Frequency of Whiplash Incidence
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 100,000 to 800 per 100,000 population per year. A Swedish study reported the annual incidence of whiplash in the local catchments area to be 4.2 per 1,000 inhabitants for grade 1 WAD and 3.2 per 1,000 grade 3. Holm et al 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.
The Insurance Institute for Highway Safety (IIHS) reported that approximately 2 million whiplash insurance claims are filed each yeah in the United States, resulting in more than $8.5 billion in insurance claims. 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.
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 no insured. Thus, the true number of cases per year in the United States us 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, 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 (ie, newly injured), but it also includes the total number of people who have it also includes the 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 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. 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, so its prevalence is undoubtedly very high. In fact, Freeman et al estimated, from a case-control study compromised 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.
This 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. 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.
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 the 45% of those with chronic neck pain considered its origin to have been a prior MVC. 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.
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. 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. Other studies, however, have found the rate of long-term WAD symptoms to be lower. 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.
Neck and should 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, paraesthesia, nausea, sleep disturbances, and ill health. 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. Furthermore, the symptoms were recurrent or persistent in almost 40% of the cases. Berglund et al concluded that whiplash injuries due to rear-impact MVCs have 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. Most subjects recovered within 3 months after the injury, and recovery rates leveled off after 3 months has 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.
Some of the Reported Risk Factors for Developing Chronic Whiplash
Category Risk Factor
Patient demographics Advancing age
Low educational level
Not being employed full-time
Presence of a compensation claim
Early intensive health care following injury
Expectation for recovery
Collision parameters Position of the occupants at impact
Being a passenger
Rear impact collision
Collision with a moving object
Head-on or perpendicular collision
Improper head restraint
Type of vehicle
Car equipped with a towbar
Previous history Headache
Widespread body pain
Presenting symptoms High neck pain intensity
High neck disability
Neck pain on palpation
Symptoms of radicular irritation
Emotional or psychological distress
Higher WAD grades
Reduced speed of information processing
In summary, not all victims of whiplash sustain injuries. “Whether or not a victim sustains an injury is a function of multiple factors: the magnitude of the impact, their posture at the time, their anatomy, and the material strength of the components of their cervical spine” (Bogduk and Yoganandan 2001).