No abstract

These data are consistent with the hypothesis concerning the possible neurodevelopmental contribution to the origins of some forms of major affective disorder, especially unipolar depressive disorder. These encouraging findings, if replicated, may suggest that some mental disorders may stem, in part, from a disturbance in the development of the fetal brain during the second trimester.

We reported previously that residents of Greater Helsinki, Finland, whose mothers were exposed to the 1957 influenza epidemic during their second trimester of gestation had a significantly elevated risk of developing adult schizophrenia. The majority of the replication studies to date have not determined whether the mothers actually contracted an infection or the stage of gestation based on mother's last menstruation. We read prenatal clinic records of the mothers of the Helsinki-born schizophrenia subjects to determine timing of infection, as noted by the prenatal clinic obstetric nurse at a time close to the actual infection. Schizophrenia subjects who were exposed in the second trimester had a significantly higher rate of definite influenza infection (86.7%) in that period compared to those who were exposed during the first and third trimesters (20.0%). These results are interpreted with caution because of the small number of cases.

Birth occurring in winter months, which are high viral infection months, have been repeatedly shown to produce a slight excess of later-diagnosed schizophrenics. As a result, some researchers have speculated on the possible aetiological effect of viral infections on some forms of schizophrenia. The implications of the viral hypothesis were indirectly tested in the context of an ongoing prospective study of Danish children at high-risk (HR) for schizophrenia. A third-order analysis of variance interaction was hypothesized. Genetically vulnerable individuals, born in winter, in an urban environment (which increases the likelihood of the presence and transmission of viruses) would be more likely, as foetuses or neonates, to have suffered some CNS damage due to the infection; thus they would show higher rates of schizophrenia in the HR-urban-winter birth condition reached 23.3 per cent, considerably above population base rates (1 per cent) or rates for the HR subjects (8.9 per cent). Alternative explanations for the results were explored.