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Injury Epidemiology Injury Epidemiology
Ten Basic Principles
Thomas Songer, PhD
University of Pittsburgh
Our class, EPI 2670, presents a quick introduction to injury epidemiology by design. My approach for teaching the class is to provide an overview of the key points of the discipline with selected examples to highlight and enhance your understanding. This lecture represents what I feel are ten key concepts that a student should recognize to have an introductory understanding of injury epidemiology. The issues presented represent basic issues that researchers recognize and practice to further injury control and prevention. So, let’s get started!
Injuries occur as
the result of
energy transfer that is
delivered in excess of
a threshold
The first basic principle in injury epidemiology is the understanding of what injuries are and, in some sense, what they are not. In general, injuries occur when humans encounter energy forces that are larger than the body’s normal tolerance levels for energy absorption. The level of energy encountered exceeds a threshold.
Energy Transfer and Injuries
- Penetrating
- Non-Penetrating
- Compression
- Burn
If the energy transfer is localized in one area, the likely outcome may be a penetrating injury. If the energy transfer is dispersed over a broad area, the result will often be a non-penetrating injury. In situations involving thermal energy transfer, the result will be a burn. And so on, depending upon the mode of energy involved.
Most injuries (74%) arise from the transfer of physical or mechanical energy. This is due to the frequency in which we come into contact with events and vehicles that involve mechanical energy. The leading causes of death from mechanical energy transfer are injuries from motor vehicle accidents, firearms, and falls.
Injuries are not
Accidents
Now, let me diverge a bit to talk briefly about what many injury professionals feel that injuries are not. “Injuries are not Accidents”. This has been a common slogan spoken by injury research professionals.
It originated some time ago to counteract the perception that injuries occurred by chance. In the past, many persons in the lay public and many legislators regarded injuries as accidents; events that you had little control over. This thought probably arose from the publicity that natural disasters receive.
However, it is now well recognized that nearly all injuries are not the result of random events. There are distinct patterns and circumstances that characterize their occurrence. We understand that injuries most often occur to certain risk groups and are fairly predictable (whether it be to certain persons, at certain times, or in common locations).
In this light, many persons in the injury field refer to automobile accidents as “crashes” rather than “accidents”. In reality, it does not matter whether or not crashes or accidents is the most appropriate term. What is important is the recognition that injury events often have identifiable characteristics, and that we may be able to prevent future injuries by intervening on one or more of these characteristics.
Public Health Model
Define the
problem
Identify
risk factors
Intervene Evaluate
Identify morbidity mortality cost
social genetic environmental health care
The public health model outlines a defined set of steps to undertake for reducing the burden of diseases or injuries. The first step of the model is to identify the relative magnitude of the problem. This is often done through the establishment of surveillance systems to quantify incidence or health outcomes.
The second step is to identify causal factors associated with the injury (i.e. risk factors). This determination might arise from information contained within the surveillance system. More often, though, it is done independently through analytic epidemiologic studies.
The third step is to utilize the information available on causal factors to develop an intervention. After implementation of the intervention, it is then appropriate to consider an evaluation of it’s effect. This evaluation may use data from the surveillance system to examine if there has been any change in the burden of disease.
Social-Ecologic Model
Individual
Community
Relationship
Society
The social ecologic model illustrates that an injury (or more commonly) a violence issue has multiple components to explain its occurrence. As shown in this slide, there are several layers of issues that can influence a situation and lead to an injury. For example, individual (or host) factors) have a role to play in injuries and violence. These individual factors may be influences by family factors, which in turn, may be influenced by community factors, ….which can be set by societal norms and standards. The social ecologic model is now gaining widespread acceptance as a perspective to consider violence in communities.
Injury Surveillance
What are injury surveillance systems? These systems represent the systematic collection of data on an injury. Over time they identify trends to show if injuries are increasing or decreasing in incidence, and which are changing in their distribution. This information is needed to identify emerging problems and also to assess the effectiveness of measures to control old problems. For more information, see Epidemiology for the Uninitiated, Chapter 1.
Sentinel Events
used to assess the
stability or change in the
health of a population.
John Last
Dictionary of Epidemiology
Surveillance systems are often used to identify sentinel events. Marked increases in these events generally provide an indication or warning to public health officials of an emerging problem. Many injury systems are set up for this purpose.
Deaths from automobile accidents are one example of a sentinel event that is followed closely by the injury community. If an increase in accident deaths were to occur, there would be an indication that someone should start to look into the factors behind it. However, many surveillance systems that monitor sentinel events, including those in the injury field, are not designed to answer research questions on the cause(s) of the sentinel event.
ICD Codes
- Diagnosis Codes (N-Codes – ICD 9; S, T Codes – ICD 10)
- Record the clinical nature of the injury and the area of the body injured
- Cause of Injury Codes
- E-Codes (ICD – 9)
- Record the external cause of the injury
- Often missing in the medical record
- V, W, X, Y Codes (ICD – 10)
Most injuries are identified at the point when an individual comes into contact with the health care system. Injuries are usually defined in medical records by one of the most widely used classification systems in the world; the International Classification of Diseases (ICD) system and codes. Two types of ICD codes are of most interest; nature of injury codes which identify the anatomy involved in the injury, and external cause of injury codes which identify the events leading to the injury.
One major issue of concern for injuries, is that monitoring systems based upon medical records do not always provide details of the cause for the injury (E-codes or V, W, X, and Y codes).
Other types of classification schemes exist for injuries as well. These include systems to classify the severity of injury; such as the Abbreviated Injury Scale (AIS) and the Injury Severity Score (ISS). Also, there is a rubric recently adopted to classify the intent underlying the injury; i.e was it intentional or unintentional.
Most injuries are
relatively minor,
and do not require
any medical
attention.
Injury severity is an important issue in injury monitoring, not only from a classification perspective, but also from the point of view of the data sources that make up the surveillance system. Nearly all injury surveillance systems utilize sources that identify relatively severe events; those that require medical attention. The reality, though, is that the vast majority of injuries are minor events that are treated at home.
Injuries and the
Factors Underlying
Injuries can be Examined
from an Epidemiologic
Framework.
The visionary of injury epidemiology and injury control was William Haddon. Dr. Haddon was the director of the National Highway Traffic Safety Administration and the Insurance Institute for Highway Safety in the 1960s and 1970s. He used these positions to play a leading role in the cause of traffic safety.
The basis behind his work was the simple argument that injuries can be examined within an epidemiologic framework. In it’s classic sense, the epidemiology triad considers the interaction of three factors in the development of disease; the host, the agent, and the environment. Dr. Haddon maintained that these factors also were key elements in the development of injuries.
Human
Environment
Mechanical Energy
Injuries and the Epidemiology Triad
(Agent)
(Host)
Here we see an illustration of the epidemiology triad as it relates to injuries. In this example, the host is the human being and their behavior in operating a motor vehicle. Physical energy is the agent in injury events. For motor vehicle events, this translates into the mechanical energy involved with motor vehicles. The environment is the milieu in which the vehicle and the human are interacting, such as the type of road, the weather conditions involved, etc.
Causal Factors for
Injuries Are Not as
Well Defined as
You Might Think
They Are
Ask someone to identify for you the primary risk factor for automobile crashes and they are likely to respond; alcohol use. Ask another person what the primary factor is behind head trauma, and they will probably indicate the non-use of a helmet. Many aspects of injury control have permeated our daily lives. The point to raise here, though, is that we should not take this and other information for granted. Many risk factors for injuries remain poorly defined. This is because they have been identified from surveillance systems that are generally poor at characterizing the population’s exposure to the factors.
Descriptive Studies
Case-control Studies
Cohort Studies
Develop
hypothesis
Investigate it’s
relationship to
outcomes
Define it’s meaning
with exposures
Descriptive studies are the most common approaches used today in injury research. A sentinel event surveillance system is one example of a “descriptive study”. It is very useful for identifying hypotheses to test in analytic studies. It is very poor at identifying causal agents.
For instance, a major flaw of studies which identify injuries from medical records, is that you only know the risk factors for people who have injuries. You do not know how these factors may differ from the people who are not injured. This is a crucial point if you want to be able to identify events that place someone at risk for an injury and the importance of that risk. This is where the importance of analytical epidemiologic studies is recognized.
Case-control studies and cohort studies are examples of analytic designs. Case- control studies are generally used to evaluate if the hypothesized factor is related to an injury. Cohort or longitudinal studies are subsequently applied to more clearly define the importance of exposure to the causal agent for the development of an injury.
