University of Bristol, Cotham House, Cotham Hill, Bristol BS6 6LJ, UK
Department of Social Medicine, University of Göteborg, Sweden
Institute of Psychiatry and GKT School of Medicine, London, UK
Community Medicine, Unit for Psychosis Research, Stockholm, Sweden
Sachsska Children's Hospital, Stockholm, Sweden
University of Wales College of Medicine, Cardiff, UK
In our recent paper we reported that signs of asphyxia at birth were associated with the subsequent development of schizophrenia (Dalman et al, 2001). Crow (2001), in his invited commentary, suggested that the birth records were assessed by midwives who were not blind as to casecontrol status. As stated in the paper, we took care to eliminate this possibility and think it highly unlikely that the midwives became unblinded. We should add that, following the Vancouver agreement (International Committee of Medical Journal Editors, 1997), the midwives were not listed as authors as they only contributed to data gathering. We understand that Professor Crow has also adopted this policy in relation to the National Child Development Study interviews (Done et al, 1991).
Why were our findings so clear-cut in relation to asphyxia? There are at least two possible reasons. First, we took care to adjust for confounders and also adjusted for the association between different pregnancy and delivery complications in order to examine for an association independent of other complications. Second, by using paediatricians to examine birth records we may have been measuring birth asphyxia more accurately than with the Apgar index, which is only poorly related to asphyxia (Sykes et al, 1982). Most of the other large studies carried out recently have relied upon routinely available data on pregnancy and birth complications. This might have introduced a random measurement error and could have obscured important associations.
Finally, the paper by Thomas et al (2001) does not contradict that of Dalman et al (2001). Thomas et al (2001) were concerned only with the possibility that pregnancy and delivery complications were more strongly associated with schizophrenia in certain subgroups. The results indicated that there were no statistically significant interactions so the association between asphyxia and schizophrenia was apparent in the whole sample.
REFERENCES
Crow, T. J. (2001) Invited commentary on: Signs
of asphyxia at birth and risk of schizophrenia/Obstetric complications and
risk of schizophrenia: Does asphyxia at birth cause schizophrenia?
British Journal of Psychiatry,
179,
415-416.
Dalman, C., Thomas, H. V., David, A. S., et al
(2001) Signs of asphyxia at birth and risk of schizophrenia.
Population-based casecontrol study. British Journal of
Psychiatry, 179,
403-408.
Done, J. D., Johnstone, E. C., Frith, C. D., et al (1991) Complications of pregnancy and delivery in relation to psychosis in adult life: data from British perinatal mortality sample. British Medical Journal, 302, 1576-1580.[Medline]
International Committee of Medical Journal Editors (1997) Uniform requirements for manuscripts submitted to biomedical journals. Annals of Internal Medicine, 126, 36-47 (http://www.icmje.org/ ).
Sykes, G. S., Molloy, P. M., Johnson, P., et al (1982) Do Apgar scores indicate asphyxia? Lancet, i, 494-496.
Thomas, H. V., Dalman, C., David, A. S., et al
(2001) Obstetric complications and risk of schizophrenia.
Effects of gender, age at diagnosis and maternal history of psychosis.
British Journal of Psychiatry,
179,
409-414.
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