Discussion
This prospective cohort study investigated associative factors for pain relief, better physical function, and improved working capacity after inpatient rehabilitation of WAD. The achieved multivariate linear models attained very good fit explaining variances between 53.3% and 72.1%. At baseline, health and quality of life of the subjects were significantly worse than expected compared to the general population. Patients reported small to moderate improvements at discharge from the 4 week inpatient pain program (ES of the three dependent variables 0.40–0.63), and 5 months later at home (ES 0.61–0.81).
Improvement was positively associated with a low baseline value for each of the three dimensions at both discharge and the 6 month follow-up (explained variances 11.4%-56.7%), meaning that persons having much pain and high disability reported higher pain relief and functional improvements at the follow-up than those with less pain and better function. The second most predictive independent variables were dimensions of affective health and pain coping. Relief of depression and low baseline depression were highly associated with improved physical function (especially at the 6 month follow-up: 20.5% explained variance) and lower with pain relief (at discharge: 3.9% explained variance). The CSQ single item, i.e. asking about the self-perceived ability to decrease pain, was an important predictor for pain relief (at discharge: 6.2 and 9.6% explained variance). Low baseline catastrophizing, and reduction of catastrophizing were associated with improvements in all three dependent variables. For improved function at discharge, reduction of catastrophizing was the most important predictor (explained variance 19.4%). These findings confirmed the main hypothesis of the study. Low baseline pain and relief of pain (up to 11.3% explained variance) were associated with improvement of function and vice versa (up to 14.8% explained variance).
This study design does not allow for causal conclusions. However, daily clinical experience suggests that pain relief, improved physical function, and working capacity are circularly associated to each other and, therefore, there may be a partial causal component since there is low to moderate evidence that specific stretching and strengthening exercise relieve chronic neck pain. Active physiotherapy, functional stability and mobility of the cervical spine may ameliorate pain in many patients but not in all. Our data suggest that patients suffering from severe pain and/or severe disability were more likely to improve and to profit from rehabilitation, because low baseline levels of pain and function were most associated with improvement in these dimensions. Physical therapy and psychotherapy were delivered during the stay and organized for continuation subsequent to discharge. Information about the disease and education on how to continue home exercises after discharge and how to transfer the newly acquired knowledge to daily life seemed to maintain improvements as was observed between discharge from rehabilitation and the 6 month follow-up at home.
During the inpatient program, patients were confronted with their lack of coping, and they learned to improve coping strategies. After the pain program, many participants were treated by psychiatrists for depression, anxiety, and coping/catastrophizing due to proven and postulated mechanisms. Psychological factors were found to have more relevance for recovery than collision severity with regard to prediction of duration and severity of WAD. Recovery and better health were associated with lower levels of pain catastrophizing, rumination, magnification, and helplessness three months or later after trauma. Catastrophizing was linked to heightened emotional distress and disability, as well as a more intense pain experience, and more pronounced displays of pain behavior. In the early stages of WAD, fear of movement was a predictor of next day pain and disability, which was significantly associated with chronic persistence of pain. The hypothesized vicious circle of more pain and less function may partly be broken by effectively modifying coping factors and depression. According to the fear and avoidance model, the results underpin the concept that pain relief can be enhanced by improvement of upper cervical function (operant behavioral therapy), and increased ability to decrease pain by learning coping strategies.
A strength of our study is its observational and "naturalistic" design without an artificial process of patient selection, allocation, or adaptation of the intervention. The study design aimed to be close to daily therapeutic and clinical reality. To measure clinical symptoms, we used standardized questionnaires which assess parameters in all dimensions of the WHO concept for International Classification of Function (ICF), like 1) Function (function/impairment of tissue or organ system), 2) Disability (functional impairment of the whole person) and 3) Health (participation/restriction in social interaction). The SF-36 measured in all three dimensions, HADS in dimension two, and the CSQ in dimensions two and three. In order to achieve a holistic bio-psycho-social approach we combined the questionnaires to cover all dimensions. The self-assessments used were validated and standardized especially for whiplash disorders. Attrition bias was assessed and the model for change of working capacity at the 6 month follow-up was corrected for attrition bias. All models fulfilled the rule that at least 10 observations (i.e. patients) per included covariate have to be available for the regression to avoid overestimated and underestimated variances.
Limitations of the study are the high drop-out rate after discharge leading to possible attrition bias. Furthermore, the design was non-randomized and not controlled, which is acceptable for an association study. Data modeling was based on linear regression on the assumption that characteristics are linearly linked. A complex polynomial approximation would be more accurate for calculation. However, in most polynomial models, the linear term is by far the most predictive term compared to quadratic and cubic terms. Linear regression makes results clinically easier to interpret. Although many findings of the study correspond to data reported in the existing literature, generalizability of the results should be limited to inpatient rehabilitation and the first phase thereafter.