Background
Whiplash-associated disorder (WAD) is a significant health problem and it results in substantial socioeconomic costs throughout the industrialised world [
1]. The term whiplash is used to describe both the acceleration-deceleration mechanism of energy transfer to the neck that results from rear-end and side-impact motor vehicle collisions
and the variety of clinical manifestations it can cause [
2,
3]. Chronic symptoms after whiplash trauma such as neck pain, headache, and/or cognitive and emotional problems are well-known [
4].
The chronic course of WAD has implications for both the individual and society. Previous studies have shown that between 19% and 60% of patients still have complaints six months after a whiplash injury, and 13–50% of patients are still absent from work or are unable to perform their usual activities at this time [
5]. It is difficult to predict which patients will develop a chronic course, but several prognostic factors have been identified, such as high pain intensity initially [
5], passive style of coping, depressed mood, fear of neck movement [
6], and pain catastrophizing [
7]. Implications of chronicity include significant socioeconomic costs for society such as long-term sick leave, disability pensions, and claims for compensation [
8,
9]. The long-term consequences of WAD have only been studied to a minor degree in Denmark, but in a study from Aarhus 12% had not returned to normal daily activity or had only returned to modified job functions one year after a whiplash injury [
10]. The results of a Dutch study agreed with this finding, as 12.6% of participants had persistent work disability after one year [
11]. From experiences in our daily clinical work we hypothesised that the disability among sick-listed due to WAD was higher compared to sick-listed due to other kinds of musculoskeletal diseases. Therefore the aim of the current study was to characterise the return-to-work (RTW) process after a period of long-term sick leave. We compared the process of RTW in people who were sick-listed due to WAD to the process of RTW in those who were sick-listed because of other musculoskeletal disorders (MSDs). Since the termination of a sick leave period does not always denote the recovery of the person, we preferred to use RTW as outcome measure. To our knowledge, this is the first Danish study with more than one year of follow-up regarding the occupational status of individuals who have been sick-listed because of WAD.
Discussion
In this study, individuals who were long-term sick-listed because of whiplash-associated disorder were slower and less likely to return to work than people who were sick-listed due to other types of musculoskeletal disorders. In both groups, return-to-work stabilised after two years of follow-up.
After two years 56% of WAD patients and 43% of MSD patients had not returned to work and this is a higher proportion than observed in other studies. In 2001, Kasch et al. reported that 12% of subjects had not returned to normal daily activity or had returned only to modified job functions one year after a whiplash injury [
10]. One Dutch study had results in line with this finding, as 12.6% of individuals had persistent work disability after one year [
11]. The low level of RTW in our cohort is primarily explained by differences in the populations. Our cohort consisted of long-term sick-listed people, while Kasch et al. included people who had been in contact with the local emergency room within two days after the trauma. Some of these people may not have been sick-listed at any time. Likewise, the Dutch study group consisted of people who had initiated compensation claim procedures and the threshold for starting such procedures is apparently low in the Netherlands [
11]. The slower RTW-rate in the WAD group during the first year could be explained by the fact that the MSD group included less severe disorders such as fractures.
In a recent Danish study early classification of patients into risk strata based on biological and psychosocial functions predicted non-recovery and decreased work ability among patients exposed to whiplash [
17]. Among patients in the high-risk group only 32% had returned to work after one year. In our analysis 34% had returned to work after one year, which indicates that our cohort includes individuals similar to the high-risk categories described by Kasch et al. [
17]. Those still sick-listed after 8 weeks (and thereby included in our study) presumably would have had high risk scores initially. Unfortunately, we are not able to classify the cohort in the proposed risk strata to compare the results.
In Denmark, there is general access to transfer income including disability pension, sickness benefit, and unemployment benefit. A person can only receive one transfer payment at a time, and there are different time limits for most types of transfer payment. Thus, estimation of the duration of the sick leave while neglecting other types of transfer payment would underestimate the risk of an unfavourable vocational prognosis [
18]. Therefore RTW was chosen as outcome, though measures of sustained RTW would have been preferred but was not possible due to the frequent switching between different types of public transfer income in Denmark.
In Figure
1 a stagnation of RTW is seen through the follow-up period and fewer individuals are sick-listed after 3 years. In Denmark, one can receive sickness benefits for one year with the possibility of extension up to two years, if there is a wait for treatment. This is one of the possible explanations for the stabilisation seen in our results after two years of sick leave. There was a tendency to regroup into other types of public transfer payments, especially flex job and disability pension in the WAD group. To be granted a disability pension in Denmark work ability has to be permanently low, and after three years, 12% in the WAD group had received disability pension, compared to 5% in the MSD group.
It has previously been reported that a large proportion of patients with WAD had been granted disability pension [
9]. A study of accident victims with WAD assessed by the National Board of Industrial Injuries in Denmark found that 29% eventually received disability pension [
8]. In a study among members of a Danish WAD patient society, more than 40% had been approved for disability pension [
19], although the higher proportion was probably due to selection of the most disabled patients, those who would join a patient society. The evaluation of chronic low work ability is a long process, and more participants in the present study would probably have a disability pension if our cohort was followed up for more than two or three years.
The predominance of women in the WAD group was surprisingly high compared to the MSD group. In line with this, a German study found that although males were involved in a greater number of rear-impact collisions, females reported more neck distortion injuries, which indicates that females are more susceptible to whiplash injury [
20]. However, on this point, the literature is inconsistent: while two systematic reviews have found evidence of an association between female gender and poor recovery after whiplash injury [
21,
22], one systematic review has found strong evidence that female gender is not associated with a poorer prognosis [
5]. In the present study, we found no change in OR for RTW when adjusting for age and gender, but gender differences in psycho-social prognostic factors could influence the result in studies where such factors are not included in the analyses. In most studies low education is a negative prognostic factor, also in studies of WAD [
22]. However the opposite effect was observed in this study, since the OR decreased after adjustment for education and we have no apparent explanation to this fact. The development of WAD and disability after whiplash is a complicated interaction between a predisposing vulnerability before the accident and multi-factorial maintaining factors after the accident [
23]. In this study only the socioeconomic consequences of WAD have been studied without knowledge of vulnerability and coping factors.
The strengths of this study were the long follow-up and the possibility of following the sick-listed individuals on a weekly basis regarding RTW and other types of public transfer income through the DREAM register. However, there is a risk of assigning unemployed individuals living on their own financial resources to the group of working people. However, as discussed by Stoltenberg and Skov [
12], this is probably the case for only a few people in the cohort due to the Danish welfare system.
One limitation of this study was the validity of the diagnostic label of WAD. While for some people we had self-reported diagnoses, others were diagnosed by their general practitioner and we have no further information about medical or diagnostic procedures. It was not possible to compare diagnoses given by GPs versus self-reports, and the validity of the diagnosis was not evaluated in this study.
Furthermore, additional weeks of sick leave could be due to diseases other than the first diagnosis of WAD or MSD used in this study. Thus, our results should be interpreted as being from a study of the RTW process in people with WAD as the initial diagnosis in a follow-up study of attachment to the labour market. Since the results show stabilisation in RTW after two years, the lack of three years of follow-up in the entire cohort appears to have had minor consequences for the overall result of this study. It would have been interesting to include some of the prognostic factors for a chronic course of WAD in the multivariate analyses, but, unfortunately we did not have that kind of information in the cohort.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
PS and AM initiated this study and CS was responsible for all analyses of data. SB made the first draft of the manuscript and JWH, PS, AM and CS contributed to the interpretation of data and all authors were involved in the revision of the manuscript. All authors read and approved the revision and the final manuscript.