Grouping Of Ill Persons--09.3.10

Gro uping of ill pers on s a nd Class if icat ion of Disease Dept Epidemiology Benfu Yang

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Introduction Cause Association Groups/Categories Grouping of events or individuals into categories is essential to causal inference

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Creation of categories of ill persons 2

• Disease entity – Symptoms and signs-cause/prognosis – Identity/cause---symptoms and signs

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process of disease classification •Ill persons are grouped into categories such that the characteristics of the members of each category permit them to be distinguished from the members of another category.

•The arrangement of the components of this nomenclature into groups thought to have common characteristics (a classification).

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Grouping of Ill Persons The creation of disease entities involves the grouping of ill persons into categories that are believed to have utility in the management of their illness or in understanding the circumstance that led to it. • categories – natural – artificial

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Manifestational and Causal Entities 1.Manifestational criteria: Ill persons are grouped according to similarity of symptoms, signs, changes in body chemistry or tissues, behavior, prognosis, or some combination of these features. e.g. fractures, diabetes mellitus, mental retardation, the common cold, schizophrenia, and breast caner.

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2. Causal criteria: Causal grouping depends on the similarity of individuals with respect to one or more experiences believed to be the cause of their illness. e.g. birth trauma, silicosis, syphilis, lead poisoning, and, in principle, AIDS.

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There is no logical reason to suppose that ill persons classified together by manifestational or causal criteria will remain as a group if the other type of criterion is used.

Tubercle bacillus

therapy / control of transmission

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Figure 3-1. Classification of persons with active tuberculosis (an etiologic entity) according to the manifestational classification of Cullen 9

• Rubeola • Rabies • Tuberculosis – Smear positive – Smear negative

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Lack of necessary congruence between the two ways of grouping ill persons is important to epidemiologists 1. Polymorphous effects (manifestations ) of newly isolated causal agents may be understood cigarette smoking is associated with several diseases other than lung cancer not specific even if cigarette smoke contained only a single disease-causing agent, diversiy of effects.

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2.

It can be understood that an agent causally associated with a certain manifestational disease entity may not be causally involved with all of the ill persons with a particular manifestation. Examination of disease subcategories may lead to refinement of the manifestational entity associated with the identified cause. e.g. different pathologyic varieties of lung cancer vary substantially in the strength of their association with cigarette smoking. No particular manifestational category of coronary artery disease has been found to be associated with cigarette smoking, although the high frequency of the disease in nonsmoking indicates clearly that cigarette smoking is not involved in all cases of the disease.

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3.

The arbitrariness of the distinction between

necessary causes, those without which the disease does not occur, and contributing causes will be realized. necessary cause: M.tuberculosis contributing factors: age, nutritional status, poverty, genetic factors M. tuberculosis may be the necessary cause of tuberculosis as it is presently defined, but if medical knowledge had developed differently, many patients now categorized as having tuberculosis might have been included in a category defined on the basis of a specific nutritional defect. In this case, the nutritional factor would have been the necessary cause and the bacillus the contributing factor.

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4. It can be understood that not all persons experiencing the cause will acquire the disease. e.g. Although there is a definite causal association between the tubercle bacillus and tuberculosis, evidence of internalization of the bacillus is found in many persons who have no evidence of illness, just as many persons who smoke cigarettes do not develop lung cancer. 14

Select ion of C rit eria for Cr eat ion of New Di se ase Ent it ies

•Particular interest of the observer

determines to a large extent the kind of criteria selected e.g. Injuries preventive therapeutic/prognosis

•In the absence of knowledge of causal

factors, manifestational criteria provide the only basis for categorization  Balkan nephropathy  Mesothelioma (asbestos)

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• causal factors are identified for manifestatinal entities, some are used to define new disease entities around etiologic factors. • Some newly discovered causal agents have not been used to develop new disease entities.

e.g., “cigarette smoker’s disease,” emphysema, carcinoma of the lung, peripheral vascular disease, coronary artery disease , bladder cancer

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• Use of one way of grouping ill persons does not exclude concurrent use of another. E.g. injuries – Hospital:

fractures, sprains, concussions, and cases of internal

injury –

prevention: automobile accidents, fire, falls, and industrial accidents.

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Role of Epidemiologic Observations • epidemiologic criteria may be used to categorize groups of persons with specific manifestational diseases. – poliomyelitis paralytic disease in Vermont (Caverly) – typhus Ireland breast-feeding infants, the elderly, and nurses and others attending the sick

– typhoid fever France and Switzerland (Lombard)

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•Epidemiological observations may be sufficient to justify separation of disease entities, even in the absence of observable manifestational differences.

•Spirochete: infectious jaundice (Weil’s disease) occurred in groups, butchers •Virus: infectious hepatitis/serum hepatitis

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• Epidemiological evidence suggests that two categories may usefully be merged, at least for the purpose of investigation of etiology. e.g. anencephaly and spina bifida gender, socioeconomic status, ethnicity, and parity, trends of frequency over time

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