The effect of neck torsion on postural stability in subjects with persistent whiplash
From: Man Ther. 2011 Jan 19. [Epub ahead of print]
Loss of balance and sensations of dizziness and unsteadiness are symptoms commonly reported by patients with persistent neck pain. Research has shown that deficits in standing balance can be present in both traumatic and non-traumatic neck disorders, although these impairments seem to be more pronounced following a whiplash injury where such disturbances may be caused by damage to the neck receptors, vestibular receptors or the central nervous system.
Nevertheless, cervical afferent input is considered to be an important contributor to balance and previous studies have demonstrated an association between postural disturbances and deficits in neck proprioceptive acuity. Balance impairments in whiplash appear to be consistent with abnormal cervical afferent input at the neck joint and muscle receptors, although damage to the vestibular apparatus is possible. Even with recent evidence showing significant differences in balance responses between subjects with whiplash and those with vestibular pathology, current clinical measures used in the assessment of postural stability are not able to specifically differentiate between the two causes. Thus it would be useful to determine whether modifying existing tests of static balance might be helpful in determining the primary origin of sensorimotor impairments in whiplash associated disorders.
The Smooth Pursuit Neck Torsion test is a measure considered to be specific for differentiating between eye movement disturbances caused by altered cervical afferent input and abnormalities arising from the vestibular or the central nervous system. Difference in eye movement control with the head and trunk in a neutral forward facing position compared to when the trunk is rotated beneath the head. The latter neck torsion position acts to stimulate the cervical receptors but not the vestibular receptors. This technique has been shown to distinguish between subjects with vestibular pathology and those with neck pain in two previous studies where a torsion difference is only seen in those with neck pain and not in those with vestibular pathology. It would be an advantage to determine whether the use of the torsion position could be useful in the assessment of the causes of balance disturbances in those with neck pain.
The aim of this study was to determine whether the neck torsion positions change balance responses in the whiplash associated disorders population when compared to asymptomatic individuals. If altered sensory input from the neck receptors can be demonstrated to have significant effects on postural stability in those with neck disorders via neck torsion, a future study could then be undertaken using this technique to compare balance between vestibular and whiplash subjects. Such knowledge would not only enhance understanding about the origins of sensorimotor disturbances but may allow clinicians to establish whether poor postural control in individuals with whiplash disorders stem from altered sensory input at the neck region or the inner ear structures. This information is clearly important in guiding the assessment and management of balance disturbances in patients with persistent whiplash.
The results of this study suggest that neck torsion positions significantly increase balance deficits in standing with the eyes closed on a firm surface in some subjects with persistent whiplash associated disorders compared to a control group. Significant differences in balance responses between the subject groups were seen in tests of comfortable stance eyes closed in the left and right neck torsion positions with whiplash subjects having greater postural sway than the healthy controls. Further, this effect was seen in the anterior–posterior direction only. The difference in balance response between the neutral position and neck rotation and torsion position (rotation and torsion difference) were also significantly different between the groups although only neck torsion demonstrated an adverse affect on balance and this only occurred in the whiplash group. Conversely, simply rotating the neck (rotation difference) tended to demonstrate on average an improvement in sway from the neutral position in both groups.
The results of this study are consistent with the findings of previous research suggesting somatosensory impairment as a likely cause of balance disturbances in the persistent whiplash associated disorders population. In tests of comfortable stance, differences in sway amplitude determined for the AP and not ML direction between the asymptomatic control and whiplash groups reflected those of a previous study comparing these populations. Increases in AP sway in those with musculoskeletal disorders are thought to be indicative of somatosensory rather than vestibular impairment.
This is further reinforced in this study where altering afferent input from the neck via the torsion position appeared to adversely affect postural stability in the subjects with whiplash but not in the asymptomatic subjects. The torsion position does not alter afferent input from the vestibular apparatus as the head is head still throughout. Further the differences in postural sway observed in this study were not caused by the head turned position as a direct comparison between the two groups revealed similar balance responses on average when the head was simply rotated, whereas the whiplash group had significantly greater sway following neck torsion. Additionally, the facts that balance deficits increased compared to a neutral head position only in whiplash subjects following neck torsion and that neck rotation tended to improve balance, further support the notion that cervical afferent disturbance is the cause of the postural control disturbances in this population. Neck torsion positions would have only stimulated cervical afferents and not allowed any vestibular contribution, whereas the neck rotation position may have also stimulated and perhaps allowed a positive contribution from the vestibular afferents.
In order to establish the overall effect of neck torsion on balance, the difference between the neutral starting position and the average of the left and right neck torsion positions was used. This is similar to the method used in the assessment of eye movement and neck torsion. The results revealed subjects with whiplash had significantly greater sway differences between the neck torsion and neck neutral positions (torsion difference) when compared to asymptomatic controls. However, this effect was similar to but smaller than that observed when torsion is used to evaluate eye movement changes which has been shown to be highly sensitive and specific in those with whiplash associated disorders. The study findings suggest that the neck torsion manoeuvre may have high specificity but only moderate sensitivity for determining abnormalities in standing balance in the persistent whiplash associated disorders population as only 50% of subjects with whiplash associated disorders compared to 10% in the control group had balance responses that were adversely influenced by the neck torsion manoeuvre. Although, a previous study found that the percentage of subjects with whiplash with balance disturbances (35%) was not as prevalent as those with eye movement disturbances (84%) which could explain why only 50% of the whiplash group demonstrated adverse effects to postural stability with neck torsion. In fact this increased percentage from 35 to 50% may indicate an important contribution of neck torsion in assessment of postural stability in whiplash associated disorders.
There also appeared to be significantly greater difference between groups in left neck torsion difference as well as rotation right difference which effectively is the same neck position. It is probable that this could be related to the predominant side of neck pain. In this study, 75% of the subjects reported the left side of the neck to be more painful than the right side. The authors contention is that the most likely cause of balance disturbances following a whiplash injury is abnormal cervical afferent input from damaged neck joint or muscle receptors. However, it remains uncertain as to whether the cause of abnormal afferent input is proprioceptive or nociceptive in nature. A recent study demonstrated that experimental neck muscle pain had significant effects on postural control during quiet standing although pain levels needed to be particularly high (VAS 7). In this study no correlations between neck disability index and balance responses could be identified although there were some significant mild to moderate correlations between pain levels which on average were about (VAS 4) at the time of testing and some of the balance measures especially in the neck torsion positions. Similar findings have been identified in previous studies. It is a likely a combination of proprioceptive and nocioceptive elements co-exist.
This study also determined no correlation between DHIsf scores and any of the balance scores. This is interesting as previous studies have demonstrated greater balance deficits in those with dizziness. Similarly those with dizziness had greater differences with neck torsion and eye movement. A general lack of difference between the subject groups for sway amplitude in neutral standing was also unexpected and against general trends from earlier studies. This study findings may be due to lack of power as only twenty subjects were tested and not all subjects reported symptoms of dizziness. While the results of this study are promising, larger subject numbers would be required to draw firm conclusions about the effect of neck torsion on standing balance especially in those complaining of dizziness and unsteadiness.
The results as a whole indicate that deficits in balance are significantly greater in subjects with neck pain compared to healthy controls. While it could be argued that an undiagnosed vestibular component may contribute to the deficits in the whiplash subjects, this seems unlikely. Overall, since the neck torsion manoeuvre acts to stimulate the cervical receptors but not the vestibular receptors, a future study using this technique will be important to compare balance between vestibular and whiplash subjects. Such knowledge would not only enhance understanding about the origins of sensorimotor disturbances but allow clinicians to establish whether poor postural control in individuals with whiplash disorders stem from altered sensory input at the neck region or the inner ear structures. This information is clearly important in guiding the assessment and management of balance disturbances in patients with persistent whiplash and differentiating these individuals from vestibular clients.
The results of this study suggest that the neck torsion manoeuvre may adversely change balance responses in some subjects with persistent whiplash associated disorders and that deficits in standing balance exist particularly in the anterior–posterior direction when compared to asymptomatic control subjects. The results also indicate that somatosensory impairment is the most likely cause of the balance disturbances following whiplash. The disturbances are probably related to a combination of proprioceptive and nociceptive factors. Overall, these findings have implications for future studies and for the clinical assessment and management of balance disturbances in persistent whiplash-associated disorders, although, further research with greater subject numbers is required.