Clinical epidemiology. Modern principles and rules conducting of clinical researches. Concept of a zero hypothesis, principles of randomization and stratification in medical researches.
Epidemiology of Chronic Non-communicable Diseases and Conditions
The Communicable Disease and Epidemiology division at the Kent County Health Department monitors the occurrence of specific diseases on a community-wide basis.
An explanation of the communicable disease and epidemiology functions at the health department would be incomplete without first providing a definition for that which serves as the foundation of this practice, epidemiology and surveillance.
Epidemiology
Epidemiology is defined as “the study of the distribution and determinants of health-related states or events in specified populations and the application of this study to the control of health problems.” To put it more simply, epidemiology is the task of using data to answer questions of:
- Who is getting sick?
- What is making people sick? And,
- How can we use this information to reduce the risk of others getting sick?
Without quality health data, it is very difficult to answer these questions. A surveillance system that serves to collect health data in a complete and timely manner is thus essential to the practice of epidemiology.
Surveillance
Surveillance is defined as “the ongoing systematic collection, analysis, and interpretation of outcome-specific data for use in planning, implementation, and evaluation of public health practice.”
Data collected in a surveillance system can be used for many purposes, including:
- To estimate the magnitude of a health problem in a population
- To understand the natural history of a disease
- To detect outbreaks or epidemics
- To document the distribution of a health event
- To test hypotheses about causes of disease
- To monitor changes in infectious organisms
Types of Surveillance
The Kent County Health Department performs surveillance activities for both communicable diseases and chronic diseases.
Communicable Disease Surveillance
Communicable diseases are those that can be transmitted from person to person (or animal to human) via direct contact with body fluids, ingesting contaminated food or water, inhalation of contaminated air, or the bite of an infected insect. Bacteria, viruses, and parasites are some of the organisms that can cause communicable diseases. Examples of communicable diseases are Hepatitis B, Salmonellosis, Measles, and West Nile Virus.
Preventing and controlling communicable disease is a necessary and critical aspect of assuring community health, and is an affirmative duty of local public health departments. Protecting the public’s health from communicable disease threats requires a proactive public health disease surveillance system, timely epidemiological assessment, and ongoing disease prevention education.
Because community disease surveillance and control is a critical component of disease prevention, the Kent County Health Department monitors the occurrence of specific diseases on a community-wide basis. Physicians, laboratories, and schools all report cases of disease to the Health Department. With this information, we are able to monitor both the incidence (number of new cases) and prevalence (number of existing cases) of disease in
Four primary types of epidemiology studies.
The “Big Picture”
In
A single case of a disease may not cause alarm in a physician’s office. However, timely and accurate reporting of communicable disease data allows health department personnel to determine whether this single case may be part of a larger problem in the community. With complete information, health department personnel can check if the disease is related to other cases as part of a cluster or is part of an outbreak (where the number of cases is greater than the number expected during a defined period of time).
Disease Detectives
Public health nurses and epidemiologists act as detectives who try to connect pieces of a puzzle in solving disease mysteries. These public health professionals monitor disease information to determine if there are more cases of a particular disease than expected. In addition, they investigate cases of disease to discover clues that may link the infected individuals and uncover the source of their infection. Once the culprit is discovered, these public health professionals provide education to those who are ill and those at risk of becoming ill to help prevent the spread of infection.
Benefits
Timely and accurate reporting of communicable disease information thus allows health department personnel to:
- Quickly identify single or multiple cases of disease occurring within a similar location or time
- Identify persons at risk of acquiring or transmitting disease
- Identify care needs and recommend appropriate prevention measures for those affected
- Provide education for future prevention
- Assess the effectiveness of public health disease prevention programs
Chronic Disease Surveillance
Chronic diseases cannot be transmitted from person to person. Examples of chronic diseases are heart disease, diabetes, and cancer. Poor health behaviors (lack of physical activity, smoking, poor eating habits, etc.) may increase a person’s risk for chronic disease.
Many of the advances in public health and life expectancy in the
Limitations
The nature of chronic diseases presents difficulty when attempting to assess their impact on a community. No formal reporting requirement exists to assist with chronic disease surveillance. Cancer registries exist that allow public health professionals to measure the incidence and prevalence of certain cancers. Beyond that, however, we rely mainly on death certificates that give an indication of causes of death. Although useful, death certificate information is limited in that it is based upon a doctor’s diagnosis, and underlying causes of death are often masked by that which was determined to be the primary cause. Assessing the impact of chronic diseases in a community is thus an inexact science.
Behavioral Risk Factor Survey
Because surveillance for chronic disease is difficult, public health professionals rely on collecting information on health behaviors to give an indication of a community’s risk for acquiring diseases such as heart disease, diabetes, and cancer. To this end, the Kent County Health Department performs a Behavioral Risk Factor Survey (BRFS) approximately every five years to quantify the presence of particular behaviors in a sample of
Other Chronic Disease Data
In addition to the BRFS,
Chronic diseases and conditions have been variously defined. A EURO symposium in 1957gave the following definition:
“An impairment of bodily structure and/or function that necessitates a modification of the patient’s normal life, and has persisted over an extended period of time”.
Another EURO symposium in 1965 observed:
“Up to now no widely acceptable definition (of acute or chronic patients) has been found. Some authors maintain that an acute illness usually consists of a simple episode of fairly short duration from which the patient returns to normal activity, whereas a chronic illness is one of long duration in which the patient is permanently incapacitated to a more or Jess marked degree. There is also the view that progress in the technology of resuscitation and haemobiology has blurred the borderline between acute and chronic conditions”.
The Commission on Chronic Illness in
a. are permanent
b. leave residual disability
c. are caused by non-reversible pathological alteration
d. require special training of the patient for rehabilitation
e. may be expected to require a long period of supervision, observation or care”.
In short, there is no international definition of what duration should be considered long-term, although many consider chronic conditions are generally those that have had a duration of at least 3 months. A practical definition should be established which will suit the particular conditions of the community.
Non-communicable diseases (NCDs) include cardiovascular, renal, nervous and mental diseases, musculo-skeletal conditions such as arthritis and allied diseases, chronic non-specific respiratory diseases (e.g., chronic bronchitis, emphysema, asthma), permanent results of accidents, senility, blindness, cancer, diabetes, obesity and various other metabolic and degenerative diseases and chronic results of communicable diseases. Disorders of unknown cause and progressive course are often labelled “degenerative”.
The problem
Chronic non communicable diseases are assuming increasing importance among the adult population in both developed and developing countries. Cardiovascular diseases and cancer are at present the leading causes of death in developed countries (e.g., Europe and
Non-communicable disease risk factors
Most epidemiologists accept that six key sets of “risk factors” are responsible for a major share of adult non-communicable disease morbidity and premature mortality. These are as follows:
1. Cigarette use and other forms of smoking
2. Alcohol abuse
3. Failure or inability to obtain preventive health services (e.g., for hypertension control, cancer detection, management of diabetes)
4. Life-style changes (e.g., dietary patterns, physical activity)
5. Environmental risk factors, e.g., occupational hazards, air and water pollution, and possession of destructive weapons
6. Stress factors
Gaps iatural history
There are many gaps in our knowledge about the natural history of chronic diseases. These gaps cause difficulties aetiological investigations and research. These are:
1. Absence of a known agent:
There is much to learn about the causes of chronic disease. Whereas in some chronic diseases the cause is known (e.g., silica in silicosis, asbestos in mesothelioma), for many chronic diseases the causative agent is not known. The absence of a known agent makes both diagnosis and specific prevention difficult.
2. Multifactorial causation:
Most chronic diseases are the result of multiple causes –rarely is there a simple one-to-one cause-effect relationship. In the absence of a known agent, the term “risk factor(s)” is used to describe certain, factors in a person’s background or life-style that (make the likelihood of the chronic condition more probable. Further, chronic diseases appear to result from the cumulative effects of multiple risk factors. These factors may be both environmental and behavioral or constitutional. Epidemiology has contributed massively in the identification of risk factors of chronic diseases. Many more are yet to be identified and evaluated.
3. Long latent period:
A further obstacle to our understanding of the natural history of chronic disease is the long latent (or incubation) period between the first exposure to “suspected cause” and the eventual development of disease (e.g., cervical cancer). This makes it difficult to link suspected causes (antecedent events) with outcomes, e.g., the possible relation between oral contraceptives and the occurrence of cervical cancer. In an attempt to overcome this problem, a search has been made for precursor, lesions in, for example, cancer cervix, oral cancer and gastric cancer. But this is not possible in all chronic diseases. However, it has now become increasingly evident that the factors favouring the development of chronic disease are often present early in life, preceding the appearance of chronic disease by many years. Examples include hypertension, diabetes, stroke, etc.
4. Indefinite onset:
Most chronic diseases are slow in onset and development, and the distinction between diseased and non -diseased states may be difficult to establish (e.g., diabetes and hypertension). In many chronic diseases (e.g., cancer) the underlying pathological processes are well established long before the disease manifests itself. By the time the patient seeks medical advice, the damage already caused may be irreversible or difficult to treat.
Integrated approach:
It is now felt that the principles of prevention of CHD can be applied also to other major non-communicable diseases (NCDs) because of common risk factors. A broader concept is emerging, that is, to develop an overall integrated programme for the Prevention and Control of NCDt as part of primary health care systems, simultaneously attacking several risk factors known to be implicated In the development of non-communicable diseases. Such concerted preventive action should reduce nut only cardiovascular diseases but also other major NCDs, with an overall improvement in health and length of life.
Measuring’ the burden of the disease
The burden of CHD may be estimated in various ways, each illustrating a different aspect of the picture.
(a) Proportional mortality ratio: The simplest measure is the proportional mortality ratio, i.e., the proportion of all deaths currently attributed to it. For example, CHD Is held responsible for about 30 per cent of deaths in men and 25 per cent of deaths in women in most western countries.
(b) Loss of life expectancy: CHD cuts short the life expectancy, Calculations have been made for the average gain in life expectation that would follow a complete elimination of all cardio-vascular deaths if other mortality rates remain unchanged. The benefit would range for men from 3.4 years to 9.4 years, and even greater for women.
(c) CHD incidence rate: This is the sum of fatal and non-fatal attack rates. Because of its different manifestations, accurate incidence of CHD rates are difficult to compute. Mortality rates can be used as a crude indicator of incidence.
(d) Age-specific death rates: When analysis is planned to throw light on aetiology, it is essential to study the age-specific rates. Age-specific death rates suggest a true increase in incidence.
(e) Prevalence rate: The prevalence of CHD can be estimated from cross-sectional surveys using ECG for evidence of infarction and history of prolonged chest pain. A useful publication to conduct such surveys is “Cardio-vascular Survey Methods” by Rose and
(j) Case fatality rate: This is defined as the proportion of attacks that are fatal within 28 days of onset. The International Society and Federation of Cardiology has suggested that “sudden deaths” be defined to include deaths “occurring instantly or within an estimated 24 hours of the onset of acute symptoms or signs”. Data collected in many industrialized countries indicate that 25-28 per cent of patients who suffer a heart attack die suddenly. In about 55 per cent of ail cardiac deaths mortality occurs within the first hour.
(g) Measurement of risk factor levels: These include measurement of levels of cigarette smoking, blood pressure, alcohol consumption and serum, cholesterol in the community.
(h) Medical care: Measurement of levels of medical care in the community are also pertinent.
Epidemicity
“Epidemics” of CHD began at different times in different countries. In United Stales, epidemics began in the early 1920s; in
Countries where the epidemic began earlier are now showing a decline. For example, in
Several European countries where the epidemic came later, have registered little or no change in rates (e.g.,
The decline in CHD mortality in US and other countries has been attributed to changes in life -styles and related risk factors (e.g., diet and diet-dependent serum cholesterol, cigarette use and exercise habits) plus better control of hypertension.
The reasons for the changing trends in .CHD are not precisely known. The WHO has recently completed a project known as MONICA “(multinational monitoring of trends and determinants in cardiovascular diseases)” to elucidate this issue. Forty-one centres in 26 countries were participating in this project, which was planned to continue for a 10 year period ending in 1994.
When CHD emerged as the modern epidemic, it was the disease of the higher social classes in the most affluent societies. Fifty years later the situation is changing; there is a strong inverse relation between social class and CHD in developed countries.
To summarize, in many developed countries, CHD still poses the largest public health problem. But even in those showing a decline, CHD is still the most frequent single cause of death among men under 65.
International variations
CHD is a world-wide disease. Mortality rates vary widely in different parts of the world (Table 1). The highest coronary mortality is seen at present in North Europe and in English-speaking countries (e.g.,
Rose calculates that the “incubation period” of CHD may be 10 years or more. This may explain the currently puzzling position of
PREVENTION OF CHD
In the 1960s the issue was whether CHD could be prevented or not. Studies were launched, reported and debated, The accumulated evidence led to a broad consensus of expert opinion that CHD is preventable. This is best expressed in a report of the WHO Expert Committee on the Prevention of CHD, which recommended the following strategies:
a. Population strategy
(i) prevention-in whole populations
(ii) primordial prevention in whole populations
b. High risk strategy
c. Secondary prevention
a. Population strategy
CHD is primarily a mass disease. The strategy should therefore be based on mass approach focusing mainly on the control of underlying causes (risk factors) in whole populations, not merely in individuals. This approach is based on the principle that small changes in risk factor levels in total populations can achieve the biggest reduction in mortality. That is, the aim should be to shift the whole risk-factor distribution in the direction of “biological normality”. This cannot obviously be done by medical means atone; it requires the mobilization and involvement of the whole community to alter its life-style practices that are associated with CHD,
Specific interventions:
The population strategy centres round the following key areas:
1. Dietary changes: Dietary modification is the preventive strategy in the prevention of CHD. The WHO Expert Committee considered the following dietary changes to be appropriate for high incidence populations:
– reduction of fat intake to 20-30 per cent of total energy intake
– consumption of saturated fats must be limited to less than 10 per cent of total energy intake; some of the reduction in saturated fat may be made up by mono- and poly-unsaturated fats
– a reduction of dietary cholesterol to below 100 mg per 1000 kcal per day
– an increase in complex carbohydrate consumption (i.e., vegetables, fruits, whole grains and legumes)
– avoidance of alcohol consumption; reduction of salt intake to
2. Smoking: As far as CHD is concerned, present evidence does not support promotion of the so-called “safer cigarette”. The goal should be to achieve a smoke-free society. Some governments (e.g.,
To achieve the goal of a smoke-free society, a comprehensive health programme would be required which includes effective information and education activities, legislative restrictions, fiscal measures and smoking cessation programmes.
3. Blood pressure: It has been estimated that even a small reduction in the average blood pressure of the whole population by a mere 2 or
4. Physical activity: Regular physical activity should be a part of normal daily life. It is particularly important to encourage children to take up physical activities that they can continue throughout their lives.
Primordial prevention:
A novel approach to primary prevention of CHD is primordial prevention. It involves preventing the emergence and spread of CHD risk factors and life styles that have not yet appeared or become endemic. This applies to developing countries in particular. These countries should seek to preserve their traditional eating patterns and life-styles associated with low levels of CHD risk factors.
Since the aetiology of CHD is multifactorial the approach to prevention should be rnultifactorial aimed at controlling or modifying as many risk factors as possible. The aim should be to change the community as a whole, not the individual subjects living in it.
Several well-planned risk factor intervention trials (e.g., the Multiple Risk Factor Intervention Trial (MRFIT) in the
b. High risk strategy
(I) Identifying risk: High risk intervention can only start once those at high risk have been identified. By means of simple tests such as blood pressure and serum cholesterol measurement it is possible to identify individuals at special risk. Individuals at special risk also include, those who smoke, those with a strong family history of CHD, diabetes and obesity and young women using oral contraceptives.
(II) Specific advice: Having identified those at high risk, the next step will be to bring them under preventive care and motivate them to take positive action against all the identified risk factors, e.g., an elevated blood pressure should be treated; the patient should be helped to break the smoking habit permanently – nicotine chewing gum can be tried to wean patients from smoking; serum cholesterol concentration should be reduced in those in whom it is raised, etc.
Several well planned high-risk intervention studies (e.g., Oslo Heart Study, Lipid Research Clinics Study, in US) have shown that it is feasible to reduce the CHD risk factors.
From a methodological point of view, however, high-risk approach suffers from the disadvantage that the intervention (e.g., treatment) may be effective in reducing the disease in a high-risk group, but it may not reduce the disease to the same extent in the general population which consists of symptomatic, asymptomatic, high-risk, low-risk and healthy people. Further, unfortunately, more than half of the CHD cases occur in those who are not apparently at special risk, and this is one limitation of the high-risk strategy. Nevertheless, recognition and treatment of high-risk cases do make an important contribution to prevention.
c. Secondary prevention
Secondary prevention must be seen as a continuation of primary prevention. It forms an important part of an overall strategy. The aim of secondary prevention is to prevent the recurrence and progression of CHD. Secondary prevention is a rapidly expanding field with much research in progress (e.g., drug trials, coronary surgery, use of pace makers).
The principles governing secondary prevention are the same as those already set out in the above sections, e.g., cessation of smoking, control of hypertension and diabetes, healthy nutrition, exercise promotion, etc. The most promising results to date have come from beta-blockers which appear to reduce the risk of CHD mortality in patients who have already suffered at least one infarct in the order of 25 per rent. None of the preventive measures discussed earlier lose their importance even after the first attack. For example, cessation of smoking is the most effective single means of intervention currently available in the management of patients after a heart attack. The risk of fatal-infarction or sudden death is reduced by 20-50 per cent. If the patient does not stop smoking, nothing else is worth doing.
Despite advances in treatment, the mortality of an acute heart attack is still high: among survivors, around 10 per cent in the first year, and 5 per cent yearly thereafter. Delay in reaching hospital is still considerable even in big cities in the West and may be as much as 3.5 hours. About 30 per cent of all deaths occur within 30 minuses of onset. This is one of the reasons why coronary care units have failed to make impact on the total coronary mortality in the community.
Each strategy – population strategy, high risk strategy, secondary prevention – has its advantages and disadvantages, but the population strategy has the greatest potential.
