Abstract
Background Manual therapy of the cervical spine has occasionally been associated with serious adverse events involving compromise of the craniocervical arteries. Ultrasound studies have shown certain neck positions can alter craniocervical arterial blood flow velocities; however, findings are conflicting. Knowledge about the effects of neck position on blood flow may assist clinicians in avoiding potentially hazardous practices.
Objective The purpose of this study was to examine the effects of selected manual therapeutic interventions on blood flow in the craniocervical arteries and blood supply to the brain using magnetic resonance angiography (MRA).
Design This was an experimental, observational magnetic resonance imaging study.
Method Twenty adult participants who were healthy and had a mean age of 33 years were imaged using MRA in the following neck positions: neutral, rotation, rotation/distraction (similar to a Cyriax manipulation), C1–C2 rotation (similar to a Maitland or osteopathic manipulation), and distraction.
Results The participants were imaged using 3T MRA. All participants had normal vascular anatomy. Average inflow to the brain in neutral was 6.98 mL/s and was not significantly changed by any of the test positions. There was no significant difference in flow in any of the 4 arteries in any position from neutral, despite large individual variations.
Limitations Only individuals who were asymptomatic were investigated, and a short section of the arteries only were imaged.
Conclusions Blood flow to the brain does not appear to be compromised by positions commonly used in manual therapy. Positions using end-range neck rotation and distraction do not appear to be more hazardous to cerebral circulation than more segmentally localized techniques.
Manual
therapy, including high-velocity thrust (HVT) manipulation, is commonly used for the management of neck pain and associated headache but has in rare cases been associated with serious adverse neurovascular events.
1–3 These adverse events most commonly involve dissection of the vertebral or internal carotid arteries (ie, the craniocervical arteries), which supply blood to the brain.
2,4–8 Dissection of these arteries may directly alter blood flow to the brain or trigger thrombus formation, potentially causing a stroke.
9,10 Although concerns often have been raised as to the safety of HVT manipulation, other manual techniques using sustained end-range positions of the neck also are potentially implicated in causing changes in blood flow in the craniocervical arteries.
11,12Where it has been possible to identify particular manipulative procedures associated with cerebrovascular complications such as dissection, rotational techniques have been most frequently described.
2,8 Dissections most commonly occur in the atlanto-axial portion of the vertebral artery and upper cervical portion of the internal carotid artery.
2,4,7 Most neurovascular injuries have been reported in individuals between 30 and 40 years of age who were healthy and had no other cardiovascular risk factors.
6,13–15
Common cervical spine manual therapeutic procedures involve moving the head and neck into various positions. It has been suggested that one factor that may contribute to adverse neurovascular events following manual treatment of the neck may be the positions of neck rotation close to the end of the physiological range, which could temporarily compromise blood flow.
2 These flow changes may be an indication of increased biomechanical stress of the arterial wall.
16 It is considered that the additional effect of an HVT on a prestressed arterial wall may cause damage such as a dissection to occur.
It has been shown using ultrasound imaging that certain neck movements, in particular cervical rotation, can alter blood flow velocities in the craniocervical arteries in some individuals.
17,18 Numerous ultrasound studies over the last 2 decades have examined blood flow in different positions of the neck, but the results have been conflicting,
17–22 with some authors reporting that blood flow was changed in contralateral rotation and others that it was unchanged. Disagreement among studies may relate to a number of methodological considerations. Most notably, ultrasound is known to be highly operator dependent, particularly when sampling blood flow parameters in small-diameter tortuous vessels such as the vertebral artery.
23,24 Studies also have commonly measured parameters involving flow velocity, which is subject to wide fluctuation, particularly if sampled close to the vessel wall.
23,24 Moreover, most studies typically examined blood flow in a single vessel, usually the vertebral artery. However, examination of blood flow in one vessel cannot provide a complete picture of blood flow to the brain.
Angiography is considered to be the reference standard for imaging the craniocervical arterial system
25 and currently is most commonly performed using noninvasive methods such as magnetic resonance angiography (MRA) or computed tomographic angiography (CTA). Magnetic resonance angiography can generate a clear image of the vessels using “time of flight” MRA. Blood flow quantification using phase contrast magnetic resonance imaging (MRI) is a robust technique, with measured errors of ≤5%.
26–29 Unlike ultrasound, MRA does not require the operator to track the vessel of interest and thus avoids many of the problems of measurement error inherent in ultrasound. Magnetic resonance angiography also provides more detailed imaging and evaluation of blood flow characteristics and cerebral perfusion than can be achieved using ultrasound.
It is important for manual
therapy practitioners to be aware of the effects their treatments may have on the craniocervical blood vessels and brain perfusion in order to avoid or minimize the use of any techniques that may have a greater effect on blood supply to the brain. It is possible that rotation combined with longitudinal distraction, a common component of manipulation techniques advocated by Cyriax and Cyriax,
30 may further increase the mechanical stress on the arteries. In contrast, it may be considered that manual techniques performed closer to the overall neutral position of the neck, perhaps involving more localized segmental rotation, such as those described by Maitland
31 and Hing et al,
32 will have less effect on the arteries.
There have been few studies evaluating the effect on blood flow in the craniocervical vessels during specific manual
therapy treatments, and all of those studies utilized ultrasound.
18,33,34 Some patient studies examined blood flow in neck rotation and extension to determine presurgical risk of stroke and showed that vertebral artery blood flow was affected by contralateral rotation in about 50% of patients, but the patients investigated in these studies were older individuals with symptomatic ischemic cerebrovascular disease.
35,36 We were unable to find any MRI studies of young individuals who were asymptomatic. Notably, no studies were identified that have investigated blood flow during specific manual therapeutic techniques using MRI.
The purpose of this study, therefore, was to examine the effect of common manualtherapy procedures on blood flow in the craniocervical arteries and blood supply to the brain in participants who were healthy using MRA. The objective was to determine whether there were any differences in craniocervical arterial flow or total blood supply to the brain between the positions involved in selected manualtherapy techniques, which may help inform risk assessment by manual therapypractitioners. The effect of the thrust component was not examined in this study.
The specific research questions were:
Do certain neck positions used in common manual therapy procedures cause a greater difference in blood flow in the craniocervical arteries compared with the neutral position than others?
Do certain neck positions used in common manual therapy procedures cause greater difference in total blood supply to the brain compared with the neutral position than others?
Method
Design
The study was an experimental MRI study examining blood flow in the vertebral and internal carotid arteries in the neutral neck position and comparing these measurements with blood flow measurements in 7 other neck positions used in common manual therapy procedures. Informed consent was obtained from all participants.
Participants
Volunteers who were between 18 and 65 years of age and had no reported mechanical neck pain or headache were recruited into the study by advertisement and word of mouth. Potential participants were screened by interview and excluded if they reported any of the following (1–4 are standard contraindications for manual therapy to the neck): (1) diagnosed inflammatory joint disease; (2) any history of serious cervical spine trauma, such as fractures; (3) any congenital disorder recognized as being associated with hypermobility or instability of the upper cervical spine; (4) diagnosed vertebrobasilar artery insufficiency (VBI); (5) claustrophobia or discomfort in confined spaces (standard contraindication for MRI); or (6) any contraindication identified by the local health authority MRI safety screening questionnaire.
Cervical range-of-motion assessment and testing for VBI as per Australian Physio
therapy Association guidelines
12 were undertaken prior to MRI examination to ensure that participants had no neck pain or limitation to neck movement and that no signs or symptoms of VBI were elicited. Participants with potential signs and symptoms of VBI were excluded because of the risk of brain ischemia imposed by the sustained neck positions required for the study.
Demographic data, including age and sex, were collected for all participants. Time of flight MRA was performed in the neutral neck position to give an image of the anatomy of the cerebral circulation. This imaging was reviewed post hoc for the presence of any vascular anomalies such as hypoplasia or aplasia of a vessel or anatomical variants such as the vertebral artery ending in the posterior inferior cerebellar artery, suggestive of an incomplete Circle of Willis. Dominance of one vertebral or one internal carotid artery was identified and was determined by visual inspection of its relative size compared with the contralateral vessel.
Experimental Conditions
The following sequence of neck positions was used:
Neutral
Left rotation: the participant was asked to turn his or her neck as far as possible to the left.
Right rotation: same as left rotation but with a different direction.
Left rotation with distraction: a strong longitudinal stretch was applied to the neck by the investigator (an experienced manipulative physical therapist); the head then was fully rotated to the left side and held in the rotated position while the distraction was maintained
30 (
Fig. 1A).
Right rotation with distraction: same as left rotation with distraction but with a different direction.
Left rotation localized to C1–C2: the C2 spinous process was stabilized in the neutral position by the thumb and index finger of the investigator, and the participant's head was rotated to the left until end-range was perceived, localizing rotation to the C1–C2 segment
31(
Fig. 1B).
Right rotation localized to C1–C2: same as left rotation localized to C1–C2 but with a different direction.
Distraction: a longitudinal stretch of the neck with the head in neutral then was applied by the investigator, with one hand under the participant's occiput and the other hand under the participant's chin.
Posttest neutral
Figure 1.
Participant position within the head coil for (A) rotation/distraction technique and (B) localized C1–C2 rotation technique.
The neck positions were selected to replicate as closely as possible common manual
therapy procedures without application of the thrust component. Neck rotation is a common component of a number of manual
therapy procedures. Neck distraction with rotation is a position described by Cyriax and Cyriax.
30 This manual
therapy procedure involves a longitudinal stretch (distraction) being applied to the neck by the practitioner and the neck then being taken to the limit of rotation. The premanipulative position for this procedure was chosen because it might be expected to apply some degree of stress to the craniocervical arteries, as highlighted by recently published international guidelines.
37 Localized rotation of the C1–C2 segment was chosen to simulate the premanipulative position for a procedure described by Maitland
31 in which the C2 vertebra is fixed by the operator's thumb and index finger and the neck rotated by the other hand. This premanipulative position was chosen because it might be expected to apply less stress to the craniocervical arteries. A second, posttest neutral position measurement was taken to assess the variability of blood flow volume.
Participants lay supine on the scanner bed with their head in a phased array head coil. This device is a rigid plastic box that encloses the head above and on either side, with a bar that passes anterior to the chin. There is a space of approximately 4 cm separating the box from the participant's head on all sides (
Fig. 2). Participants were asked to report any restriction to rotation imposed by the box and were observed by the operator. Participants were monitored closely throughout and immediately following the procedure for any symptoms or signs of discomfort, claustrophobia, or vertebrobasilar insufficiency, in the case of which the examination would have been terminated.
Figure 2.
Participant positioned in scanner showing head and neck coil.
Measurement of Blood Flow in the Craniocervical Arteries
Blood flow in each of the 4 craniocervical arteries was measured with MRI using a phase-contrast flow quantification sequence. All participants were imaged on a 3-T superconducting magnet (Siemens Magnetom Verio, Siemens AG, Erlangen, Germany). Participants were scanned with T1-weighted sagittal and axial images and 2-dimensional time of flight angiography. A retrospective cardiac-gated phase-contrast flow quantification sequence was used (repetition time=29 milliseconds, echo time=7 milliseconds, flip angle=30°, slice thickness=6 mm, matrix=192 × 512, field of view=200, and number of excitations=1). This is a standard sequence available on this imaging unit.
38 A velocity encoding value of 100 cm/s was used. The arterial plane of section was selected to intersect the top of the atlas loop of the vertebral arteries at the level of the C1 vertebra, with imaging extending to just below the atlas loop. The atlas loop segment was chosen because this is where most manipulative injuries have been reported to occur and, therefore, where most changes in blood flow might be expected to occur. The acquisition time was approximately 2 minutes.
Blood flow measurements were analyzed post hoc using the proprietary software syngo Argus (Siemens AG). In order to analyze blood flow using the Argus system, a region of interest was placed around each artery for each of the neck positions. In order to assess the effect of neck position on blood flow, average blood flow volume measured in milliliters per second was used as the primary test variable and was analyzed in neutral and each of the neck positions for each artery. Average blood flow volume in each artery then was compared between the neutral position and each of the experimental neck positions to determine whether blood flow volume changed from neutral. Velocity was not used for comparison because it is subject to wide variation depending on the area of sampling, which is particularly a problem in small tortuous vessels.
14,26,39
Measurement of Total Blood Supply to the Brain
Total blood supply to the brain was determined from the sum of average flow volume (mL/s) in both vertebral and both internal carotid arteries. The total blood supply for each of the neck positions then was compared with neutral. A difference in average total supply (increase or decrease) of ≥10% compared with the neutral position was considered to be clinically important.
40
Data Analysis
Descriptive statistics were used to summarize the demographic data. Average blood flow in all 4 arteries for the neutral position and each of the experimental neck positions was analyzed with descriptive statistics and tested for normal distribution. A linear mixed-effects model was fit for each artery (left, right) using a single-effect variable (neck position) to assess whether there were any differences between positions. The models were fitted using SAS version 9.2 (TS2M2) and SAS 2008 Proc Mixed technique (SAS Institute Inc, Cary, North Carolina) with restricted maximum likelihood estimation and with the Kenward-Roger adjustment for downward bias in the variance-covariance matrix. Compound symmetry was used. If the effect of a neck position was significant, follow-up testing of pairs of means was undertaken in 2 ways using Dunnett's adjustment to compare the neutral position with the other neck positions and to examine all pairs of means using a Bonferroni adjustment to the significance level. For post hoc power analysis, the variabilities were determined using a random-effects mixed model for each artery, combining data for all 4 arteries, to determine standard deviations due only to measurement and neck position sources of variation.
Role of the Funding Source
The study was funded by a grant from The University of Newcastle.
Results
Twenty participants (10 male, 10 female) with a mean age of 33.1 years (SD=11.9) were recruited into the study. All participants had normal anatomy of their craniocervical arterial circulation (
Fig. 3). Three participants (15%) had dominance of one vertebral artery. No participants experienced any signs or symptoms of vertebrobasilar insufficiency in any of the neck positions. One participant was excluded due to previously unknown claustrophobia when scanning commenced.
Figure 3.
Surface-rendered 3-dimensional multiplanar reformatted image of the carotid and vertebral arteries of a participant showing (A) normal anatomy and no dominance of any vessel and (B) hypoplastic right vertebral artery and dominance of left vertebral artery.
Effect of Neck Position on Blood Flow
Measurements of average blood flow volume (mL/s) for the internal carotid and vertebral arteries in the neutral condition and each of the experimental conditions are presented in
Table 1.
Table 1.
Mean (SD) Average Blood Flow Volume (mL/s) in the Craniocervical Arteries for Each Neck Position and the Mean Difference (mL/s, %) Between Each Neck Position and the Neutral Position With Linear Mixed Model Testing by Artery (P Value)a
In order to compare differences in average volume flow between all 9 positions to see if any of the neck positions had greater effect on any artery than another, we analyzed flow volume using a linear mixed-effects model with one categorical variable with 9 levels (neutral, left rotation, right rotation, left rotation/distraction, right rotation/distraction, C1–C2 left rotation, C1–C2 right rotation, distraction, and a second posttest neutral position). The significance level was set at α=.05, and following a significant effect, post hoc testing was carried out. The results showed no significant differences in flow volume for either of the vertebral arteries (right vertebral artery,
P=.28; left vertebral artery,
P=.47) but a statistically significant difference between positions for the right internal carotid artery (
P<.001) and left internal carotid artery (
P=.01) (shown at bottom of
Tab. 2).
Table 2.
Total Blood Inflow to the Brain Measured by Sum of Average Flow Volume (mL/s [95% Confidence Interval]) in Internal Carotid and Vertebral Arteries: Difference Between Neck Position and Neutral (mL/s, P Value, and Percentage Difference)
Further analysis of the significant results for the left internal carotid artery using the Dunnett test, in order to adjust for multiple tests and comparing neutral with all the other positions, showed no significant differences, despite the significant overall test result. Similarly, analysis of the right internal carotid artery values using the Dunnett test showed no significant differences. Further exploration of all pair-wise differences using a Bonferroni adjustment to control for family-wise error rate (α=.05/36=0.0014) showed no difference for the left internal carotid artery, but 4 combinations were significantly different for the right internal carotid artery. All combinations involved left rotation. Mean average flow volume for the right internal carotid artery in left rotation was unusually low and statistically different from the other positions. Removing left rotation from the analysis resulted in a nonsignificant result for the right internal carotid artery. As the Dunnett test was negative for all test positions compared with neutral, and because significant changes were not similarly demonstrated for the left internal carotid artery in right rotation, it is likely that this finding for the right internal carotid artery in left rotation is due to a statistical anomaly.
Although mean values of average flow volume were not significant for any position, there were certain individuals with marked flow changes in some positions.
Figure 4 shows parallel plots of flow volume in all positions to demonstrate individuals with large changes.
Figure 4.
Individual parallel plots of flow volume (mL/s) in (A) vertebral arteries and (B) internal carotid arteries for all participants, with specific plots highlighted to show individuals with large variation. For each, one individual with stable flow is shown for comparison. R=right, L=left, VA=vertebral artery, ICA=internal carotid artery, RVAneut=right vertebral artery neutral, RVAneut2=right vertebral artery posttest neutral, RVArr=right vertebral artery right rotation, RVAlr=right vertebral artery left rotation, RVAlrdist=right vertebral artery left rotation/distraction, RVArr=right vertebral artery right rotation, RVAc1lr=right vertebral artery left rotation at C1–C2, RVAc1rr=right vertebral artery right rotation at C1–C2, RVAdist=right vertebral artery distraction, LVAlr=left verterbral artery left rotation, LVArr=left verterbral artery right rotation, LVAlrdist=left vertebral artery left rotation distraction, LVArrdist=left vertebral artery right rotation distraction, LVAc1lr=left vertebral artery left rotation at C1–C2, LVAc1rr=left vertebral artery right rotation at C1–C2, LVAdist=left vertebral artery distraction, LVAneut=left vertebral artery neutral, LVAneut2=left vertebral artery posttest neutral, RICAneut=right internal carotid artery neutral, RICAneut2=right internal carotid artery posttest neutral, RICAlr=right internal carotid artery left rotation, RICArr=right internal carotid artery right rotation, RICAlrdist=right internal carotid artery left rotation/distraction, RICArrdist=right internal carotid artery right rotation/distraction, RICAc1lr=right internal carotid artery left rotation at C1–C2, RICc1rr=right internal carotid artery right rotation at C1–C2, RICAdist=right internal carotid artery distraction, LICAlr=left internal carotid artery left rotation, LICArr=left internal carotid artery right rotation, LICAlrdist=left internal carotid artery left rotation distraction, LICArrdist=left internal carotid artery right rotation distraction, LICAc1lr=left internal carotid artery left rotation at C1–C2, LICAclrr=left internal carotid artery right rotation at C1–C2, LICAdist=left internal carotid artery distraction, LICAneut=left internal carotid artery neutral, LICAneut2=left internal carotid artery posttest neutral.
Blood Supply to the Brain
The total blood inflow (mL/s) to the brain for each position is shown in
Table 2. The total inflow to the brain did not vary substantially from neutral in any test position. Flow generally decreased slightly for both the end-range rotation and distraction positions but increased in the other positions. The total flow volume for all positions was analysed with a linear mixed effects model. No difference was found among positions (
P=.06). In addition, flow changes were all less than 10%, which is considered to be the normal variation for cerebral inflow.
40
Post Hoc Power Calculation
We were unable to perform a power calculation prior to the study due to the lack of published studies using MRI, but with the benefit of the data collected, we performed a post hoc power analysis. The standard deviations for each artery due to the sum of measurement and position sources of variation were 0.40 for the vertebral artery and 0.54 for the internal carotid artery. The power calculation was based on a comparison of 2 neck positions with an independent samples t test. For the vertebral artery, for a 50% change in average flow volume between neutral and any neck position, power was 0.97; for a 36.4% change from the neutral position, power was 0.80. For the internal carotid artery, for a 50% change in flow volume, power was 1, and even for as low a value as 16.4% change, power was still 0.80.
Discussion
This comparative MRI study examined blood flow in the craniocervical arteries in different neck positions and compared the measurements of blood flow with that in the neutral position to identify if any neck positions were potentially more hazardous than others. The study showed that blood flow was not negatively affected by any of the neck positions used and that no position had significantly greater effect on blood flow than any other. Total cerebral inflow also remained fairly constant in all positions, suggesting that cerebral perfusion similarly was not negatively affected by any of the neck positions. To date, there have been no previous studies of young individuals who were healthy investigating blood flow in the craniocervical vessels in different neck positions using MRI. The results of this study suggest that common neck positions used in manual therapy practice do not, in and of themselves, appear to pose a risk to blood flow to the brain, and concerns about the safety of commonly used neck positions may be unfounded, at least on the grounds of their effects on blood flow.
Blood flow in the craniocervical arteries varied between neck positions but was not significantly changed by any neck positions used in common manual
therapyprocedures compared with the neutral position. In addition, no position had any significantly greater effect on blood flow than any other, including those using potentially more stressful positions of end-range rotation or rotation and distraction. Previous studies examining blood flow changes during neck rotation have generally examined flow in specific arteries only, notably the vertebral arteries,
10,17,18 which does not allow consideration of the cerebral circulation as a whole. Concerns have been raised that flow reduction in rotation may represent a risk factor for neurovascular complication subsequent to cervical manual
therapy.
41 Moreover, it has been suggested that clinicians should measure flow in the vertebral arteries as part of premanipulative screening of the cervical spine.
12The current study contrasts with some previous studies where reduction in flow velocity was demonstrated in the vertebral arteries associated with contralateral rotation.
10,17,18 However, these were all ultrasound studies that did not measure actual flow volume. Using MRA, Weintraub and Khoury
35 showed that blood flow may be reduced in the vertebral artery on contralateral rotation, but their study was of older patients with advanced cerebrovascular disease who do not represent the population of interest for the current study aim, and even in this group only about 50% showed changes in flow. Interestingly, although not statistically significant, the current study also suggests that when flow is decreased in one vessel by a neck position, it appears to be compensated by increased flow in another. For example, in left rotation, there was marked reduction in flow in the right internal carotid and vertebral arteries but with a marked increase in flow in the left vertebral artery. Measurement of flow in the vertebral arteries for premanipulative screening purposes, therefore, would not seem to be particularly useful.
Studies also have demonstrated greater reduction in blood flow with increasing range of rotation
16,18 and suggested that end-range contralateral rotation positions might have a greater effect on blood flow than more localized segmental rotation. The results of this study suggest that common manual
therapyprocedures including end-range neck rotation and rotation/distraction do not appear to be any more hazardous in terms of blood flow than more localized positions. It is important to consider, however, that although a healthy artery should have the capacity to easily withstand such mechanical stress, the effects on arteries that are in a weakened state either from underlying arteriopathy or a temporary friable state due to infection or the potential presence of proinflammatory factors in the circulation are as yet unknown in the human model. Future research into the effects of neck position on blood flow, therefore, probably is not warranted. However, other factors such as the state of the arterial wall and the effect of the manipulative thrust may be more important determinants of risk from manual
therapy applied to the neck, and future studies could investigate these factors.
The present study showed that total blood inflow to the brain was not significantly changed from the neutral position by any of the test positions, and none of the positions showed a compromise to total cerebral blood supply. In the manipulation therapeutic positions, flow volume was increased somewhat. This finding was further supported by the fact that no signs or symptoms of cerebral ischemia were evident despite marked reduction of flow in the vertebral arteries of some individuals. A previous ultrasound study also demonstrated marked reduction or even cessation of vertebral artery flow on neck rotation in some individuals, yet no signs of vertebrobasilar insufficiency being elicited,
42 suggesting cerebral blood supply was not compromised. This conclusion is not surprising given the homeostatic function of the Circle of Willis in maintaining a constant blood supply to the brain. The position of C1–C2 rotation to the left showed a large change in flow volume from neutral, which may help account for the overall
P value being close to significance (.06); however, this change was actually an increase in flow (7.9%) and does not suggest a risk to brain perfusion. As shown in a recent study, small fluctuations (<10%) in flow volume to the brain normally occur according to an individual's arousal state and increase if the individual is anxious or doing a mental task, which is not considered to be detrimental to brain function.
40Clinically, the results of this study suggest that even if blood flow in one vessel is markedly altered by a neck position or an individual has low or absent flow in one vessel, it may still be adequately compensated for via the other vessels, so concerns about the safety of particular neck position in terms of brain ischemia may generally be unwarranted. However, if the Circle of Willis is not intact (eg, missing a posterior communicating artery), reduction in total blood flow to the brain may be possible, particularly if there is no other source of collateral circulation. In the absence of radiological imaging, clinicians can generally rely only on production of signs or symptoms of brain ischemia to identify individuals at potential risk of an adverse neurovascular event.
Strengths and Weaknesses
The main strengths of this study are the use of MRI, which provides high-quality images of the craniocervical arteries, and the ability to track these images in different neck positions, which has not been undertaken previously. The advantage of using MRI is that flow can be measured simultaneously in all 4 vessels, which allows comparison of the overall effect of the different neck positions on blood flow, as well as the estimation of total cerebral inflow. The study used neck positions designed to replicate those used in typical manual therapy practice, so the results can be related to the clinical situation. In addition, the neck positions selected allowed comparison between potentially more stressful end-range rotation and rotation/distraction positions with more localized segmental rotation positions in order to see whether any were more hazardous than another in terms of effects on blood flow.
There were some limitations of the study, including the scanner setup, with the participant's head enclosed within a head and neck coil. In order to achieve the manipulation positions of the neck as described, it was necessary to modify some hand positions: (1) to have the top hand around the chin and the underneath hand under the occiput for the rotation-distraction position and (2) to rotate the head from the vertex of the skull rather than the chin for the C1–C2 rotation position. However, only one hand was in the coil for the manipulation positions and was positioned under the head and neck, so did not limit the amount of rotation available. Although it was more challenging to perform the technique, it was possible to replicate the technique position as described. Due to MRI safety restrictions, it was not possible to formally evaluate range of rotation inside the coil. In addition, in order to minimize the time spent in each position, only a short segment of the arteries was examined at the level of C1; however, it is possible that during the rotation positions the arteries moved slightly cephalad, causing a less straight section of artery to be imaged. This approach may have made it more difficult in some cases to accurately identify the center of the arterial lumen in cross-section to undertake flow analysis. Flow analysis, therefore, may have been performed nearer the arterial wall in some instances where flow might be more variable. Future studies could image a longer section of artery to minimize this problem when associated acquisition times are reduced by improvements in technology.
Future Research
One of the key components of neck manipulation is the HVT. It was not possible during this study to look at the effects of the manipulative thrust on blood flow as the participant's head would have had to be removed from the head coil for the procedure and then returned to it, making it difficult to compare flow measures because the baseline neutral position would be slightly different. Future studies could examine the effects of the manipulative thrust on blood flow in the craniocervical arteries. The study examined relatively young participants who were healthy and in whom it was assumed arteries were healthy and without any pathology. The effect of neck position on blood flow in arteries with some form of arteriopathy may be different and could be investigated in the future. Future studies also could look at other neck positions used in manual therapy, such as extension or lateral flexion. The post hoc power calculation may be useful to determine appropriate sample size for future flow studies.
Conclusion
Although concerns have been raised about the safety of manual therapy applied to the neck, in particular the upper cervical spine segments, none of the positions tested in this study demonstrated any significant change in blood flow volume from the neutral position. Moreover, no position including end-range rotation, upper cervical rotation, or strong distraction had any greater effect on blood flow than any other. Total blood supply to the brain was not adversely affected by any positions, and in most positions relating to common manual therapy procedures such as rotation/distraction and C1–C2 rotation, supply was increased somewhat. Reduction in flow in one vessel appeared to be compensated for by an increase in another. This finding suggests that the neck positions themselves are not inherently hazardous in terms of compromise to blood flow in the craniocervical arteries, and it is more likely, therefore, that other factors such as the state of the arteries and the effect of the manipulative thrust may be more important. Future imaging studies focusing on blood flow in normal or individual craniocervical arteries may not be particularly useful.
Footnotes
All authors provided concept/idea/research design. Ms Thomas, Dr Rivett, Dr Stanwell, and Professor Levi provided writing. Ms Thomas, Dr Bateman, and Dr Stanwell provided data collection. Ms Thomas, Dr Rivett, Dr Bateman, and Professor Levi provided data analysis and project management. Ms Thomas provided study participants. Dr Bateman provided facilities/equipment. Dr Rivett, Dr Bateman, and Professor Levi provided institutional liaisons. Dr Stanwell and Professor Levi provided consultation (including review of manuscript before submission).
The study protocol was approved by The University of Newcastle Human Research Ethics Committee.
The study was funded by a grant from The University of Newcastle.
- Received December 7, 2012.
- Accepted June 21, 2013.
- © 2013 American Physical Therapy Association
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