Epidemiological studies investigate the frequency of psychiatric disorders, their relationship to social factors, and their natural history. They are carried out on whole populations rather than individual patients. Sources of information include the following:

• population surveys, in which every member of a defined population, or a random sample of it, is studied by interview or questionnaire

• GP consultation records

• case registers, which are kept in some health districts to provide information on all contacts with psychiatric services

• hospital statistics.

The frequency of a disorder may be expressed as point prevalence (the percentage of subjects with the disorder in question at one point in time) or period prevalence (the total percentage who receive the diagnosis during a defined period).

Incidence means the number of new cases within a defined population in a given period (rate). Lifetime expectation expresses the risk of an individual's developing the condition concerned sometime during their life.

The results of different surveys vary greatly. Community interview surveys find the highest rates, invariably detecting many psychiatric 'cases' that are not known to GPs, let alone to specialist services. GP and hospital statistics may be inaccurate because they reflect different definitions of a psychiatric 'case', or variations in local treatment policies. There remains a fundamental distinction between subjects who have presented themselves for medical attention, and therefore have identified themselves as patients, and those who have not so presented themselves.

Most community surveys report that 10-20 per cent of the population meet diagnostic criteria for psychiatric disorder at any one time, and that a person's lifetime risk of disorder may be as high as 50 per cent.

Several socio-demographic variables have an association with psychiatric morbidity, but the direction of cause and effect is not always clear-cut. For example, consider the following variables:

• Sex. Women have higher rates of most psychiatric disorders than men. This particularly applies to the common (non-psychotic) forms of anxiety and depression. Possible explanations include the following:

- Women use health services of all kinds more frequently than men. They may be more willing to acknowledge emotional complaints and seek medical treatment, whereas men tend to express their distress through other means such as antisocial behaviour or alcohol misuse.

- Doctors are more likely to diagnose women's symptoms as psychiatric.

- Women suffer more psychosocial stress than men because of their role in society.

- Biological differences, such as genetic constitution and sex hormone profile, play a role.

For other mental disorders, including schizophrenia, the sex ratio is equal.

• Marital status. For men, rates of psychiatric disorder are lower among the married than the single, divorced, or widowed. Possible explanations include the following:

- Married life is beneficial for men's mental health.

- Men with psychiatric disorder tend to remain single.

- Psychiatric disorder results in marital breakdown.

- Widowhood and divorce are stressful life events that may lead to psychiatric disorder.

For women, the pattern is different than for men. Young working-class housewives with several small children have high rates of depression and neurosis, whereas single women in paid employment have lower rates.

• Residential area. Urban areas, especially poor inner-city districts, have higher rates of psychiatric morbidity than rural areas. Possible explanations include the following:

- The lack of stable social networks in inner-city areas contributes to psychiatric disorder.

- The stresses of city life, such as overcrowding, high crime rates, and noise, contribute to psychiatric disorder.

- Psychiatric disorder causes people to lose their jobs and social supports, and forces them to move to poorer areas.

• Unemployment is associated with psychiatric disorder. Possible explanations include the following:

- The socio-economic adversity and loss of self-esteem of the unemployed contributes to psychiatric disorder.

- Workers with psychiatric disorder are liable to lose their jobs.

• Social class. Manual workers show higher psychiatric morbidity than the professional/managerial classes. Possible causes include the following:

- genetic factors

- a stressful and unhealthy environment

- 'drift down the social scale' caused by mental illness.

• Nationality, and issues of'transcultural' psychiatry. Diagnostic statistics vary around the world. Organic brain syndromes and somatic presentations of 'functional' conditions are more common in developing societies. Suicide rates vary greatly between countries. Some of these observed differences are genuine. Others are artefactual, depending on what kinds of behaviour are considered abnormal in the culture concerned, and disappear when uniform diagnostic criteria are applied. For example, until the 1970s, the diagnosis of schizophrenia was made much more frequently in the USA than the UK. Yet, since introduction of more rigorous diagnostic criteria, it appears that schizophrenia and other major psychotic disorders occur about equally frequently in both countries, and also in most other parts of the world. Migrants show high psychiatric morbidity, being especially prone to develop a range of conditions including psychotic disorders and post-traumatic stress disorder (PTSD); refugees, who have been forced to migrate rather than doing so by choice, are most at risk. Possible explanations include the following:

- Pre-existing psychiatric disorder causes people to emigrate.

- Stressful circumstances in the country of origin precipitate both emigration and psychiatric illness.

- 'Culture shock' in the new country, including a strange language and customs, and discrimination against immigrants.

- Over-diagnosis occurs as a result of mental health professionals' unfamiliar-ity with the culture of immigrant groups and language difficulties.

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