Cognitive function and prenatal exposure to ionizing radiation

It is clear from the many studies of the prenatally exposed survivors of th e atomic bombing of Hiroshima and Nagasaki that exposure to ionizing radiat ion during gestation has harmful effects on the developing human brain, par ticularly if that exposure occurs at critical stages in the development of the neocortex. Data on a variety of measures of cognitive function, includi ng the occurrence of severe mental retardation as well as variation in the intelligence quotient (IQ) and school performance, show significant effects on those survivors exposed 8-15 weeks and 16-25 weeks after ovulation. Stu dies of seizures, primarily those without known precipitating cause, also e xhibit a radiation effect on those individuals exposed in the first 16 week s after ovulation. The cellular and molecular events that subtend these abn ormalities are still largely unknown although some progress toward an under standing has occurred. For example, magnetic resonance imaging of the brain of some of the mentally retarded survivors has revealed a large region of abnormally situated gray matter, suggesting an abnormality in neuronal migr ation, but cell killing could also contribute importantly to the effects on cognitive function that have been seen. The retardation of growth in statu re observed in individuals exposed in the first and second trimesters of pr egnancy suggests that the development of an atypically small head size, wit hout conspicuously impaired cognitive function, may reflect a generalized r etardation of growth. (C) 1999 Wiley-Liss, Inc.