This meta-review, summarising the results of twelve systematic reviews, indicates that the outcome of patients with acute whiplash injury is associated with post-injury symptoms and some psychosocial factors, and not injury-related physical or mechanical factors. These findings are consistent with a previous meta-review that explored prognostic factors of neck pain in general . To summarise and simplify the result of this meta-review, a ‘typical’ whiplash patient with a poor outcome (that is, prolonged pain and disability) can be depicted as having severe pain and anxiety, and is seeking or has sought legal advice and early healthcare use. The type of accident, findings on physical examination, or radiological investigations will not affect the prognosis. Thus, a patient suffering chronic pain and disability post-whiplash can potentially be involved in a minor car accident with no motor dysfunction or radiological abnormality. The association of some psychosocial factors with the chronicity of whiplash injury is in accordance with previous studies involving chronic pain patients, which indicate a similar association between psychosocial factors and the course of chronic pain in general , and other forms of chronic pain such as non-specific low back pain [25, 26].
It is also notable that current evidence is conflicting or lacking on factors such as demographic factors (age, gender and education), three psychological factors and pain prior to accident. It is notable that Walton et al. concluded in their meta-review, with moderate confidence, that age has no effect on the outcome of whiplash . This contrasts with our analysis, which concluded controversial evidence based on an association reported by Cote et al. . This lack of conclusiveness might be explained by differences in the methodologies of various studies, such as different sample frames (normal population, insurance population or hospital emergency departments) [13, 16, 23]. In addition, the effect of demographic factors is not usually direct, but is mediated by other factors ; therefore, future studies should consider the role of confounding factors, such as comorbid mental health problems, while exploring the association of demographic factors with the prognosis of whiplash injury.
All twelve papers included in this review emphasised the need for more rigorous evidence, and made suggestions for future work in this field. These included the need for further studies on some of the prognostic factors, the need to explore the causal effect of other factors, and studies assessing the possibility of using prognostic factors in the prevention or treatment of whiplash whenever possible, as discussed below.
Carroll et al. reported a lack of high-quality studies on the association of the following items with the prognosis of whiplash: occupation type, disc degeneration, cultural factors, pre-injury fitness or exercise, and pre-existing or new incidence of widespread body pain or fibromyalgia . Cote et al. emphasised that, based on current evidence, it is not clear whether the course of whiplash differs in patients recruited from the general population compared to those recruited from emergency departments or primary care practice . Spearing et al. could not find any studies that directly explored the role of receiving compensation payment on the prognosis of whiplash patients . Finally, Williamson et al. reported a lack of high-quality evidence on the association of psychological factors and chronicity of acute whiplash injury . These areas should be investigated in any future studies.
The association of a factor with the prognosis of whiplash does not necessarily reflect a causal relationship; such associated factors cannot therefore be necessarily used as a basis for the treatment or prevention of whiplash. More studies are necessary to investigate the potential role of prognostic factors on aetiology, prevention and treatment of whiplash. For example, although cold hyperalgesia is associated with pain and disability in whiplash patients, more studies are needed to investigate whether cold hyperalgesia can be considered as a cause of pain, or if there are other confounding factors . Another example is related to the role of compensation, which is associated with poor health outcome [29, 30]; however, studies have yet to explore reverse causality, that is, the poor outcome being the cause of compensation-seeking [16, 31].
In addition, future studies should explore whether a patient’s outcome can be improved by removing a prognostic factor. For example, while whiplash patients who report back pain following a car accident are more likely to have a poor outcome, more studies are needed to determine if treating the back pain can improve the outcome of whiplash .
Considering the complexities that exist around the association of factors with outcome of a health condition such as acute whiplash injury, complete elaboration of such associations would be beyond the scope of a single study, and different phases of research might be needed to identify, confirm and understand prognostic associations . It is also necessary that future studies employ rigorous methodology (such as using validated and objective measures) and reporting standards (including the use of magnitude of associations) [8, 15, 19, 33].
We did not identify any recent systematic reviews (within the past 5 years) that examined psychological factors, early healthcare use and motor dysfunctions. It would be helpful to undertake updated systematic reviews to explore the association of these factors with the prognosis of whiplash.
Our methodology had the benefit of relying on the best available evidence provided by the systematic reviews included, but this has limitations. More recent studies would not have been captured by the included reviews. In addition, by including all the prognostic factors explored by the systematic reviews, this meta-review maps the field and provides an overall picture, but in doing so, it necessarily reduces the depth of analysis for each individual factor.
In conclusion, this meta-review provides a comprehensive overview of the state of the high-level evidence available concerning the factors associated with the outcome of patients with whiplash injuries. The predictors of poor outcome after acute whiplash injury are early pain and some psychosocial factors, whereas physical factors are not associated with the outcome of acute whiplash.