Challenging the hypothesized link to season of birth in patients with schizophrenia

Abstract

The cause of schizophrenia is unknown; however, one hypothesis is that seasonality of birth contributes to its development, with an excess of winter-spring births observed in those with schizophrenia. There are over 200 studies exploring this issue at the writing of this article with most of the studies revealing a decrease in late summer births and an increase number of winter-spring births of those individuals with the disease.The primary objective of this study was to evaluate the seasonality of birth for 376 institutionalized patients with schizophrenia receiving clozapine treatment in a New York State psychiatric hospital. This was a retrospective review of a clozapine database accessed between January 1, 2001 to January 1, 2011, which included date of birth for the national clozapine registry, confirming the accuracy of the information. The psychiatric diagnoses for the patients included in this study were documented to ensure that this research included individuals with schizophrenia. The author hypothesized that due to the long-term institutionalization often required for refractory schizophrenia, the New York state hospitalized patients were assumed to be of Northern Hemispheric birth origin. The study sample included 376 patients, 16 of which did not have a definitive diagnosis of schizophrenia. The author chose to evaluate all 376 patients in the sample due to the introduction of clozapine therapy, which would substantiate the presence of a refractory psychotic disorder, such as schizophrenia. The author found that the seasonality distribution did not reflect any difference in percentage from that which would be expected in the general population, challenging the hypothesis that seasonality of birth contributes to the development of schizophrenia. The author provides a review of literature and discusses some of the theories surrounding the season of birth hypothesis. A larger patient sample should be studied to confirm the findings of this smaller population sample.

Keywords: Schizophrenia; month of birth; season of birth; seasonality.

Figure 1

The relative risk of schizophrenia established by Moretson et al. History of schizophrenia in a parent or sibling was associated with the highest risk, with place of birth and season of birth accounting for a significant portion of the relative risks.

Figure 2

Relative risk of schizophrenia according to the month of birth (Mortenson et al). Investigators depict their data with points and vertical bars documenting the relative risks and confidence intervals (95%) with the month of birth analyzed as a categorical variable. The curve reflects the relative risk as a function of the reference category as the month of December.

Figure 3

Adjusted mean monthly percentage of birth for patients with schizophrenia compared with general population. Parker et al demonstrated that despite the equatorial nature of the environment in Singapore, this region did show a seasonal difference between those births for patients with schizophrenia and for the general population. The peak of births for this region was the fall. The blue arrow indicates those with schizophrenia; the red arrow indicates the general population.

Figure 4

Diagnosis breakdown

Figure 5

Definitions for New York seasons—fall: September, October, November; winter: December, January, February; spring: March, April, May; summer: June, July, August

Figure 6

Seasonality distribution based on 40-week conception dates—fall: September, October, November; winter: December, January, February; spring: March, April, May; summer: June, July, August

Figure 7

Seasonality distribution based on 36-week conception dates—fall: September, October, November; winter: December, January, February; spring: March, April, May; summer: June, July, August