Our hypothesis, that individuals with chronic WAD who did not respond to FB procedures (WAD_NR), would have greater sensory, sensori-motor and psychological features than responders (WAD_R) was largely rejected; with few between group differences demonstrated. However, the results did reveal that both WAD groups were different to the healthy controls (HC). Possible reasons for these findings are discussed.
Our participants with WAD presented similar profiles to previous studies and support findings that chronic WAD demonstrates a complex clinical presentation including sensory hypersensitivity, sensori-motor dysfunction and psychological distress [66, 67]. Pain and disability levels were comparable to other patients undergoing MBB [17, 18, 22, 68]. Some individuals reported an extensive duration of neck pain, and although the literature indicates the episodic nature of neck pain over time , all individuals reported that their symptoms were attributable to an original MVC. In concert with other studies, our participants reported lower pain thresholds to pressure and thermal stimuli [70–72] heightened responses bilaterally to BPPT [73, 74], reduced NFR thresholds [9, 10], decreased cervical ROM [35, 36, 75] and impaired control of cranio-cervical flexion [11, 36, 76]. Our healthy control data were likewise similar to that previously reported [11, 77, 78]. The psychological profile of our whiplash participants is also consistent, with high levels of psychological distress [15, 16], moderate post traumatic stress symptoms  and levels of pain catastrophizing  evident.
The presence of sensory hypersensitivity likely reflects central nervous system hyperexcitability [81, 82] indicating that similar nociceptive processes underlie the conditions of both groups. Higher levels of pain and disability have been associated with the presence of these sensory features in WAD  and 82% of our participants reported moderate to severe levels of pain related disability. Thus, it could be expected that sensory hypersensitivity would be a feature of both groups irrespective of responsiveness to the joint block techniques. There were also no differences in measures of motor function between the two whiplash groups. Loss of neck movement and impaired performance on the CCFT are also features of other neck pain conditions including non-traumatic idiopathic neck pain and cervicogenic headache [35, 83]. Whilst there may be some relationship with levels of pain and disability , the uniform presence of motor dysfunction across neck pain conditions suggest that our findings are not unexpected.
Levels of psychological distress as measured with the GHQ-28 were no different between our whiplash groups and are not surprising considering the levels of pain and disability reported by the participants. Whilst not reaching statistical significance, a greater proportion of non-responders fulfilled the criteria for a PTSD diagnosis on the PDS questionnaire (44% of non-responders versus 29% of responders) and reported higher symptom severity levels. The lack of statistical significance may be a consequence of the sample size of the study and this factor requires further investigation, especially given recent studies that demonstrate a relationship between PTSD, and pain/disability in WAD [84–86].
There was one notable difference between the two whiplash groups. Higher levels of pain catastrophization were demonstrated in the WAD_NR group. Catastrophization has been associated with enhanced pain reports, concurrent disability [80, 87] and lower pain threshold/tolerance levels, but is not significantly related to nociceptive flexion reflex (NFR) threshold in healthy and clinical pain samples [10, 88]. Sullivan et al.  reported that higher levels of catastrophization predicted higher levels of pain following medical procedures, such that these individuals may actually be less responsive to invasive interventions. It is possible that the higher levels of catastrophization and tendency towards higher psychological distress and post traumatic stress symptoms observed in the WAD_NR group may have contributed to the lack of response to the facet joint injection. The exact mechanisms responsible for this lack of responsiveness require further investigation, but may even include diminished placebo responses, where individuals may not ‘believe’ in the blocks or invasive procedures. Alternately, the higher PCS scores in our non-responder group may be a consequence of the study methodology. PCS scores were obtained following diagnostic facet joint procedures in both whiplash groups. It is possible that a lack of response may increase levels of catastrophization.
The WAD_NR group reported greater medication intake than the responder group and this was the case for all medication types. Given that pain and disability levels were no different between the groups, it could suggest that higher levels of catastrophization may explain the need for increased medication; or alternately, the lack of effectiveness of medication in reducing pain and disability may result in higher levels of catastrophization. There is some data available to support the initial claim suggesting that catastrophization is associated with greater medication intake . However, this requires further investigation.
The few differences found between the two groups in both physical and psychological measures would seem to indicate that similar processes are contributing to the clinical presentation, regardless of whether or not facet joint nociception is involved. It is possible that the WAD_NR group may have nociception arising from other structures. Cadaver and biomechanical studies indicate that various cervical spine structures can be potentially injured during whiplash trauma mechanisms and structures other than the cervical facet joints may be responsible for ongoing nociception [89–91]. However, it has also been proposed that factors other than peripheral nociception, for example physiological stress responses, can induce hyperalgesic responses and these may explain the presence of various symptoms in individuals with chronic WAD [92–94]. Future studies are currently underway to investigate the attenuation of the physical and psychological features of chronic WAD following modulation of facet joint nociception, to assist in understanding this relationship further.
Wasan et al.  previously demonstrated that psychiatric co-morbidity is associated with reduced pain reduction following MBB, however they utilized different scales (Hospital Anxiety and Depression Scale); focussing on symptoms of anxiety and depression whereas this current study evaluated psychological distress (GHQ) and post traumatic stress symptoms (PDS). It may be that affective/anxiety symptoms have a greater association with response to MBB. Additionally, symptoms may not be as important as actual diagnosis in predicting response to MBB. There was certainly a trend towards an increased proportion of PTSD diagnoses in the WAD_NR group that may be of significance in a larger study. Therefore, further investigation of psychological diagnoses, and the role of pain catastrophization and posttraumatic stress symptoms in outcomes following procedural interventions would be indicated.
Consideration must be given to the diagnostic facet joint blockade procedures and ‘cut-points’ used in our study. The use of comparative local anaesthetic blocks or placebo blocks has been advocated to guard against false positive responses . In this study, two diagnostic injection procedures were used, IAB followed by MBB. This combination of diagnostic techniques possesses a similar construct to comparative MBB’s, with individuals reporting relief of their predominant pain for the duration of the anaesthetic. Target specificity was ensured with each procedure by the use of radiographic confirmation of contrast medium (without note of radiate spread) to ensure needle location . The responder patients in this study reported a consistent response to both procedures (50% or greater decrease in pain intensity).
Whilst placebo blocks are preferred for ensuring diagnostic accuracy in the cervical region , this was not possible at the clinic where our study was conducted. Therefore, whilst the approach used in our clinic was stringent, we cannot fully exclude a placebo effect in responders or a nocebo effect in non-responders. A lack of differences between the whiplash groups may have also resulted from the criterion standard utilized in our study for determining ‘success’ of the intervention. The clinic used in the study refers individuals for RFN if they report ‘greater than 50% relief of pain’ following confirmatory MBB. This cut-off may not be sufficiently sensitive to detect differences between the responder and non-responder groups. Eighty percent pain relief has been suggested for use in research studies , but our study was required to use 50% to adhere to the protocol required by the clinic involved. Of note, previous research has shown no difference in clinical outcomes following RFN when 50% versus 80% pain relief from FB was used as the criterion standard .
It was also noteworthy that more individuals who failed to respond to the MBB were lost to follow-up. As Figure 1 demonstrates, 23/55 (42%) people who did not respond to IAB were lost to follow-up, compared to only 11/69 (16%) of those who responded. Comparison of these individuals was not possible and the effects on the results are not known.
Another possible limitation of this study was that the measures performed in this study were performed by the study author, who was aware of the study hypotheses, however considerable care was made to avoid describing study aims to the participants during the study (and expectations of results were unknown given it was a descriptive study); however bias is possible when examiners are not blinded.
This study was a preliminary cross-sectional study to investigate any physical or psychological differences in a cohort of individuals with chronic WAD who did and did not respond to cervical FB procedures. The design has limitations, but the results serve to inform future predictive studies. Inclusion of the physical measures (i.e. sensory and motor measures) in future prospective studies, may be necessary for profiling patients, but is unlikely to be predictive of response. Our findings do suggest that a wider raft of psychological measures be explored, given some differences in these domains. In addition, the inclusion of measures such as locus of control, coping styles and expectations, may ultimately assist the clinical selection of patients for FB procedures.