Grouping of Ill Persons--09.3.10

8/14/2019 Grouping of Ill Persons--09.3.10

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Gro uping of ill pers on s a ndClass if icat ion of Disease

Dept Epidemiology

Benfu Yang


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Introduction

Cause

AssociationGroups/Categories

Grouping of events or individuals into

categories is essential to causal inference.

Creation of categories of ill persons


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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 thatthe characteristics of the members of eachcategory permit them to be distinguished from the

members of another category.

The arrangement of the components of this

nomenclature into groups thought to have commoncharacteristics (a classification).


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Grouping of Ill

Persons

categories

natural artificial

The creation of disease entities involves thegrouping of ill persons into categories that arebelieved to have utility in the management oftheir illness or in understanding thecircumstance that led to it.


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Manifestational and Causal Entities1.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 oftheir illness.

e.g. birth trauma, silicosis, syphilis, leadpoisoning, 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 activetuberculosis (an etiologic entity) according to themanifestational classification of Cullen


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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 specificeven 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 causallyassociated 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 distinctionbetween

necessary causes, those without which the

disease does not occur, and contributing causes

will be realized.

necessary cause: M.tuberculosiscontributing 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.


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Select ion of C r it e r ia for Cr eat ion of New Di se a seEnt it ies

Particular interest of the observerdetermines to a large extent the kind ofcriteria selected

e.g. Injuries preventivetherapeutic/prognosis

In the absence of knowledge of causalfactors, manifestational criteria provide theonly basis for categorization

Balkan nephropathy

Mesothelioma (asbestos)


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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 Irelandbreast-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 (Weils

disease) occurred in groups, butchers

Virus: infectious hepatitis/serum hepatitis


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Grouping of Ill Persons--09.3.10

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