Lead poisoning overt

What are these subclinical effects Very simply, they are effects that occur at blood lead levels below those that produce clinically measurable effects - they occur in the absence of any sign of overt lead poisoning. These effects can be detected only by studying various forms of behavior, such as degree of hyperactivity and classroom attention span, and performance on various tests of intelligence and mental development. Deficits in neurobehavioral development, as measured by two widely used tests - the Bayley and McCarthy Scales - have been reported in children exposed prenatally (via maternal blood) to blood... 

Lead interferes with vitamin D metabolism, since it inhibits hydroxylation of 25-hydroxy-vitamin D to produce the active form of vitamin D. The effect has been reported in children, with blood levels as low as 10-15 pg/lOO cm3 (WHO, 1986). Measurements of the inhibitory effects of lead on heme synthesis are widely used in screening tests to determine whether medical treatment for lead toxicity is needed for children in high-risk populations who have not yet developed overt symptoms of lead poisoning. 

It does, however alleviate overt symptoms in the extremely rare case when lead poisoning is so severe as to be symptomatic. It also prevents the disease from progressing. Older children (4 to 6 years old) are more difficult to chelate because the lead stored in their bones has become part of the bony matrix and more tightly bound than in a younger child (1 to 2 years old). ... 

Many papers have attempted to define a normal blood lead level and seem to imply that values not diagnostic of lead poisoning are normal. Indeed, most papers equate the lowest symptomatic blood lead level with the upper limit of normal. A level not associated with overt clinical evidence of toxicity is not necessarily normal. However, most children with increased and therefore abnormal blood leads are reported as asymptomatic. This may be due to the absence of classical symptoms and the failure to recognize other less obvious clinical features. The evidence supporting low lead toxicity is nevertheless slight and is prone to criticism, as other variables, e.g. genetic, perinatal, nutritional and socioeconomic factors may be just as relevant in causing the sequelae detected. 

The clinical picture of overt lead poisoning in young children has long been recognized. Detailed information has been published and levels of exposure at which this may occur are well known. From these, laboratory guidelines have been produced to indicate whether excessive exposure has occurred. Periodically revised downwards, these recommendations use empirically established criteria to define exposure. Although dentine and hair lead, and urinary porphyrins have been used, blood lead, normally expressed in /tg/lOOml blood, remains the most common and convenient measure of exposure. 

Childhood clinical Pb poisoning in its most fulminant forms has typically presented in the medical literature as a cluster of overt injuries in principally three systems CNS, kidneys, and the blood-forming system. Other signs and symptoms arose from effects on the GI tract and the cardiovascular system. Some of the principal clinical characteristics of overt neurotoxicity are presented in Table 12.1. The primary sources date to the early to mid-twentieth century, the time when recognized clinical, severe lead poisonings were at their peak in the United States. 

The environmental health history of regulation of Pb in human diets dates to erratic and typically limited efforts to control or eliminate periodic outbreaks of overt lead poisonings associated with contaminated foods or beverages other than water. As chronicled by writers such as Nriagu (1983, 1985), contamination episodes were more likely than not purposeful adulteration, occasioned by an intent to cloak poor quality or other problems with foods or beverages in the marketplace. Typically, controls on such practices were highly localized, short-lived, rarely disseminated more broadly, and often ineffective. 

In summary, pertinent literature demonstrates that lead poisoning with encephalopathy results in a greatly increased incidence of permanent neurological and cognitive impairments. Also, several studies showed that children with overt, symptomatic lead intoxication in the absence of encephalopathy experience persisting neurological and behavioural impairments. 

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