Lead Toxicity: Its Effects on Fetal and Infant Development
Lead toxicity has been an area of unending research in recent years. There have been positive and negative correlation’s relating its toxic effects to both child developmental deficiencies and adult regression problems. This review will focus on the problems associated with the children. It will discuss various routes of entry of lead into the child’s system, both prenatally and postnatally, the mechanisms employed by lead to cause the dysfunction’s, and some of the neurological deficits believed to be caused by the lead exposure.
The development of a child begins in utero and continues following birth; thus both of these time frames must be examined as possible periods of lead intoxication. During development, the fetus is at the mercy of its mother. If the mother has high blood lead levels during pregnancy, the developing fetus will have the same. This is due to the lack of a transplacental barrier to lead. Thus, the maternal levels are consistently equal to fetal levels throughout pregnancy. The mode of transport is not clearly understood. However, it has been suggested that it is a matter of simple diffusion for several reasons (1). First, is the close quantitative relationship between maternal and fetal blood lead levels. Second, is the experimentally modeled linear relationship between the transfer of lead from the mother to the fetus and the umbilical blood flow rate. An increase in blood flow rate coupled with the increased surface area of the placental barrier, 2 m2 to 11 m2, over the gestational period increases the transplacental diffusion (1). With this direct correlation in mind, it then becomes important to discuss possible sources of increased maternal lead levels in blood.
There are several possible compartments where lead can be found in the mother. The most easily measured is the blood levels. Maternal exposure to lead through the diet and inhalation increases the absorption of lead into her blood stream through the intestinal and alveolar walls, respectively. The induced elevation in maternal blood lead concentrations results in an equivalent increase in the fetal blood levels. A second compartment where lead can found is the maternal bone structure. In this case, lead and calcium must be discussed together due to the chemical similarities between the two metals. Lead can be incorporated into the bone structure of the mother as a result of previous lead exposure, up to thirty years before in some cases. Thus, whenever, net bone resorption occurs to increase blood calcium levels, lead may also be released into the circulation. During gestation, there are two such periods. The first is in the first trimester when maternal blood volume increases, thus increasing the need for calcium to hold a constant concentration. The second is the third trimester when fetal ossification begins, thus increasing the fetal requirement for calcium (2). Both cases can result in...