Causation in Epidemiology PDF

Assistant professor Dr.Abdulfatah Hamarahm Hawramei M.Sc, F.I.B.M.S/Comm.Medicine College of Medicine-Community and Family Medicine Department General epidemiology - Family and Community Medicine department A major focus of epidemiology is informing efforts to prevent and control disease and pr...

Assistant professor Dr.Abdulfatah Hamarahm Hawramei M.Sc, F.I.B.M.S/Comm.Medicine College of Medicine-Community and Family Medicine Department General epidemiology - Family and Community Medicine department A major focus of epidemiology is informing efforts to prevent and control disease and promote health. We need to know the causes of disease or injury and the ways in which these causes can be modified. General epidemiology - Family and Community Medicine department The concept of cause me fffee but Rf is not An understanding of the causes of disease or injury is important not only for prevention, but also for correct diagnosis and treatment. wrios The process by which we make causal inferences – judgments linking postulated causes and their outcomes – is a major theme of the general philosophy of science, and the concept of cause has different meanings in different contexts. General epidemiology - Family and Community Medicine department A cause: something that made a difference i.e. contributes to a causal mechanism. A cause will express itself only if when a sufficient causal mechanism is complete, if the complementary components of a causal factor are absent, the disease/outcome will not become manifest. Factors work together in a given causal mechanism is said to interact causally or causally interdependent. General epidemiology - Family and Community Medicine department Direct cause: is a causal factor & is very close to the pathogenic event. Indirect cause: is not directly related to pathogenic events (in the body), but does have a causal role via it interconnection to other factors. Sufficient causal constellation: a set of factors 54 that makes disease inevitable in an individual. Causal web: the causal metaphor that links direct & indirect cause of disease into a hierarchy. a Necessary cause: a factor that is always present with a given disease; the disease cannot occur without this type of factor. General epidemiology - Family and Community Medicine department Establishing the cause of a disease Causal inference is the term used for the process of determining whether observed associations are likely to be causal.  The process of judging causation can be difficult. It has been argued that causal inference should be restricted to the measurement of an effect, rather than as a criterionguided process for deciding whether an effect is present or not. Before an association is assessed for the possibility that it is causal, other explanations, such as chance, bias and confounding, have to be excluded. General epidemiology - Family and Community Medicine department The ASSOCIATION between two factors can be either: DIRECT –CAUSAL- ASSOCIATION NON-CAUSAL ASSOCIATION: due to one or more of the following: 1.chance. 2.confounder 3.bias General epidemiology - Family and Community Medicine department Assessing the relationship between a possible cause and an outcome OBSERVED ASSOCIATION How No No Could it be due to selection Or measureme nt bias? Could it be due to confoundin g? EE uis or to judge confirm PROBABLY NOT Could it be a result of chance? Could it be causal? To Apply guidelines and make judgement iEEEdffec.net 1 General epidemiology - Family and Community Medicine department 1.CHANCE. 2.CONFOUNDER :it is a third (independent) factor that its presence with exposure will also affect the outcome (positively or negatively), e.g. irradiation is a confounder on studying the risk of getting ca lung from smoking, and smoking is a confounder on studying the risk to develop cardiac ischemia from hypercholesterolemia. 3.BIAS :is (are) (a) systemic error(s) in the design &/or the conduct of the study, that will affect the estimation of exposure-disease relationship. As in depending only on hospital based records which do not represent the real population and are not designed for research purposes. General epidemiology - Family and Community Medicine department Association Vs. Causation Association refers to the statistical dependence between two variables The presence of an association…in no way implies that the observed relationship is one of cause and effect General epidemiology - Family and Community Medicine department Types of causes Sufficient causes: iii a set of conditions without any one of which the disease would not have occurred not usually a single factor, often several Necessary cause: must be present for disease to occur, disease never develops in the absence of that factor. a component cause that is a member of every sufficient cause General epidemiology - Family and Community Medicine department A necessary cause is a causal factor whose presence is required for the occurrence of the effect. If disease does not develop without the factor being present, then we term the causative factor “necessary”. Sufficient cause is a “minimum set of conditions, factors or events needed to produce a given outcome. The factors or conditions that form a sufficient cause are called component causes. General epidemiology - Family and Community Medicine department For example, in a study of an outbreak of foodborne infection, it may be found that chicken salad and creamy dessert were both sufficient causes of salmonella diarrhoea, but the ingestion of Salmonella bacteria is a necessary cause of this disease. Similarly, there are different components in the causation of tuberculosis, but the infection with Mycobacterium tuberculosis is a necessary cause . However, a causal factor on its own is often neither necessary nor sufficient, such as tobacco use as a factor for cerebrovascular disease. General epidemiology - Family and Community Medicine department Genetic factors Causes of tuberculosis Tissue Exposure invasion to bacteria Increased susceptibility poverty Crowded housing Risk factors of TB TB Infection fff.EE si _cause Rfg Mechanism of TB General epidemiology - Family and Community Medicine department The usual approach in epidemiology is to begin with a disease and search for its causes, although it is also possible to start with a potential cause (such as air pollution) and search for its effects. Epidemiology encompasses a whole set of relationships. FOR example, social class is associated with a range of health problems. Low social class, as measured by income, education, housing and occupation, leads to a general susceptibility to poor health, rather than to a specific effect. General epidemiology - Family and Community Medicine department A complete scale of specific causes of disease could explain why poor people have poor health, among them excessive exposure to infectious agents due to overcrowding, lack of clean water and sanitation, insufficient and unsafe food, and dangerous working conditions. In addition, being at the bottom of the social ladder is in itself associated with poorer health even after taking all the other factors into account. General epidemiology - Family and Community Medicine department A causal pathway Prevention strategies often need to be directed simultaneously at more than one factor. Causes can be linked to a causal pathway where one factor leads to another until eventually the specific pathogenic agent becomes present in the organ that gets damaged; this can also be called a hierarchy of causes. General epidemiology - Family and Community Medicine department Genetic factors Causes of cholera Exposure to contaminated water Increased susceptibility Effect of cholera toxins on bowel wall cells Infection by Vibrio cholera cholera poverty Crowded housing Risk factors of cholera Mechanism of cholera General epidemiology - Family and Community Medicine department Single and multiple causes Pasteur’s work on microorganisms led to the formulation, first by Henle and then by Koch, of the following rules for determining whether a specific living organism causes a particular disease: • the organism must be present in every case of the disease; • the organism must be able to be isolated and grown in pure culture; • the organism must, when inoculated into a susceptible animal, cause the specific disease; • the organism must then be recovered from the animal and identified. General epidemiology - Family and Community Medicine department for many diseases, both communicable and noncommunicable, Koch’s rules for determining causation are inadequate. Many causes act together, and a single factor – such as tobacco use – may be a cause of many diseases. In addition, the causative organism may disappear when a disease has developed, making it impossible to demonstrate the organism in the sick person. Koch’s postulates are of most value when the specific cause is a highly pathogenic infectious agent, chemical poison or other specific factor, and there are no healthy carriers of the pathogen: a relatively uncommon occurrence or General epidemiology - Family and Community Medicine department Web of Causation There is no single cause Causes of disease are interacting Illustrates the interconnectedness of possible causes Chain of causation Complexity of origin is web Multiple factors promote or inhibit Emphasizes multiple interactions between host and environment General epidemiology - Family and Community Medicine department Web of Causation General epidemiology - Family and Community Medicine department I refus Web of Causation -Income CHD use im ear His 1 Fuse RS Bhopal General epidemiology - Family and Community Medicine department Factors in causation Four types of factors play a part in the causation of disease, all may be necessary but they are rarely sufficient to cause a particular disease or state: • Predisposing factors, such as age, sex, or specific genetic traits that may result in a poorly functioning immune system or slow metabolism of a toxic chemical. • Enabling (or disabling) factors such as low income, poor nutrition, bad housing and inadequate medical care may favour the development of disease. The social and economic determinants of health are just as important as the precipitating factors in designing prevention approaches. e General epidemiology - Family and Community Medicine department • Precipitating factors such as exposure to a specific disease agent may be associated with the onset of a disease. • Reinforcing factors such as repeated exposure, environmental conditions and hard work may aggravate an established disease or injury. General epidemiology - Family and Community Medicine department The term “risk factor” is commonly used to describe factors that are positively associated with the risk of development of a disease but that are not sufficient to cause the disease. The concept has proved useful in several practical prevention programmes. Some risk factors (such as tobacco smoking) are associated with several diseases, and some diseases (such as coronary heart disease) are associated with several risk factors. General epidemiology - Family and Community Medicine department Risk factors common to major non-communicable diseases off new plan Iv General epidemiology - Family and Community Medicine department Interaction The effect of two or more causes acting together is often greater than would be expected on the basis of summing the individual effects. It is illustrated by the particularly high risk of lung cancer in people who both smoke and are exposed to asbestos dust . The risk of lung cancer in this group is much higher than would be indicated by a simple addition of the risks from smoking (ten times) and exposure to asbestos dust (five times); the risk is multiplied fifty times. General epidemiology - Family and Community Medicine department A hierarchy of causes Multiple causes and risk factors can often be displayed in the form of a hierarchy of causes, where some are the proximal or most immediate causes (precipitating factors) and others are distal or indirect causes (enabling factors). Inhaled tobacco smoke is a proximal cause of lung cancer, while low socio-economic status is a distal cause that is associated with smoking habits and indirectly with lung cancer. Various frameworks have been devised for visualizing the relationships between the distal and proximal causes and the eventual health effects. One such multi-layer framework, DPSEEA (driving forces, pressure, state, exposure, effect, action), was used by WHO to analyse different elements of causation, prevention and indicators in relation to environmental health hazards . General epidemiology - Family and Community Medicine department General epidemiology - Family and Community Medicine department Epidemiological criteria (guidelines) for causality An association rarely reflects a causal relationship but it may. Criteria for causality provide a way of reaching judgements on the likelihood of an association being causal. causation Lilian I General epidemiology - Family and Community Medicine department causation Hill’s Criteria for Causal Relation Strength of association Consistency of findings Specificity of association Temporal sequence Biological gradient (dose-response) Biological plausibility Coherence with established facts Experimental evidence General epidemiology - Family and Community Medicine department Strength of association association Does exposure to the cause change disease incidence? The strength of the association is measured by the relative risk. The stronger the association, the higher the likelihood of a causal relationship. Strong associations are less likely to be caused by chance or bias General epidemiology - Family and Community Medicine department Consistency of findings Consistency refers to the repeated observation of an association in different populations under different circumstances. Causality is more likely when the association is repeated by other investigations conducted by different persons in different places, circumstances and time-frames, and using different research designs. General epidemiology - Family and Community Medicine department Specificity of association RF weak diseas specific a for criteria It means that an exposure leads to a single or characteristic effect, or affects people with a specific susceptibility -easier to support causation when associations are specific, but -this may not always be the case  as many exposures cause multiple diseases This is more feasible in infectious diseases than in noninfectious diseases, which can result from different risk agents. General epidemiology - Family and Community Medicine department Temporal sequence (temporality) Did the cause precede the effect? it OUFF'd factor Temporality refers to the necessity that the cause must precede the disease in time. This is the only absolutely essential criterion. It is easier to establish temporality in experimental and cohort studies than in case-control and cross-sectional studies. General epidemiology - Family and Community Medicine department Biological gradient c Does the disease incidence vary with the level of exposure? (dose-response relationship) Changes in exposure are related to a trend in relative risk A dose-response relationship (if present) can increase the likelihood of a causal association. General epidemiology - Family and Community Medicine department Biological plausibility Is there a logical mechanism by which the supposed cause can induce the effect? Findings should established understanding not disagree of with biological processes. General epidemiology - Family and Community Medicine department Coherence Coherence implies that a cause-and-effect interpretation for an association does not conflict with what is known of the natural history and biology of the disease General epidemiology - Family and Community Medicine department Experimental evidence It refers to evidence from laboratory experiments on animal or to evidence from human experiments Causal understanding can be greatly advanced by laboratory and experimental observations. General epidemiology - Family and Community Medicine department Judging the causal basis of the association  No single study is sufficient for causal inference  It is always necessary to consider multiple alternate explanations before making conclusions about the causal relationship between any two items under investigation.  Causal inference is not a simple process -consider weight of evidence -requires judgment and interpretation General epidemiology - Family and Community Medicine department Evaluating Evidence of Causal vini relationship Major Criteria hills a. Temporal relationship b. Biologic plausibility c. Consistency of Results d. Alternative explanations m a ÉÉÉ Other criteria weak egg a. Strength of association b. Dose-response relationship Infected they c. Cessation of effects when General epidemiology - Family and Community Medicine department Pyramid of Associations Causal Non-causal Confounded Spurious / artefact Chance RS Bhopal General epidemiology - Family and Community Medicine department Thank you General epidemiology - Family and Community Medicine department