Abstract
Cervical pain is a prominent symptom in both acute whiplash injury and late whiplash syndrome. However, no systematic analysis of post-traumatic pain development covering several weeks has yet been performed in whiplash patients. It was the aim of the present study to analyse the duration and course of post-traumatic muscle pain due to whiplash injury in a prospective follow-up examination with short investigation intervals. A recovery of initially increased muscle pain after whiplash injury within 1 month was hypothesized. Pressure pain of the splenius and trapezius muscles was recorded using PC-interactive pressure algesimetry. Whiplash patients were investigated during the acute injury stage and after 3, 4, and 6 weeks and compared with matched controls. We found significantly increased pressure pain of the splenius and trapezius muscles in the acute stage of whiplash injury. After 4 weeks patients' scores of pain parameters were comparable to those of healthy control subjects. Within the patient group the first changes of pressure pain were observed within 3 (splenius) and 4 weeks (trapezius). For most patients the recovery dynamics lasted 4-6 weeks. A minority of patients did not show any improvement after 6 weeks. The present study shows that the dynamics of pressure pain due to whiplash injury can be quantified by means of PC-interactive pressure algesimetry. Our results confirm the clinical experience that the acute post-traumatic cervical syndrome normally subsides within weeks.
Keywords
Introduction
The acute cervical syndrome following whiplash injury consists of neck pain resulting from painful neck and shoulder muscle sprain due to a motor vehicle collision, often accompanied by post-traumatic tension-type-like headache (1). Clinical experience shows a fast remission of the post-traumatic cervical and cephalic pain within days or weeks. Chronic post-traumatic syndromes with sustained neck pain and headache are uncommon and develop in only 10–20% of whiplash patients (2). Our current knowledge about the time course of symptoms due to acceleration trauma is mainly based on retrospective studies performed up to 17 years after the collision, with unselected patients suffering from chronic symptoms and often involved in litigation. Most of these studies did not compare more than two examinations (e.g. 3–7), over time. Prospective studies on the temporal recovery of post-traumatic neck pain are rare and mostly based on subjective statements of the patients in terms of pain questionnaire entries (e.g. 8–12). There is a lack of quantitative data and of data acquired closer in time on recovery after whiplash injury. This is of special importance considering the background of models about pain chronification assuming a reciprocal nature of pain and psychological variables. Whereas the contribution of psychological factors has been discussed extensively (e.g. 13, 14), little attention has been paid to the precise development of post-traumatic muscle pain. PC-interactive pressure algesimetry is a valid and reliable diagnostic tool to quantify muscle pressure painfulness after whiplash injury (15, 16). Standardized temporal analysis of recovery dynamics of the strained neck and shoulder muscles with computer-aided pressure algesimetric quantification methods has to our knowledge not yet been performed. Thus, it was the aim of the present study to determine the duration and time course of recovery from post-traumatic muscle pain after whiplash injury in a prospective follow-up examination with short investigation intervals, using also a control group.
Methods
Subjects
A total of 20 whiplash patients of grade 1 and 2 according to the Quebec Task Force (17) took part on this prospective investigation of post-traumatic pain [mean age (± SD) 28.75 ± 12.08 years, range 18–53 years; 11 women, nine men]. Initially 23 patients were examined, but three failed to take part in all examinations and were therefore excluded. They were recruited from emergency rooms in the area. All patients were involved in motor vehicle collisions (rear-end collisions) as drivers with attached seat-belts. Patients drove passenger cars with a self-reported speeds < 30 km/h, leaving the passenger cab undamaged. In 35% of cases the motor vehicles were stationary before the collision. Table 1 provides further details on whiplash patients. After the collision patients suffered from a cervicocephalic pain syndrome including both painful tension in the cervical muscles and a dull and bilateral occipital pain. Whiplash patients received non-specific physical therapy (ice, moist, heat, massages, and exercise). Pharmacological intervention included the application of pain relievers, anti-inflammatories, or muscle relaxants. Patients were off medication 24 h before the examination. Exclusion criteria were a direct head impact or direct neck trauma, painful accompanying pain syndromes such as back pain or pain resulting from other collision-related injuries, and a previous neurological or psychiatric history or chronic and episodic pain syndromes, including different kinds of headache (i.e. migraine, chronic tension-type headache and occurrence of tension-type headache or cervical and neck pain for more than 1 day per month). Previous medical history, actual symptoms and complaints, and collision history were taken by means of standardized interviews and history questionnaires. According to the Quebec Task Force grade 1 and 2, no patient in the clinical investigation had focal neurological symptoms or deficits.
Socio-demographic and collision-related background data of whiplash patients
Of the total of 24 healthy age- and sex-matched volunteers, one dropped out. Eleven female and twelve male control persons aged on average 28.83 ± 10.44 years, range 19–53 years, were examined. Exclusion criteria were a prior whiplash injury, acute neck pain or headache, and otherwise the same as for the whiplash group. Except for oral contraceptives, none of the controls took medication (especially central acting drugs or analgesics).
Comparability of age and sex distribution of the two compared groups (independent t-test, P = 0.98; χ2 (1); P = 0.76) was given. All subjects were instructed about the purpose of the examination and gave their informed consent.
Data acquisition
The first examination (T1) in patients took place during the early post-traumatic phase, on average 7 days (± 2.96 SD, range 4-maximally 14 days) after the collision. The follow-up investigations were carried out after 3, 4, and finally 6 weeks (T2–T4). Controls were assessed at the same time intervals.
The pressure algesimetry further developed by our group has been described in a previous publication (15). Subjects had to tolerate a constant pressure of 1310 kPa (400 g), induced in the left and right splenius and trapezius muscles for 180 s. The position of the algometer stylus was determined in a standardized manner to avoid pressure application to tender points. For the trapezius muscle the contact point was half the distance between the processus spinosus of C7 and the acromion, for the splenius muscle the contact point was two-thirds of the distance between the processus spinosus of C7 and the mastoid. Pressure pain induction was terminated if the maximally tolerable pain or the time limit was reached. Splenius and trapezius muscles were investigated separately while patients were seated comfortably in a chair, the lower arms supported by arm rests. For stimulation of the splenius muscles the head was supported by an anatomically shaped polystyrene block in a slight inclination position. Participants could familiarize themselves with the pressure algometer before the recordings started.
During pressure application subjects rated the experienced pain intensity continuously on a visual analogue scale (VAS) on a PC display. Pain could be evaluated from 0 (‘no pain at all’) to 10 (‘unbearable pain’). Subjects were instructed to press the cursor pointing upward (downward) whenever they felt that the pain has reached an increased (reduced) level on the VAS. The time course of the rated pressure pain intensity was recorded and calculated offline. The following parameters of the acquired graph served for quantification of pressure painfulness: (i) time until pain perception (pain threshold), (ii) pain maximum in VAS units, (iii) time until reaching the maximally tolerable pain intensity (tolerance threshold), (iv) slope of the pain intensity–time function, and (v) integral of the pain intensity–time function.
Mean values of these pain-related parameters were computed for each muscle and for each investigation. The influence of group membership on the dependent variables was tested for significance (multivariate analysis of variance,
A detailed description of all results is beyond the scope of this paper. We restricted the presentation of results to the maximum and the integral of the acquired pain intensity–time function. Further information will be provided by the authors on request. The maximum represents the absolute pain intensity, the integral summarizes the pain intensity graph.
Results
An overview is given in Fig. 1, illustrating the time-dependent averaged pain maximum and integral for whiplash patients as well as for control subjects.
Time course of pressure painfulness of the left and right splenius and trapezius muscle for whiplash patients (WP) in comparison with control subjects (CG). The figure illustrates the averaged integral and maximum of the acquired pain intensity–time function in the acute stage (T1) and after 3 (T2), 4 (T3), and 6 weeks (T4). A decrease of initially highly increased pressure painfulness could be observed for whiplash patients. In contrast, control persons showed similar values at each examination.
Acute muscle pain
A detailed presentation of pain characteristics of whiplash patients in the acute stage in comparison with a sample of healthy control subjects has been given in previously presented data (15). In short, analysis of acute pressure pain perception revealed an increased pressure painfulness of the left and right splenius and trapezius muscles for nearly all pain-related parameters. The shoulder muscles (trapezius) of whiplash subjects were more painful left than right, which can be ascribed to the seat-belt position. In contrast, the paravertebral muscles (splenius) showed equally increased muscle painfulness on both sides. In healthy participants the side of pressure pain induction had no influence on the results.
Follow-up measurements
Intergroup comparison
The multivariate comparison at each investigation time revealed significant differences between patient and control group for the first (Λ= 0.22, P < 0.001) and second (Λ= 0.3, P = 0.001) investigation. After 4 (Λ= 0.51, P = 0.158) and after 6 weeks (Λ= 0.46, P = 0.067) no statistically relevant differences could be seen.
Regarding the dependent variables separately, significantly increased pain-quantifying parameters in whiplash patients were found in both splenii and left trapezius muscle at T1 and T2. For the right trapezius muscle there was a tendency towards a significant group difference in the acute stage (integral). No significant deviations of patients’ mean values from those of healthy controls could be observed after the third examination (see Table 2).
Intergroup comparison of pain maximum and integral in the acute stage (T1), after 3 (T2), 4 (T3), and finally after 6 (T4) weeks
Post hoc t-tests:
P < 0.05, corrected for multiple comparisons. WP, whiplash patients; CG, control group; NS, non significant.
Intragroup comparison
To analyse the influence of elapsed time since the motor vehicle collision, we computed, separately for each group, repeated measures
Repeated measures
F-values with degrees of freedom (df, in parentheses). Significance of F-values was assessed using Greenhouse–Geisser correction.
P < 0.05. WP, whiplash patients; CG, control group; NS, non significant.
Subsequent paired comparisons (t-tests) were performed to investigate further the significant time effect in whiplash patients. Values of the pain parameters at different investigation times were compared. This analysis revealed that whiplash patients showed the first significant decreases in pressure painfulness within 4 weeks (T2 vs. T3) in the left and right splenius and trapezius muscles (Table 4). However, visual inspection of the pain intensity–time functions (see Fig. 1) suggests that first declines of increased post-traumatic pressure painfulness in the splenius muscles already occurred within 3 weeks, even if the paired comparison (t-test) of the first with the second investigation did not reach statistical significance. Results for the comparison of the third with the last examination were mixed. Recovery dynamics of statistical relevance could be observed for the integral of the left splenius muscle. Maximum of the left as well as integral and maximum of the right splenius muscle narrowly missed the statistical threshold. The trapezius muscle did not show significant improvements when comparing pain parameters of the third with the fourth examination.
Paired comparisons (post hoc t-tests) within the patient group between different examinations (T1–T4)
P < 0.05, corrected for multiple comparisons. WP, whiplash patients; CG, control group; NS, non significant.
Deviant recovery after whiplash injury
Four patients (20%) perceived applied pressure pain as unbearable 6 weeks after the whiplash injury. This extreme pressure painfulness affected the four examined muscles in detail as follows. Maximal pain was experienced by one patient (5%) in the left and by two patients (10%) in the right splenius muscle. Four patients (20%) rated pressure pain in the left and three (15%) in the right trapezius muscle with a VAS score of 10.
Discussion
This prospectively designed longitudinal study analyses spontaneous recovery from the post-traumatic cervical syndrome and shows that pressure painfulness of the strained shoulder and neck muscles following whiplash injury can be quantified during follow-up examinations by means of PC-interactive pressure algesimetry. Using the established pressure algesimetry we have been able to quantify and standardize the painfulness of the neck and shoulder muscles following whiplash injury, a proceeding which can be biased by varying examination techniques and assessment qualities of different investigators in the standard clinical examination. As expected, we could demonstrate an initially increased pressure painfulness of the left and right splenius and trapezius muscles after whiplash injury. Pain parameters were significantly higher in whiplash patients in the acute stage compared with control subjects. After 3 weeks these differences were still present. The only exception was the right trapezius muscle reaching normal values as early as T2. This might be due to the fact that the right trapezius muscle is less exposed to sprain in left-seated motor vehicle drivers. After 4 weeks the initially increased pressure painfulness of neck and shoulder muscles had receded to such an extent that both groups showed comparable values in all pain parameters. To conclude, there was, as hypothesized, a complete remission of the initially increased post-traumatic pressure pain of the neck and shoulder muscles after 4 weeks. Our data confirm the clinical experience that, in general, the acute post-traumatic cervical syndrome subsides within weeks. Pearce (19) reports on 62% of patients that were free of pain 1 month after whiplash injury. After 6 months the percentage of patients without injury-related symptoms was 82%. The present results are also in accordance with animal models of soft-tissue healing, that claim inflammatory reactions within the first 72 h, repair and regeneration from 72 h up to 6 weeks, and processes of remodelling and maturation which may last up to 1 year (17).
In patients with cervical pain following whiplash injury there was a continuous improvement of the initially significantly increased pressure painfulness. Repeated measures
For the trapezius muscle a significant decline of pressure painfulness could be shown only for the comparison of the second with the third investigation. There were no further improvements towards T4. For the trapezius muscle the main decrease of pressure painfulness occurred between the second and the third investigation visit, and obviously its recovery dynamics ceased 4 weeks post trauma.
Four patients (20%) were still highly pressure pain sensitive 6 weeks after the whiplash event. These patients reached the upper limit of 10 on the VAS during pain application, indicating that they experienced intolerable pressure pain. Regarding the examined muscles separately, the percentage of unimproved, maximal pressure painfulness ranged between 5 and 20. This proportion is in accordance with the literature, reporting 10–20% of whiplash patients developing chronic symptoms (2, 4, 19). According to the consensus findings of the Quebec Task Force (17), symptoms after 6 months are designated as chronic and continued symptoms after 45 days are considered as important predictors of chronification. Scholten-Peeters et al. (20) stress the role of high initial pain intensity as an important predictor of delayed recovery after whiplash injury. Accordingly, those patients, who still suffer from unchanged, highly increased pressure painfulness at the fourth examination visit in our study, might be prone to develop chronic symptoms. A long-term follow-up was necessary to clarify whether patients at risk of developing chronic symptoms can be identified already after about 6 weeks with the help of computer-aided pressure algesimetry. If this was possible, at-risk patients could take part in multimodal and multiprofessional therapeutic interventions long before symptoms become by definition (>6 months) chronic.
Several authors discuss a biopsychosocial model to explain persistent symptoms following whiplash injury (13, 21, 22). Within this model chronic symptoms are explained as a product of real pain and various psychosocial factors. In this context, ‘pain’ refers to pain due to the traumatically caused muscle strain as well as precollision symptoms and life's daily aches. Depending on cultural knowledge, focusing on symptoms, amplification and misattribution of symptoms, fear, distress, as well as life/job satisfaction are discussed (among others) as psychological components. In the scope of the present study we tried to elucidate the development of one possible contributing factor, namely muscle pain, resulting from traumatic muscle strain due to a motor vehicle collision in a time frame close to the injury. To our knowledge this has not been done before in terms of quantitative data. Our data provide empirical evidence for a remission of whiplash injury-related muscle pain within approximately 1 month. Thus, other factors may contribute to the long-lasting symptoms seen in some patients. Processes of central sensitization, finally manifested by structural changes (23, 24), as well as psychosocial factors, both influencing the response to painful stimuli (21, 25), provide the basis for the transition from acute to chronic muscle pain. Increased excitability of central neurons due to sensitization may in turn influence the regulation of peripheral processes, as shown by Ashina et al. (26).
Whiplash patients of the present study had to meet strict inclusion criteria to exclude comorbidity and neurological signs. This was essential to prevent uncontrolled variables from confounding the dependent variable. Therefore, we investigated only whiplash patients of grades 1 and 2 with general, non-specific symptoms about the neck, or neck symptoms plus signs limited to musculoskeletal structures. Beyond that, this patient group is of special interest since its symptoms can not be explained purely biologically, and thus the problem of chronification of symptoms is most frequently discussed regarding these patients. At the same time, our inclusion criteria led to patient selection bias. As a result, the present study can not provide evidence for the impact of various concomitant signs on pressure pain after whiplash injury. Patients with more and severe symptoms are likely to bear a divergent risk of developing longer lasting complaints. Further studies are necessary to clarify this question.
The results of the present study can finally be discussed in light of the prevalence of chronic whiplash syndromes in different populations. Estimates of the number of patients developing chronic symptoms vary for patient samples in different parts of the world. For example, it has been claimed that the chronic whiplash syndrome is uncommon in Lithuania, Greece, and Germany. Epidemiological evidence suggests that chronic whiplash prevalence in Lithuania, Greece, and Germany is 10% or less at 6–12 months (6, 11, 27–29). In contrast, this figure approaches 50% in Canada for the same time period (30, 31). In the present study we investigated a patient sample from Germany, and even though the examined population is selected and small, our results seem to affirm that the recovery rate is very rapid in Germany. Future studies with larger populations, less selected cohorts, and additional outcome measures will be necessary to verify this finding.
Footnotes
Acknowledgements
We would like to thank two anonymous reviewers for their helpful comments. Parts of this work were granted by German Federal Ministry of Education and Research (P.S.) and by the S.M. Freiburg-Fund (K.N.).