PCB levels but not with breast milk PCB levels after control for confounding (Jacobson et al., 1985~.
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From page 236... ...
This hypothesis rests on partial similarity of dialysis dementia to presenile and senile features of Alzheimer's disease, in both clinical and pathological terms. It has been shown, for example, that in Alzheimer's patients, aluminum selectively accumulates in the nucleus of the brain cells that form the neurofibrillary tangles, typical of the Alzheimer condition; neurofibillary tangles have also been observed subsequent to injection of aluminum salts in cats and rabbits.
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From page 237... ...
As yet it is not clear whether repeated prenarcotic exposure over years may eventually give rise to irreversible brain damage. It has, however, been shown that for some compounds such as trichloroethylene, styrene, and carbon disulfide, as well as solvent mixtures, chronic low-level exposure is associated with perceptual and motor retardation which, from the very design of the different studies, could not be explained as an acute reversible effect.
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From page 238... ...
, e.g., increase of task difficulty, should also be studied and exploited in a more systematic manner to increase the sensitivity and the comparability of psychological outcome measures and, possibly, to clarify their validity. One such example is the interaction of task difficulty and lead-induced neurobehavioral deficit in serial choice reaction performance (Winneke et al., 1989~: It was shown that lead-induced deficit occurred for high but not for low signal rates.
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From page 239... ...
Even in prospective studies of early developmental lead exposure, the Bayley Scales have been used, which are known to possess poor predictive validity. Instead, the work of Pagan and coworkers on visual recognition-memory (Fagan and McGrath, 1981)
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1984. Prenatal exposure to polychlorinated biphenyls: Effect on birth size and gestational age.
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1985. The effect of intrauterine PCB exposure on visual recognition memory.
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1985. The Bender Gestalt Test as a neurobehavioral measure of preclinical visual-motor integration deficits in children with low-level lead exposure.
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