No Association between the Deficit Syndrome in Psychosis and Summer Birth in a Southern Hemisphere Country

Joy Welham, David Chant, S. Saha, J. McGrath , Queensland Centre For Mental Health Research, The Park Centre for Mental Health, Wacol, Australia, Brian Kirkpatrick, Maryland Psychiatric Research Center, University of Maryland, Baltimore, Maryland, US, and David Castle, Mental Health Research Institute, Melbourne, Australia:

The deficit syndrome (i.e. primary, enduring negative symptoms) defines a distinct subgroup of patients with schizophrenia. Those with the deficit syndrome differ from their non-deficit counterparts in signs and symptoms, course, biological correlates, treatment response, and putative aetiologic factors [1]. A number of studies suggest that the deficit syndrome is associated with summer birth. This association has been found in studies from the United States [2, 3], England [4] and Scotland [5, 6]. Furthermore, a recent pooled analysis of these studies confirmed this pattern [7]. While a study by Dolfuss et al. [8] failed to find such an association, the sample was not population-based. This study extends this work to examine whether such an association is found (i) when a broader definition of psychosis is used; and (ii) in a Southern Hemisphere country, where a previous meta-analysis of Australian studies found an attenuated season-of-birth effect [9].

Data were obtained for Australian-born individuals with non-affective psychotic disorders (here defined as DSM-III-R schizophreniform disorder, schizophrenia, delusional disorder and atypical psychosis). These subjects were identified from the four catchment areas (south-east Queensland, Australian Capital Territory, Melbourne, Perth) included the National Survey of Mental Health and Well-being; Low-Prevalence (Psychotic) Disorders – full details of this study are published elsewhere [10]. All subjects provided written informed consent and ethics approvals were obtained from the relevant institutional ethics committees. Following previously published methods [1], we defined the deficit group as including those who had negative formal thought disorder or restricted affect, excluding those who also had dysphoria in the past year or positive formal thought disorder. The remaining patients with non-affective psychoses were defined as non-deficit. A logistic regression analysis was used to ascertain whether being born in summer (December, January, February) in the Southern Hemisphere compared with all other months altered the odds of having the deficit syndrome.

The survey included 446 individuals with nonaffective psychosis who were born in Australia: of these 171 (38.3%) were classified as deficit, with the remaining 275 (61.7%) classified as non-deficit. These numbers and proportion with deficit syndrome compare well with those suggested necessary by Messias et al. [7] for sufficient power. While we found the deficit group had more negative symptoms (by definition), poorer premorbid functioning and had a higher proportion of males, they did not differ on other characteristics such as duration of illness, level of psychosis, current age or age-of-onset (data not shown).

We found no association between summer birth in the southern hemisphere and odds of having the deficit versus non-deficit syndrome (odds ratio = 0.75; 95% CI = 0.49–1.16). The deficit syndrome was also unrelated to other definitions of ‘summer’ (such as December to January, January to February, or January to March). We also examined the subgroup with schizophrenia (n = 372), however, the results did not change (data not shown).

While we found no association between psychosis and summer birth, we did find a greater proportion of individuals with the deficit syndrome in this psychosis group (38%) than that reported for the deficit syndrome in schizophrenia (15–20%) [7]. We have previously shown that the size of the season-of-birth effect in schizophrenia is larger in sites further away from the equator [11]. We speculate that the association between summer birth and the deficit syndrome may also be more readily detected at higher latitudes in contrast to the four sites included in the Australian study (latitude from 28° to 32° south). From this study, we speculate that factors that influence the association between summer birth and the deficit syndrome may vary across geography and/or latitude. Further work to examine the influence of geography and other factors – such as the effects of diagnosis and winter gestation – is warranted.