Motor control patterns in whiplash and chronic neck pain
Altered motor control patterns in whiplash and chronic neck pain
From: BMC Musculoskelet Disord. 2008 Jun 20;9(1):90 [Epub ahead of print]
Whiplash associated disorders have been studied mainly in comparison to asymptomatic subjects and it remains controversial whether whiplash represent a diagnostic entity different from chronic neck pain. whiplash patients are separated from chronic neck pain merely on the history of trauma. The trauma designation is however not reflected in structural injuries of the cervical spine or the cerebrum. Consequently, somatosensory and motor deficits have gained research attention. There are consistent findings of hypersensitivity and widespread pain in whiplash compared to healthy subjects, and when comparing whiplash patients to chronic neck pain patients. While this points to centrally mediated somatosensory alterations in whiplash, it is not clear whether motor areas are also affected. If such changes exist, altered motor control as well as kinaesthetic change should be present in whiplash, which would also provide important information for clinicians.
There is conflicting evidence for presence of kinaesthetic change in whiplash patients, as measured by head repositioning or joint position error in comparison to healthy controls, and the reported repositioning errors are generally small. The smooth pursuit eye movement test, believed to reflect cervical afferent dysfunction, has also shown inconsistent results.
There is consistent documentation for reduced standing balance and increased sway in whiplash patients compared to healthy subjects. Small differences, and only for difficult balance tasks, have been found between whiplash and non-traumatic neck pain patients. Less attention has been given to local motor control in the neck. Proprioceptive information from the neck greatly influence head and trunk position sense and motor control and mechanisms controlling head motion may be different from those that control the trunk. whiplash patients present with reduced cervical range of motion (neck range of motion) relative to asymptomatic individuals, but they also show increased variability in neck range of motion. These tests may however be affected by motivational factors. For trajectory head movement patterns whiplash patients showed greater variation between days than asymptomatic controls. More jerky movement patterns were found in whiplash and chronic neck pain patients compared to healthy subjects, but there were no differences between the two pain groups. Although the latter two studies address relevant motor control issues, they were both small and neither could provide evidence for a difference between whiplash and chronic non-traumatic neck pain. In fact, of the above studies on postural control only two included non-traumatic chronic neck pain patients in addition to whiplash and healthy subjects. In other words, the other studies were not designed to reveal whether postural control deficits are related to the trauma or just long-standing pain. If whiplash patients are distinguished by altered motor control strategies, their movement patterns should be different from that seen in both nontraumatic neck pain patients and in healthy controls.
Pain has been shown to induce decreased variability in postural strategies in low back pain, and stiffening of spinal movements. Cervical movements in the associated planes relative to the primary movement plane have been named conjunct motion, and might reflect protective postural control strategies. Conjunct motion was explored in this study to get an impression of motor function or “freedom of movement” in the neck. Conjunct motion at the end of primary range was investigated in a previous study which found small differences between whiplash and healthy controls, but the study did not include non-traumatic neck pain patients. In addition, movement deviations would more likely be found during execution of the primary motion which from a motor control perspective is more complicated.
In order to explore motor function characteristics of whiplash patients in particular, control groups of non-traumatic neck patients and asymptomatic subjects were included in this study. The main purpose was to investigate motor control deficits in whiplash compared to the two control groups in relation to conjunct motion, joint position error, neck range of motion and neck range of motion variability.
The main finding in this study was that patients with chronic neck pain showed altered motor control in the cervical spine. We found no indications for differential effects between traumatic and non-traumatic chronic neck patients in terms of conjunct motion. No group differences were found in neck range of motion-variability or joint position error. Maximal cervical range of motion in the primary planes was however significantly reduced among the whiplash patients compared to both control groups.
Conjunct motion was included in this study to get an impression of motor function or motion stiffness during execution of primary plane movements, believed to reflect central nervous motor control strategies. Conjunct motion in the cervical spine has been studied in asymptomatic subjects, but only two studies have included whiplash and healthy subjects. Marginal reductions in conjunct motion in the whiplash group were observed in both studies. A limitation in the study by Dall’Alba et al was that conjunct motion was recorded only at end of primary motion. In our study, conjunct motion was recorded during primary movements and is to our knowledge the first to compare whiplash patients to both non-traumatic neck pain and healthy subjects. The study groups were separated on most conjunct motion measures in the unadjusted analyses. It is possible that conjunct motion mainly happens towards end of physiological range and is therefore reduced due to reduced primary neck range of motion. However, differences between the pain groups and the healthy controls remained after controlling for neck range of motion in the analysis. Neither could the differences between the pain groups and healthy subjects be explained by current neck pain intensity. This indicates that stiffer or more guarded movement patterns are a response to long-standing pain. This response was irrespective of a trauma history. For all groups the largest conjunct motion ranges were found during primary cervical rotation (horizontal plane) and, interestingly, stiffer movement patterns in the pain groups were also most evident during cervical rotation. This complies with a general understanding that cervical rotation demands more complex physiological coupled motions due to the anatomic structures, such as the orientation of cervical zygapophysial and uncovertebral joints. Changes in motor control due to chronic pain may thus be observable only in more complex movement patterns.
Whether reduced conjunct motion reflects altered motor control strategies needs further investigation. If confirmed, it remains to reveal if such changes are peripherally driven or merely confined to central processing mechanisms. Perceived position of the head is greatly influenced by proprioceptive signals from the neck. Hypothetically, cervical microtrauma involving mechanoreceptors may cause corrupted afferent information and sensorimotor mismatch. The head relocation test, equivalent to the joint position error test, was introduced to detect alterations in cervical proprioception. We thus expected that disturbances in proprioception would be reflected in the joint position error test, but we found no difference between the groups.
Consequently, proprioceptive dysfunction gives a poor explanation for the observed alterations in conjunct motion in the pain groups. Since current pain intensity could neither account for reduced conjunct motion, it may be argued that the observed changes in motor control reflect central nervous motor processing adaptations due to long-standing pain rather than disturbances in concurrent afferent inputs. Musculoskeletal pain has been reported to induce cortical reorganization in both somatosensory and motor areas Maladaptive cortical sensimotor integration with chronic neck pain has been suggested and neuromagnetic imaging has shown altered responses in the motor cortex due to chronic pain. Although local trauma to mechanoreceptors in the neck cannot be excluded, our findings favour centrally driven alterations in motor control due to long-standing pain.
Kinaesthetic sense deficits in patients with cervical pain have in previous studies been demonstrated as reduced accuracy, or increased cervical joint position error, when relocating the head to the initial position after a maximal or sub-maximal cervical movement. The evidence is inconclusive, however, as other studies have found only small or no differences in joint position error among neck pain patients or greater joint position error only among those with the highest levels of pain and disability. Various procedures have been used to record joint position error of the cervical spine and the variation in set-up may account for the conflicting findings. In our study, group differences in joint position error were very small and statistically non-significant.
As expected, and in agreement with other studies, primary neck range of motion differentiated between whiplash patients and healthy controls also in this study. The whiplash patients also had significantly less cervical neck range of motion compared to the chronic neck pain group, even after adjusting for current pain intensity, gender and age. Although neck range of motion differentiated between the two groups of neck pain patients, the diagnostic value of neck range of motion-testing can be questioned. Patients could intentionally simulate less motion for beneficial reasons in cases of unclarified insurance compensation. However, the subgroup of whiplash patients with ongoing compensation claims in this study performed similarly to the rest of the whiplash group, which leaves no suspicion of simulation of abnormal motion.
Compared to asymptomatic controls both whiplash and chronic neck pain patients showed reduced conjunct motion, particularly during primary cervical rotation. We found no indications for a difference between traumatic and non-traumatic neck pain patients. Reduced conjunct motion was not explained by variation in current neck pain or range of motion in the primary plane. Less flexible movement patterns in the two pain groups compared to the asymptomatic group may reflect motor control adaptations, possibly due to long lasting pain. joint position error and maximal neck range of motion-variability did not separate the study groups. The whiplash patients showed reduced maximal cervical neck range of motion compared to both control groups, and the chronic neck pain group had less neck range of motion compared to the asymptomatic controls. No significant differences in neck range of motion and pain intensity were found between whiplash patients with ongoing insurance compensation claims and the rest of the whiplash group.
