Intrauterine radiation exposures and mental retardation.

Small head size and mental retardation have been known as effects of intrauterine exposure to ionizing radiation since the 1920s. In the 1950s, studies of Japanese atomic-bomb survivors revealed that at 4-17 wk of gestation, the greater the dose, the smaller the brain (and head size), and that beginning at 0.5 Gy (50 rad) in Hiroshima, mental retardation increased in frequency with increasing dose. No other excess of birth defects was observed. Otake and Schull (1984) pointed out that the period of susceptibility to mental retardation coincided with that for proliferation and migration of neuronal elements from near the cerebral ventricles to the cortex. Mental retardation could be the result of interference with this process. Their analysis indicated that exposures at 8-15 wk to 0.01-0.02 Gy (1-2 rad) doubled the frequency of severe mental retardation. This estimate was based on small numbers of mentally retarded atomic-bomb survivors. Although nuclear accidents have occurred recently, new cases will hopefully be too rare to provide further information about the risk of mental retardation. It may be possible, however, to learn about lesser impairment. New psychometric tests may be helpful in detecting subtle deficits in intelligence or neurodevelopmental function. One such test is PEERAMID, which is being used in schools to identify learning disabilities due, for example, to deficits in attention, short- or long-term memory, or in sequencing information. This and other tests could be applied in evaluating survivors of intrauterine exposure to various doses of ionizing radiation. The results could change our understanding of the safety of low-dose exposures.