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Prevalence of schizophrenic disorders

Systematic review
Results
Prevalence
Incidence
Comment

How many people have schizophrenia or schizophrenic disorders? A systematic review using rigorous methodology derives reasonably consistent estimates both for prevalence and for incidence [1], and explores differences between different estimates.

Systematic review

Two databases were searched for English language studies published between 1980 and 2000. Studies were selected for inclusion based on several features.

They had to be community surveys of the general population, or, for incidence studies, case-register methods that surveyed at least primary care general medical services. The age range for included studies was adults between the ages of 18 to 65 years, though for incidence studies the age of 15 and over was used. Studies also had to have a reasonable size, and only those with a denominator sample of 450 or more were included. The diagnostic criteria were the ICD-9 or DSM-III systems or later, and case definition had to be explicit.

Results

Prevalence

Sixteen prevalence reports were found, examining mental disorders, with samples ranging from 500 to 20,000, and most used structured interviews administered by lay interviewers, with algorithms applied to derive diagnoses.

Best estimates for one-year and lifetime prevalence of schizophrenic disorders and schizophrenia are shown in Table 1. For schizophrenia, for instance, the best estimate for a one-year prevalence was about 340 per 100,000 adults.

Table 1: Prevalence of schizophrenic disorders, schizophrenia and schizophreniform disorder in adults

Prevalence per 100,000 adults
(95% CI)

Diagnosis
One year
Lifetime
Schizophrenic disorder
600 (380 to 910)
1450 (80 to 2370)
Schizophrenia
340 (220 to 500)
550 (370 to 800)
Schizophreniform disorder
90 (50 to 140)
110 (60 to 170)

There was little difference in rates between the sexes. Prevalence was influenced by world region, with Asian studies having lower rates (lifetime prevalence of 400 per 100,000 or below) than those in other regions (300 to 1,600 per 100,000).

Incidence

Eight studies provided one-year incidence rates for schizophrenia. The sizes ranged from 73,000 to over five million.

Most estimates of incidence were between five and 23 per 100,000, with a best estimate of 11 per 100,000 (95% confidence interval 8 to 16).

There was a tendency for more recent studies, with assessments in the 1990s, to have higher incidence rates, but this was not a strong trend.

Comment

What this paper does, apart from tell us much about prevalence and incidence of schizophrenic disorders, is to educate us more about methods. It is an object lesson on how to think through obtaining quality information because it has thought about what constitutes quality before starting the review.

What might these figures mean to a UK population? In every 100,000 people about 80,000 are aged 16 or more. That means we might expect there to be about 250 adults with schizophrenia at any time, with another eight or so being diagnosed every year.

It is also interesting to question whether standard textbooks agree with these estimates. Bandolier's bible, the Oxford Textbook of Medicine, provided an annual schizophrenia incidence of 0.1 or 0.2 per 1,000, and a prevalence of 3 per 1,000. That's 10-20, and 300 per 100,000 respectively, which is in excellent agreement with the figures in this systematic review.

References:

  1. EM Goldner et al. Prevalence and incidence studies of schizophrenic disorders: a systematic review of the literature. Canadian Journal of Psychiatry 2002 47: 833-843.

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