Summary: A systematic review of medical literature reveals schizophrenia is overall less common than previously thought. Researchers say the 0.5% estimate for lifetime prevalence of schizophrenia, often reported in textbooks, is significantly overestimated. Additional findings from the review were that schizophrenia is more prevalent in developed countries than poorer countries, and immigrants have higher rates of schizophrenia diagnoses than native-born people.
Schizophrenia is a devastating mental illness and a major contributor to the global burden of disease, but how many people are actually suffering from the disease worldwide? John McGrath and colleagues from the University of Queensland have conducted the most comprehensive analysis on the topic and now report their findings in the May issue of the international open-access journal PLoS Medicine. The researchers have done what is called a systematic review of the medical literature, which turned out a few surprises. And knowing about prevalence–defined as the number of people suffering from the disease at a given time or within a time interval–of such an important disease is crucial to health policy strategies. They found that schizophrenia is more common in developed than in poorer countries but overall less common than previously thought. Across countries, immigrants have higher rates of the disease than native-born individuals.
The scientists have analyzed a total of 1,721 estimates from 188 studies covering 46 countries and calculated the following median prevalence estimates: 4.6 per 1,000 for point prevalence (defined as prevalence during any interval of less than a month), 3.3 for period prevalence (defined as prevalence during a period from 1 to 12 months), 4.0 for lifetime prevalence (the proportion of individuals in the population who have ever manifested the disease and who are alive on a given day), and 7.2 for lifetime morbid risk (which attempts to include the entire lifetime of a birth cohort, both past and future, and includes those deceased at the time of the survey).
The point prevalence numbers are consistent with key policy documents, but the 0.5% estimate for lifetime prevalence given in many textbooks is a significant overestimate. “This,” the authors suggest, “is another example where the research community needs to review their belief systems in the face of data.” Another often quoted statistic, namely that “schizophrenia affects about one in a hundred” most sensibly refers to lifetime morbid risk data. Here as well, the systematic analysis suggests that the reality is somewhat lower, and the authors suggest that “if we wish to provide the general public with a measure of the likelihood that individuals will develop schizophrenia during their lifetime, then a more accurate statement would be that about seven to eight individuals per 1,000 will be affected.”
However, the analysis makes clear that “many people with schizophrenia have persisting symptoms, despite the best mix of interventions we can offer.” It has been estimated that current interventions can at most reduce 25% of disease burden. Thus, the authors conclude that “this is a powerful argument for investing in applied and basic research.”
John McGrath – PLOS
The image is in the public domain.
Original Research: Open access
“A systematic review of the prevalence of schizophrenia”. Sukanta Saha, David Chant, Joy Welham, John McGrath.
PLOS Medicine doi:10.1371/journal.pmed.0020141.
A systematic review of the prevalence of schizophrenia
Understanding the prevalence of schizophrenia has important implications for both health service planning and risk factor epidemiology. The aims of this review are to systematically identify and collate studies describing the prevalence of schizophrenia, to summarize the findings of these studies, and to explore selected factors that may influence prevalence estimates.
Methods and Findings
Studies with original data related to the prevalence of schizophrenia (published 1965–2002) were identified via searching electronic databases, reviewing citations, and writing to authors. These studies were divided into “core” studies, “migrant” studies, and studies based on “other special groups.” Between- and within-study filters were applied in order to identify discrete prevalence estimates. Cumulative plots of prevalence estimates were made and the distributions described when the underlying estimates were sorted according to prevalence type (point, period, lifetime, and lifetime morbid risk). Based on combined prevalence estimates, the influence of selected key variables was examined (sex, urbanicity, migrant status, country economic index, and study quality).
A total of 1,721 prevalence estimates from 188 studies were identified. These estimates were drawn from 46 countries, and were based on an estimated 154,140 potentially overlapping prevalent cases. We identified 132 core studies, 15 migrant studies, and 41 studies based on other special groups. The median values per 1,000 persons (10%–90% quantiles) for the distributions for point, period, lifetime, and lifetime morbid risk were 4.6 (1.9–10.0), 3.3 (1.3–8.2), 4.0 (1.6–12.1), and 7.2 (3.1–27.1), respectively. Based on combined prevalence estimates, we found no significant difference (a) between males and females, or (b) between urban, rural, and mixed sites. The prevalence of schizophrenia in migrants was higher compared to native-born individuals: the migrant-to-native-born ratio median (10%–90% quantile) was 1.8 (0.9–6.4). When sites were grouped by economic status, prevalence estimates from “least developed” countries were significantly lower than those from both “emerging” and “developed” sites (p = 0.04). Studies that scored higher on a quality score had significantly higher prevalence estimates (p = 0.02).
There is a wealth of data about the prevalence of schizophrenia. These gradients, and the variability found in prevalence estimate distributions, can provide direction for future hypothesis-driven research.