Incidence of psychotic illness in London: comparison of ethnic groups

Entry to the study

All people aged 16 to 54 years who were resident in the catchment area of St Ann's Hospital and made contact with health services between 1 July 1991 and 30 June 1992 were eligible to take part. Information on potential cases was obtained from community, domiciliary, outpatient, casualty, and inpatient services. The community services included all local general practitioners, a mental health clinic, community psychiatric nurse case managers, and the community psychiatric team. All 280 general practitioners in the 125 practices of the Enfield and Haringey Family Health Services Authority were circulated with a letter (which included the screening criteria) every month for the year of recruitment. Members of staff of the community mental health teams and the regional interim secure unit and medical staff in Brixton and Pentonville prisons, the main remand prisons for the population, were contacted monthly by post. Medical staff in all inpatient facilities were contacted monthly by post and visited personally every two weeks by a member of the research team. All the professionals concerned were advised that we were seeking all patients with new onset of psychotic illness, but our specific hypotheses were not discussed. Patients from the catchment area who were admitted outside the area were monitored by a search of the integrated district and regional information system for the North East Thames Health Region.

We screened the information on potential cases for possible features of psychotic disturbance. The screening criteria applied were those used by Harrison et al.11 In summary, evidence was required for at least one of the following: hallucinations, delusions, obvious thought disorder, psychomotor disorder, bizarre or grossly abnormal behaviour, and a definite change in personality or behaviour. Exclusion criteria were acute or chronic organic brain syndromes and conditions attributable to alcohol abuse. Patients meeting these criteria were approached to take part in a detailed, structured, face to face assessment which was carried out by two of us (EC and AH). EC is of black African origin born in England and AH is white and also born in England.

The study was approved by all relevant ethical practices committees. Each patient gave informed consent; consent was also obtained from the next of kin of patients detained under a section of the Mental Health Act 1983.

Assessments

Face to face interview was used to collect demographic details, a conventional psychiatric history using a semistructured questionnaire, an assessment of psychiatric symptoms using the present state examination,13 and an assessment of how the patient had sought help. A key informant, usually a close relative, was interviewed to corroborate the history and provide details of onset of disturbance and how care had been sought.

Drug use - Each subject was asked standardised questions about the use of non-prescribed drugs and was asked to give a urine sample, which was screened for drugs.

Ethnic group was assigned in the same way as the 1991 census. When patients were unable to assign themselves an ethnic category the interviewer assigned a category following the guidelines laid down by the Office of Population Censuses and Surveys. The ethnic ratings used in the 1991 census were arrived at after extensive consultation with ethnic minority organisations and piloting for acceptability during the 1980s.14 It provides the most accurate measure yet of the ethnic composition of the British population.

Psychiatric diagnosis was assigned to each patient according to the ninth edition of the International Classification of Diseases (ICD-9)15 and the revised third edition of the Diagnostic and Statistical Mannual of Mental Disorders (DSM-III-R) of the American Psychiatric Association.16 When there was uncertainty, an alternative diagnosis was also recorded. Each diagnosis was the consensus of discussion between at least three clinical members of our research team (MK, EC, EJ-S, and AH). Only the original interviewer (EC or AH) was aware of the subject's ethnic origin. On the basis of the ICD consensus diagnosis, the degree of certainty for the diagnosis of schizophrenia was established as described previously.11,17

To estimate incidence a broad classification of non-affective psychosis was derived; this included schizophrenia (ICD-9 code 295) and paranoid states (ICD-9 code 297) and other non-organic psychosis (ICD-9 code 298). ICD- 9 codes 297 and 298 were, however, also examined in their own right. The CATEGO class, a computer estimate derived from the present state examination, was used to measure symptoms in the previous month.18

Onset of disorder - The mode of onset of disturbance was established from information given by the patient and key information at interview and from case notes. As much independent information as possible was used to assess time since the first detectable change in personality and behaviour or the development of unequivocal psychotic symptoms, or both. Onset was further categorised as acute or insidious according to the classification established by the World Health Organisation.19

Analysis

Univariate analyses applied were the X² statistic for proportions, the Mantel-Haenszel test for linear association, and Student's t test and analysis of variance for differences in means. Incidences were calculated from the 1991 census figures for the catchment area, which were made available to us by the health authority. Rates were standardised in two broad age bands (16 to 29 and 30 to 54 years) using figures for England supplied by the Office of Population Censuses and Surveys. Confidence intevals for incidences were calculated with the computer program confidence intervals analysis,20 and rates were compared by linear logistic modelling with the GLIM computer program.21

Response rates

One hundred and three subjects satisfied the screening criteria for having a possible first onset psychotic disorder. Three patients refused to take part and seven could not be contacted. A search of the integrated district and regional information system found a further three psychotic patients who had been missed. Thus, 93 patients (88%) were successfully recruited, five of whom were unable to undergo a present state examination owing to disturbed behaviour and concentration. An ethnic classification was attempted on the 10 patients who satisfied criteria but refused to participate or could not be contacted. Four were white, three black Caribbean, two black African, and one black other.

Demography

The mean ages were 31.9 years and 27.2 years for women and men respectively (mean difference 4.7 years (95% confidence interval 1.1 to 8.4), t=2.53, df=91, P=0.013) (table I), The median time of residence in the area was 39 months, 14 patients having lived there for six months or less. Ten (11%) patients had refugee status compared with 5% of people (of all ages) in the population of the catchment area.22 Fifty four patients were born in the United Kingdom; in 40 cases one or both parents had been born abroad, in 11 both parents had been born in the United Kingdom, and in three the parents' place of birth was uncertain.

Ethnic group

Sixteen patients were unwilling or unable to decide on their ethnic group, which was therefore assigned by the interviewer (table I). Census figures for the St Ann's catchment area are shown in table II. Of the 39 white patients, 18 were of British origin, five were Irish, seven were Greek or Turkish Cypriot, and nine were of other European origin.

There were no significant differences between the ethnic groups as categorised in the 1991 census (white, black Caribbean (African Caribbean), black African, black other, Asian (Indian, Pakistani, and other Asian) and other) in terms of sex, marital state, social class of head of household, level of education, employment, period of residence in the area, or homelessness. White patients were older than the others (mean difference 3.4 years, P=0.08).

Incidences of psychotic illness

Sixty two patients were given a diagnosis in one of the categories of non- affective psychosis, including schizophrenia (ICD-9 codes 295, 297, and 298). Thirty eight patients were assigned a restrictive (certain or very likely) diagnosis of schizophrenia (table III). There was strong agreement between ICD-9 and DSM-III-R; only one of these 38 patients received a discrepant diagnosis. When the Mantel-Haenszel test for trend was applied there was no significant relation between the degree of certainty of schizophrenia (five categories in table III) and ethnic group.

The annual incidences of schizophrenia and non-affective psychosis were standardised for age with a cut off point of 29 years for ease of comparison with other studies in which incidences have not been standardised (table IV).11 The age standardised annual incidence of schizophrenia was 2.2 per 10 000 population (1.5 to 2.9). Linear logistic modelling (controlling for age) showed significant overall differences between the white population, black Caribbeans, black Africans and black others, all Asians and others in the rate of schizophrenia (X²=19.54, df=4, P=<0.001) and non-affective psychosis (X²=30.51, df=4, P<0.001). To avoid multiple comparisons of small numbers, we report rate ratios only for the white population against all other ethnic groups. The incidence ratio for schizophrenia in all ethnic minority groups compared with the white population was 3.6 (1.9 to 7.1); the corresponding ratio for non- affective psychosis was 3.7 (2.2 to 6.2).

Nine patients (five white, two black African, one black other, and one other) three of whom were refugees, received a diagnosis of mania (ICD-9 codes 296.0, 296.2, and 296.4). The age standardised annual incidence of mania in the population of the catchment area was 0.5 (0.2 to 0.8) per 10 000 population.

Nine of the 10 refugees (five white, four black African, and one other) were given a diagnosis of affective psychosis (eight of ICD-9 code 296 and one of ICD-9 code 298). One was given a diagnosis of non-affective psychosis (ICD-9 code 297.8, paranoid state); in this case there was a degree of uncertainty about the diagnosis.

Psychiatric state

Doubtful interviews, in which the adequacy of ratings of key symptoms in the present state examination was questionable, occurred no more or less often in any particular ethnic group. The ethnic group of the interviewer was not a factor influencing the rate of doubtful interviews. On the basis of the CATEGO class there were no unusual or distinctive syndromes predominating in any particular ethnic group.

The distribution of the time that had elapsed between the first sign of a change in behaviour or personality and the first contact with psychiatric services was highly skewed, with a mean of 90 weeks and a median of 28 weeks, but it did not vary significantly between the white, black, Asian, or other ethnic groups. In particular, there was no difference between black Caribbean and other patients in time since the first onset of schizophrenia or all psychoses (median difference 2 weeks longer for black Caribbeans, P=0.33). There was no difference between the ethnic groups with regard to the type of onset of the disorder, broadly defined as acute or insidious.

Fourteen patients had a first degree relative who had had a psychotic illness. Neither size of sibship nor prevalence of psychotic illness in first degree relatives differed significantly between ethnic groups.

Black patients were overrepresented among those with less common psychotic syndromes: five black Caribbean, three black African, and two white patients had diagnoses with ICD-9 codes 295.4, 298.3, 298.4, 298.8, and 298.9). Only five of these, however, had developed the disorder acutely (in two weeks or less).

All nine black Caribbeans with schizophrenia were aged 29 years or under, whereas there was a broader age distribution in the other ethnic groups.

More white patients (33/39, 85%) had been admitted to hospital at ascertainment compared with black patients (23/38, 61%). Asians (7/11, 64%), or patients in the census classification other groups (2/5, 40%) (X²=7.93, df=3, P=0.047). There were no differences between the ethnic groups in terms of compulsory admission under the Mental Health Act or the police being involved in admission.

Use of non-prescribed drugs

Cannabis was the main drug misused. Fourteen (36%) white, 12 (32%) black, and three (27%) Asian patients reported having used cannabis at one time. Six white and seven black patients used cannabis at least weekly. Urine samples were screened for drugs in 63 patients; six tests were positive for cannabinoids, four of which were from black patients. There was no association between ICD-9 diagnosis of schizophrenia and use of cannabis.