Prevalence incidence and disease expectancy

Prevalence

Prevalence is defined as the number of cases per 1000 persons at risk present in a population at a given time or over a defined period. Point prevalence refers to the 'active' (i.e. symptomatic) cases on a given date, or within a brief census period. Since asymptomatic cases (e.g. persons in remission) will be missed in a point prevalence survey, it is useful to supplement the assessment of the present mental state with an enquiry about past episodes of the disorder to obtain a lifetime prevalence index. In schizophrenia, which tends to a chronic course, estimates of point and lifetime prevalence will be closer to each other than in remitting illnesses.

An overview of selected prevalence studies of schizophrenia spanning a period of 60 years is presented in Tab!e...2, The studies differ in many aspects of methodology but have in common a high intensity of case-finding. Several of them are repeat surveys in which the original population was reinvestigated following an interval of 10 or more years (the resulting consecutive prevalence figures are indicated by arrows).

Table 2 Selected prevalence studies of schizophrenia

The majority of studies have produced prevalence estimates in the range 1.4 to 4.6 per 1000 population at risk. However, these are usually crude prevalence figures that are difficult to compare because of the demographic differences between populations with regard to factors such as age-specific mortality and migration. Therefore the modal prevalence of 1.4 to 4.6 per 1000 is unlikely to reflect the extent of variation that exists in the prevalence of schizophrenia in different populations.

There are populations and groups which deviate from the central tendency. Unusually high rates (two to three times the national or regional rate) have been reported for several genetic isolates such as an area in northern Sweden and several areas in Finland, and for an area in Croatia characterized by a high level of out-migration during the nineteenth and early twentieth centuries (see Table...?.). At the other extreme, a virtual absence of schizophrenia and a relatively high rate of depression has been claimed for the Hutterites of South Dakota, a Protestant sect whose members live in close-knit endogamous communities sheltered from the outside world. (3Z) Negative selection for schizoid individuals who fail to adjust to the lifestyle of the majority and eventually migrate without leaving progeny has been suggested (but not definitively proven) as an explanation.

Low prevalence rates have also been reported for certain Pacific island populations, (38) but uncertainties about the extent of case-finding makes the interpretation of such reports problematic. Two carefully planned surveys in Taiwan (see Table.3) were separated by 15 years during which major social changes had taken place. While the total mental morbidity increased, the prevalence of schizophrenia decreased from 2.1 to 1.4 per 1000. In both surveys, the aboriginal Taiwanese had significantly lower rates than the mainland Chinese who had migrated to the island after the Second World War.

The results of the Epidemiologic Catchment Area study in the United States,(12) which indicate a higher prevalence rate compared with most other studies, should be treated with caution. There are inconsistencies among the study areas that are difficult to interpret (such as a 13-fold difference in the rates for age group 18-24 years across the sites), which suggests that the diagnostic procedure, involving the DIS administered by lay interviewers, may have elicited a number of false-positive diagnoses. In the more recent National Comorbidity Survey, diagnoses of 'non-affective psychosis' by computer algorithm based on a version of the CIDI were found to agree poorly with clinicians' diagnoses based on telephone reinterviews, resulting in discrepant estimates of the lifetime prevalence of both 'narrowly' and 'broadly' defined psychotic illness.(39)

The question of whether major differences exist in the prevalence of schizophrenia in different populations has no simple answer. In the great majority of studies the prevalence rates are similar. On the other hand, there are a small number of populations that clearly deviate from the average. However, the magnitude of these deviations is modest compared with the 10- to 30-fold differences in prevalence observed in other multifactorial diseases (e.g. ischaemic heart disease, diabetes, multiple sclerosis) across populations.

Incidence

The incidence rate (the annual number of first onset cases in a defined population per 1000 individuals at risk) is of greater interest for the search of risk factors than the prevalence since it is a better estimate of the so-called force of morbidity (the probability of disease occurrence at a point in time) in a given population. Its estimation depends on how reliably the point of onset can be determined. Since it is not possible at present to determine the time of onset of any cerebral dysfunction or biochemical lesion underlying schizophrenia, onset of the disorder is usually defined as the point in time when its clinical manifestations become recognizable and diagnosable according to specified criteria. The first hospital admission, which has been used in many studies, is not a good approximation to the 'true' onset because of the variable time lag between the earliest appearance of symptoms and the first admission. A better approximation is provided by the first contact, i.e. the point at which any psychiatric or general medical service is accessed by symptomatic individuals for the first time.

TableS presents the essential features of 13 incidence studies of schizophrenia. The comparison of studies using a 'broad' definition of schizophrenia (e.g. ICD-8 or ICD-9) suggests that the variation in rates based on first admissions or first contacts is about threefold, in the range from 0.17 to 0.54 per 1000 population per year. Studies using restrictive criteria such as the Research Diagnostic Criteria ( RCD),(51) DSM-III and its successors, and ICD-10 have reported incidence rates that are two to three times lower than those based on 'broad' criteria.

To date, the only study that has generated directly comparable incidence data for different populations is the WHO 10-country investigation. (11> Incidence counts in the WHO study were based on first-in-lifetime contacts with any 'helping agency' (including traditional healers in the developing countries) monitored prospectively over a 2-year period. Potential cases and key informants were interviewed by clinicians using standardized instruments, and the timing of onset was ascertained for the majority of the patients. In 86 per cent of the 1022 patients the first appearance of diagnostic symptoms of schizophrenia was within the year preceding the first contact, and therefore the first-contact incidence rate was accepted as a reasonable approximation to the onset rate. Two definitions of 'caseness', differing in the degree of specificity, were used to determine incidence: a 'broad' clinical class comprising ICD-9 schizophrenia and paranoid psychoses, and a restrictive definition including only 'nuclear' schizophrenia manifesting with so-called Schneiderian first-rank symptoms. (52) The rates for eight catchment areas are shown in Table4.

study(11)

The differences between the rates for 'broadly' defined schizophrenia (0.16-0.42 per 1000) were significant ( p < 0.001, two-tailed test); however, those for 'nuclear' schizophrenia were not. Since the mean incidence rates for the 'nuclear' cases were lower than the mean rates for 'broadly' defined schizophrenia, the confidence intervals for the lower rates would be expected to be wider than the confidence intervals for the higher rates. In fact, the opposite was true, which suggests that 'nuclear' schizophrenia comprises a more homogeneous subset of cases than the 'broad' diagnostic class and occurs with a similar frequency in these different populations. However, no differences were found between cases meeting the 'broad' criteria and the 'nuclear' cases with regard to either age or type of onset (acute or insidious), or 2-year course and outcome. Therefore it is unlikely on clinical grounds that 'nuclear' and 'broadly' defined cases represent two different disorders.

In recent years, replications of the design of the WHO 10-country study, including its research instruments and procedures, have been carried out with very similar results by investigators in India, the Caribbean, and the United Kingdom ( T§ble3).

Disease expectancy (morbid risk)

This is the probability (usually expressed as a percentage) that an individual born into a particular population will develop the disease if he or she survives the period of risk for that disease. In the instance of schizophrenia the period of risk is defined as either 15 to 44 or 15 to 54 years. If the age- and sex-specific incidence rates are known, disease expectancy can be estimated directly by a summation of the age-specific rates within the period of risk. Disease expectancy can be estimated indirectly from prevalence data.

The estimates of risk produced by a number of studies are fairly consistent across populations and over time. Excluding outliers, such as the northern Swedish isolate

(Table 2), the risk ratio (ratio of highest to lowest disease expectancy) is about 5.0; for the WHO study (11,) it is 2.9 ('broad' diagnostic class) and 2.0 ('nuclear' schizophrenia). The frequently cited 'rule of thumb' estimate of disease expectancy for schizophrenia at around 1 per cent (or slightly less) seems to be consistent with the evidence.

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