The Biopsychosocial Model & the Web of Causation
The late George Engel promulgated a theoretical model, based on general systems theory, of the etiology of mental disease. Research had demonstrated, and indeed continues to demonstrate, that unitary explanations are not adequate to explain disease etiology or thus indicate appropriate prevention and treatment strategies. Engel suggested an interrelatedness among biological, psychological, and social factors. Biological factors included anatomic and molecular factors and those factors related to gender, age, ethnicity, and genetics. Psychological factors related to the individual’s personality. According to Engel’s theory, social factors included family, society, culture, and environment; other authors would include religious and spiritual as well as economic factors in this group. Engel also believed that the physician’s contribution, through a psychosocial presence, to this “collaborative pathway to health” was often inseparable from that brought by the patient.
Concept of the “Case"
Epidemiologists, for better or for worse, typically allocate people to being either a “case” or a “non-case."Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV) and other psychiatric diagnostic systems do virtually the same thing. For example, either an individual meets criteria for a diagnosis of major depressive disorder or he or she does not. To identify a case, one must have criteria for identifying cases, but these criteria may vary from one nomenclature to another. For example, the criteria for a case in DSM-IV differ from those in DSM-III in some circumstances.
The use of the concept of a case in epidemiology makes it easier for practicing clinicians to interpret the types of studies performed by epidemiologists, although by arbitrarily assigning an individual to a category of either “case” or “non-case,” one loses considerable data. Several early epidemiologic studies were cognizant of this dilemma and attempted to assign patients to groups based on how well they met the predetermined criteria for each group; these researchers recognized that the ability of clinicians to assign individuals as either cases or non-cases was not perfect and was more applicable to a probability function than to a simple yes or no decision. Similarly, the use of symptom rating scales, such as the Hamilton Depression Rating Scale, does not require that an individual be assigned to a case or non-case category but rather permits the assessment of depressive psychopathology as a continuum.
Other Epidemiologic Concepts
To discern the relationship(s) between what has been described as the web of causation, and to determine relevant patterns, the clinical investigator must study not only the impaired individual but also persons within the context of their community. The concept of a web of causation is that specific relationships, such as the relationship between social stressors and a mental disorder, may be connected through a variety of intervening variables that interrelate in a way best illustrated by a web consisting of nodes (etiologic factors) and strings (interrelationships of these etiologic factors). Epidemiology here is defined as a “. . . method of reasoning about disease that deals with biological inferences derived from observations of discrete phenomena in population groups.” Epidemiology allows more exact determinations as to the contribution(s) that biological, psychological, sociodemographic, and cultural differences may contribute to the prevalence and incidence of mental disorders. Table 5-1 defines these and other key terms used in epidemiology.
According to Morris, epidemiology has several vital uses: (1) in the study of the historical health of communities and the estimation of morbidity for different disorders, (2) in the assessment of the efficiency of health programs and services, (3) in the determination of individuals at risk of acquiring a disease or disability in all their various presentations, (4) in the identification of syndromes as the unified collection of related signs and symptoms, and (5) to assist in “. . . the search for the causes of health and disease.” Proper epidemiologic studies can promote sound health policy, enable more rational health care planning, and facilitate cost-effective prevention and treatment. An essential component of all these uses is the determination of valid denominators to compare the characteristics of populations with and without disease.
The study of comorbidity required advanced statistical methods, available only relatively recently for the study of disease in populations. Epidemiology and statistics have become vital partners in the development of approaches to delineate and disentangle the interrelatedness of psychiatric disorders, or the web of causation.
