Schizophrenia

Descriptive Epidemiology

The most credible data on the epidemiology of schizophrenia come from registers, including inpatient and outpatient facilities for an entire nation, in which the diagnosis is typically made carefully according to the standards of the World Health Organization's International Classification of Diseases and in which treatment for schizophrenia in particular and health conditions in general is free (e.g., Denmark). The global point prevalence of schizophrenia is about 5 per 1,000 population. Prevalence ranges from 2.7 per 1,000 to 8.3 per 1,000 in various countries. The incidence of schizophrenia is about 0.2 per 1,000 per year and ranges from 0.11 to 0.70 per 1,000 per year. The incidence of schizophrenia peaks in young adulthood (15 to 24 years, with females having a second peak at 55 to 64 years). Males have about 30% to 40% higher lifetime risk of developing schizophrenia than females.

Natural History

The onset of schizophrenia is varied. In 1980, Ciompi found that about 50% of cases had an acute onset and about 50% had a long prodrome. About half of the individuals had an undulating course, with partial or full remissions followed by recurrences, in an unpredictable pattern. About one third had a relatively unremitting course with poor outcome; and a small minority had a steady pattern of recovery with good outcome. Several studies have shown that negative symptoms and gradual onset predict poor outcome. There is variation in the course of schizophrenia around the world, with better prognosis in the socalled developing countries. Although there is a long literature on the relation of low socioeconomic position to risk for schizophrenia, it seems likely that the association is a result of its effects on the ability of the individual to compete in the job market. Recent studies suggest that the parents of schizophrenic patients are likely to come from a higher, not lower, social position.

Some individuals with schizophrenia differ from their peers even in early childhood in a variety of developmental markers, such as the age of attaining developmental milestones, levels of cognitive functioning, neurological and motor development, and psychological disturbances, but no common causal paths appear to link these markers to schizophrenia. Minor physical anomalies, defined by small structural deviations observed in various parts of the body (e.g., hands, eyes, and ears), are more prevalent in individuals with schizophrenia and their siblings as compared with the rest of the population. This evidence on developmental abnormalities is consistent with the hypothesis that schizophrenia is a neurodevelopmental disorder, with causes that may be traced to early brain development.

Risk Factors

A family history of schizophrenia is the strongest known risk factor, with first-degree relatives having about 5to 10-fold relative risk and monozygotic twins having about 40to 50-fold relative risk. In addition to family history, there are several risk factors that have been identified in the past 50 years, including complications of pregnancy and birth, parental age, infections and disturbances of the immune system, and urban residence.

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