Chapter 9 Epidemiology of Whiplash Injuries
Epidemiology has been defined as “the science concerned with the study of the factors determining and influencing the frequency and distribution of disease, injury, and other health-related events and their causes in a defined human population for the purpose of establishing programs to prevent and control their development and spread.”1 Hence, this chapter will cover the numbers of persons who experience whiplash injuries, mainly in the United States, as well as factors that are thought to influence the risk of becoming injured in a motor vehicle crash (MVC) or of developing chronic symptoms post-injury. Many such factors have been suggested, including the injured person’s age, gender, anthropometry, certain pre-existing conditions, position within the vehicle, the vehicle’s crashworthiness, and others. Clinicians should be familiar with the significant risk factors in order to formulate more personalized treatment plans and prognoses in whiplash patients.
Epidemiological studies have been used extensively to investigate the whiplash phenomenon, primarily because the condition is not amenable to investigation using randomized controlled trials, which are considered to be the gold standard of clinical research. This is primarily because it would be unethical to expose people to the kinds of forces that are capable of causing injuries involved in MVCs. Thus, whiplash studies typically compare a group of people who have already been involved in MVCs with another group who have not. Researchers either follow the groups forward in time (i.e., cohort studies) or assess various aspects of their medical history retrospectively, before as well as after the time of the crash (i.e., case-control studies).
Studies have been carried out where human volunteers were exposed to whiplash-like forces (i.e., crash tests) similar to what would be expected in a real-life MVC. However, they have always simulated relatively low-speed crashes and rarely resulted in symptoms.2–4 When symptoms did occur, they have been mild and short-lived. Also, these types of studies have never randomized subjects to groups for comparisons.
Epidemiology is commonly used in the field of public health, which focuses on the study of populations with the purpose of identifying the causes of diseases and conditions. Once the causes are identified, strategies can be developed that are designed to prevent the diseases from occurring. Public health interventions, such as advising people about the best ways to avoid risk factors or how they can integrate protective factors into their lifestyles, can effectively prevent future injuries and illness, as well as enhance the healing process when injury or illness has already occurred. Traffic safety matters, including whiplash, are of interest to public health practitioners, health care providers, as well as local, state, and national governments because of the potential to prevent or reduce the severity of crash-related injuries using these preventive methods.
One aspect of epidemiology involves collecting data from various sources and then analyzing it. However, the data pertaining to the epidemiology of whiplash is at times conflicting—sometimes because of inconsistencies in the way the data were gathered and other times in the way they were analyzed. It is also possible, and not that uncommon, for data collection and analysis to go awry at the same time. These inaccuracies can often be resolved by relying on higher-quality information sources, though even high-quality sources are not always in agreement. In addition to the potential flaws associated with the collection, analysis, and reporting of data, the way data are interpreted is often inconsistent. This chapter will call attention to some of these controversial aspects in relation to whiplash, relying on the highest-quality studies available.
Not only is the way data are collected and interpreted inconsistent in this area of investigation, the term “whiplash” itself is controversial: whereas some researchers use it to describe a mechanism of injury in which the head and neck are forcibly moved beyond the limits of anatomical integrity, others use it to describe the injury that often results. Consequently, the Quebec Task Force on Whiplash-Associated Disorders coined the term “whiplash-associated disorders” (WAD) in 1995 to describe an array of symptoms that have been reported in association with whiplash injuries.5 According to the Quebec Task Force, WAD symptoms may include neck pain and stiffness, headache, dizziness, tinnitus, memory loss, deafness, dysphagia, upper extremity syndromes (e.g., radicular symptoms and carpal tunnel syndrome), temporomandibular joint disorder, and others.
The incidence of whiplash refers to the number of new cases that are diagnosed each year in a given population. Due to the previously mentioned problems with collecting whiplash-related data, estimates of whiplash incidence vary greatly, ranging from 3.4 per 100,0006 to 800 per 100,0007 population per year. A Swedish study reported the annual incidence of whiplash in the local catchment area to be 4.2 per 1,000 inhabitants for grade 1 WAD and 3.2 per 1,000 for grade 3.8 Holm et al9 reported that the incidence of reported WAD in the Western world is probably at least 300 per 100,000 inhabitants per year. A commonly quoted statistic that refers to the rate of whiplash in the United States is 1,000,000 total cases per year.10
The Insurance Institute for Highway Safety (IIHS) reported that approximately 2 million whiplash insurance claims are filed each year in the United States, resulting in more than $8.5 billion in insurance claims.11 In fact, neck sprains and strains are the most frequent type of injury claim reported to insurance companies in the United States, comprising 25% of all injury-related claim dollars paid out by insurers each year. The institute also reported that about 10% of whiplash injuries result in long-term medical problems. In the Canadian province of Saskatchewan, 83% of traffic injury claims were for whiplash in 1994-1995, resulting in an annual incidence of 677 insurance claims per 100,000 adult population.12
In addition to the cases that were included in the IIHS data, there are likely other cases of whiplash that occur but do not give rise to an insurance claim. This is because injured persons may not choose to open a claim (perhaps they do not want their insurance rates raised or they think their injuries are minor) or they are not insured. Thus, the true number of cases per year in the United States is almost certainly higher than 2,000,000, especially when non–traffic related whiplash injuries are factored in.
There are quite a few reasons why the various data sources so often generate different numbers. For instance, the National Accident Sampling System (NASS) provides data on all types of MVCs, including those that result in whiplash injuries. NASS data are collected from police-reported traffic crashes,13 which is very problematic when trying to obtain an estimate of the actual number of whiplash injuries that occur in the United States. Many MVCs that give rise to whiplash injuries are not reported to police because they do not typically investigate crashes that involve minor vehicle damage, and many whiplash-causing crashes are associated with minor to no vehicle damage. These injured persons “slip through the cracks” and are not included in the NASS database, and this results in an underestimation of the annual number of injuries by possibly hundreds of thousands. Even when police do investigate MVCs that result in whiplash, sometimes symptoms are delayed for hours and days; consequently, these cases are also unreported.
The prevalence of WAD includes the previously mentioned incident cases (i.e., newly injured), but it also includes the total number of people who have persistent symptoms and physical impairments that are not included when only incident cases are counted. It represents the estimated number of persons in a population who manifest WAD symptoms at any given time. Many people experience residual problems for years after a whiplash injury and some never recover. These cases keep mounting in numbers until some of them recover or die. In either case, they are no longer included in calculation of prevalence.
The duration of a disease has an effect on its reported incidence and prevalence. For instance, short-duration diseases like the common cold tend to have a high annual incidence but low prevalence. Because people recover so quickly, not many will have the condition at any one point in time. On the other hand, long-duration diseases like diabetes may have a relatively low annual incidence, yet its prevalence is quite high because the total number of cases keeps accumulating. This principle applies to WAD too, because many affected people experience long-term symptoms. A study from the Netherlands reported that the highest prevalence of MVC-related neck sprain was 28.3 per 100,000 and occurred in those who were in the 25- to 29-year-old age group, with the 40- to 44-year-old group a close second at 27.9 per 100,000.6 As mentioned earlier, the incidence of WAD in the United States is probably into the millions, and approximately half of those with WAD continue to report neck pain 1 year after their injuries,14 so its prevalence is undoubtedly very high. In fact, Freeman et al15 estimated, from a case-control study comprised of 419 chronic neck pain cases and 246 chronic low back pain controls, that about 6.2% of the U.S. population may have chronic neck pain attributable to a whiplash injury.
There is a great deal of controversy and debate surrounding the determination of which risk factors actually contribute to chronic WAD (aka, late whiplash syndrome) and which ones are merely associated by chance. Furthermore, some even question the legitimacy of chronic WAD, considering it to be a psychosocial phenomenon rather than being physically based.16 As a result of this dichotomy, there has been much debate about this issue in the whiplash-related literature. As stated so well by Dr. Murray Allen, “There are two great puzzles in this world that foster debate among humans. One is the wonder of the universe, the other is whiplash.”17
A large proportion of persons with chronic neck pain in the United States were initially injured in an MVC. This estimate was based on a case-control study involving 419 cases and 246 control subjects which reported that 45% of those with chronic neck pain considered its origin to have been a prior MVC.15 On the other hand, a study that was based on a random sample of 6,000 subjects from two counties in Northern Sweden reported that 42% had chronic neck pain and only about 8% of them attributed their condition to a previous whiplash injury.18
Most WAD patients recover in time, although many have long-lasting and even permanent pain and impairment. For instance, a cohort of 2,627 persons with whiplash that resulted from an MVC in Canada was followed for up to 7 years.19 The median time to recovery for the overall group was 32 days, although 12% of the subjects still had symptoms at 6 months. Several risk factors for chronic symptoms were identified in this study, including neck pain on palpation, muscle pain, pain or numbness radiating from the neck to the upper extremities, and headache. Females over 60 years old who had the identified risk factors required a median of 262 days to recover compared with only 17 days for younger males without any risk factors.
Several studies have reported that approximately 50% of WAD patients continue to complain of symptoms 1 year following injury.20–22 Other studies, however, have found the rate of long-term WAD symptoms to be lower,8,23–25 and some have reported it as being much lower.26–28 To complicate the issue further, one study found the prevalence of long-term pain following whiplash injuries to be very close to the same level as the prevalence of chronic neck pain in the general population.29
Neck and shoulder pain are commonly reported symptoms of chronic WAD. Symptoms involving other bodily regions and overall health have been reported as well, including headache, back pain, jaw pain, fatigue, dizziness, paraesthesiae, nausea, sleep disturbances, and ill health.30,31 Depression has also been reported following whiplash injuries. In one study, 42.3% of 5,211 subjects who did not have pre-injury mental health problems reported depressive symptoms within 6 weeks of the injury.32 Furthermore, the symptoms were recurrent or persistent in almost 40% of the cases. Berglund et al30 concluded that whiplash injuries due to rear-impact MVCs have a substantial impact on health complaints, even a long time after the injury.
A systematic review and meta-analysis involving 38 cohort studies that followed subjects with acute whiplash reported that recovery rates were extremely variable across studies.33 Most subjects recovered within 3 months after the injury, and recovery rates leveled off after 3 months had elapsed. The review’s authors suggested that data concerning prognostic factors thought to be associated with a poor recovery were difficult to interpret because of the dissimilar ways studies assessed associations, differences in their methods of reporting data, as well as differences in the outcome measures that were used.
Numerous studies have assessed the influence of a variety of risk factors on the probability of developing WAD subsequent to an MVC. Example risk factors include position of the occupant within the vehicle, head position (e.g., rotated), female gender, head-to-neck ratio, prior neck injury, use of safety belts, and crash severity. The results of these studies have been conflicting for the most part, but a few WAD risk factors have emerged as being important.
The underlying mechanism of whiplash is best explained by distortion of the neck that results from sudden movement of the head in relation to the torso. Thus, automobile head restraints and reactive seat backs are designed to minimize the difference between the rates of acceleration of the head compared with the torso. If there is very little or no movement of the head relative to the torso, the neck will probably not be injured, even in severe crashes that result in a lot of vehicle damage. In fact, studies have reported that the incidence of whiplash is surprisingly low in high-energy crashes, as well as in crashes where occupants sustain multiple traumas.34,35
A recent systematic review9 that included studies pertaining to the determinants of developing WAD following MVC found several relevant factors, including the occupant’s seat position (i.e., being a front-seat versus a rear-seat passenger) and the direction of collision impact. The authors found preliminary evidence suggesting that head restraints and/or car seats that limit head extension have a preventive effect on reporting WAD, although this relationship was mainly evident in females. Being a younger person and/or female seemed to be associated with filing claims or seeking care for WAD, although the evidence was not consistent. Preliminary evidence presented in one of the studies included in the review reported that the elimination of insurance payments for pain and suffering were associated with a lower incidence of WAD injury claims.
Holm et al36 conducted a study in Sweden wherein questionnaires were mailed to 1,187 persons who reported WAD after an MVC and filed an insurance claim for injuries. The questionnaire asked about prior health, details about the collision, and symptoms after the collision. The following factors were reported to be significantly associated with severe initial neck pain intensity: low educational level (OR 2.8), being sole adult in the family (OR 1.6), prior neck pain (OR 2.9), prior headache (2.2), prior poor general health (OR 2.6), and exposure to rollover MVC (OR 1.9). The authors concluded that sociodemographic and economic status, pre-injury health status, and collision-related factors were associated with the degree of participants’ initial neck pain intensity.
Several factors, including job style, severity of injury, and social class, were also found to have an impact on the time taken off work following whiplash injuries.37 The data for this study were extracted from the files of 800 medicolegal cases in a private orthopedics practice. The mean time off work corresponded to the severity of injury: 10.6 days for minor injuries, 12.1 days for moderate, 13.8 days severe, and 24.9 days very severe. Heavy manual workers were off work an average of 20.5 days, 15.7 days light manual, 13.9 days for drivers, 9.2 days secretarial, and 12.8 days sedentary. Analysis for each social class showed that professionals were off 7.0 days, 16.1 days for skilled nonmanual workers, and 34.2 days skilled manual, and 11.5 days unskilled manual. Approximately 31% of the 800 cases took no time off work, about 52% returned to work after only 4 days off, and 90% returned to work after 30 days off. Only 4.9% were still off work after 12 weeks.
Sturzenegger et al interviewed a sample of 137 consecutive WAD patients soon after their MVC and found three attributes of crash mechanisms that were associated with more severe symptoms. These included occupant unpreparedness, a rear-end collision vector, and rotated or inclined head position at the time of impact.38
Some studies have suggested that the introduction of automobile safety belts has increased the incidence and/or severity of whiplash injuries,39–41 although others have not found an association.6,38 A recent best evidence synthesis did not find a relationship between the use or type of safety belt and the prognosis for recovery from WAD.14 A report from the early 1990s indicated that there was a progressive increase in the number of whiplash patients seen in emergency departments in the United Kingdom since 1982 when safety belt laws were established,42 but the authors suggested that the increase was not necessarily due to the introduction of seatbelts. Thus, the evidence seems to point away from safety belts having an impact on the incidence of whiplash injuries. Their use should always be encouraged, however, because they have been shown to significantly reduce the risk of serious injury and death in MVCs.43,44
Risk factors for developing chronic symptoms following whiplash include some of the same that increase the risk of being injured in the first place. A number of risk factors have been investigated that were initially thought to increase the chance of developing chronic WAD symptoms (e.g., advancing age, female gender, neck pain on palpation, pain or numbness radiating from the neck to the upper extremities, headache, pretraumatic neck pain, low educational level, and position of the occupant at impact). However, the findings of most studies pointing to a positive association are not robust, are sometimes conflicting, and many are of poor methodological quality.45 Ultimately, only a few risk factors have emerged as being significant. Table 9-1 provides a list of commonly reported risk factors for developing chronic whiplash.
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