Schizophrenia, schizoaffective and bipolar disorder within an epidemiologically complete, homogeneous population in rural Ireland: small area variation in rate.

While a controversy has endured as to whether schizophrenia evidences the geographical variations in rate that characterise essentially all medical conditions, even less is known of such fundamental aspects of the epidemiology of schizoaffective and bipolar disorder. Within an ethnically and socioeconomically homogeneous region of rural Ireland, population 29,542, several methodological refinements were adopted to seek an epidemiologically complete population of 'all' cases of these disorders, with each potential case interviewed and diagnosed. Prevalence and morbid risk were calculated over the region as a whole and for each of the 39 constituent District Electoral Divisions [DEDs], by place at birth and by place at onset. Using multiple sources of information, 115 cases of schizophrenia, 33 of schizoaffective disorder and 77 of bipolar disorder were identified. Unremarkable overall prevalence and morbid risk values obscured marked variation between District Electoral Divisions for schizophrenia. No such variation was observed for bipolar disorder. These data indicate, using improved methodology, that what is often interpreted as an invariant overall rate of schizophrenia across countries and cultures may not apply to spatial microstructure; macroscopic rates can obscure small area variations when ethnic and socioeconomic diversity are minimised and effects of urbanicity are absent. Under these conditions, small area variations in bipolar disorder may be limited.