This study showed that three sessions of UC-TSM resulted in a short-term increase of headache-related quality of life and upper cervical range in patients with CEH.
Evidence for headache-related quality of life changes following cervical manual therapy interventions
The present study demonstrated that UC-TSM may be effective for an improvement of short term headache-related quality of life in subjects with CEH. One month after the intervention, UC-TSM group showed a statistically significant reduction in HIT-6 score compared to the control group (10.0 (SD 10.6) in UC-TSM vs. 0.2 (SD 2.9) in the control group), with large effect size and superior to the MDC of 8 points (Castien et al., 2012). HIT-6 comprises 6 items (Kosinski et al., 2003): severe pain during headache, the limitations in daily activities, the need to lie down during headaches, tiredness due to headaches, irritation from headaches, and difficulty concentrating during headaches. By that reason, the improvements of the UC-TSM group in HIT-6 score may involve benefits in all these aspects of headache-related quality of life. The impact in the headache-related quality of life was reinforced by the statistically significant reduction in the headache intensity, frequency and duration produced by the UC-TSM group compared to the control group at one month follow-up. Nevertheless, only reduction in the maximum headache overcame the recommended minimal clinically important difference (MCID) on the VAS of 1–2 points (McCormack et al., 1988; Kelly, 2001). In any case, the results of the present study in terms of headache intensity and MCID should be interpreted and compared with caution. Some have argued that the MCID value varies depending on baseline pain score, with the MCID increasing for higher baseline pain score (Hawker et al., 2011).. In case of low baseline scores as in the current study (mean baseline headache intensity of about 1.5), a difference of 0.5 may be considered a clinically relevant change (Rowbotham, 2001). Considering this aspect and the different methodologies, our study supports the effects of manual therapy in the cervical spine without and within a multimodal approach for the reduction of headache intensity, duration and frequency (Nisson et al., 1995; von Piekartz et al., 2011; Bodes-Pardo et al., 2013).
Evidence for cervical mobility changes following cervical manual therapy interventions
The present study demonstrated that UC-TSM may be effective for an improvement of cervical mobility in subjects with CEH with small to moderate effect sizes. The improvements in each plane overcame the lower bound of the standard error of measurement established between 2º and 7º (Sterling et al., 2002). These increases in cervical ROM were superior to the studies of Hidalgo et al. (2016) and Lluch et al. (2013), that used our same technique but with a lower dosage (1 session of 10 minutes and 3 minutes respectively). Our sample presented a total cervical ROM lower than what is considered normal for an asymptomatic sample of similar age (353º (SD = 46)) (Prushansky et al., 2006). In T1, the UC-TSM group almost achieved the normal total cervical ROM (344.07º (SD = 51.38)) from the value in T0 (314.51º (SD = 59.23)) and reduced the total cervical ROM to 336.83 (SD = 53.42) in T2, with statistical signification compared to the control group in both T1 and T2.
In spite of the shown cervical ROM effects, the indication of the translatoric mobilization of the occipital-C1 segment is mainly to restore upper cervical dysfunction (Hidalgo et al., 2016). Due to the involvement of the upper cervical spine in CEH, especially the C1-C2 segment (Bogduk, 2001), quantification of the upper cervical mobility is considered more important in the assessment of CEH patients. FRT suppose a valid and reliable tool for testing C1-C2 mobility (Hall et al., 2008; Takasaki et al., 2011). In the present study, increases of FRT in UC-TSM group exceeded the minimal detectable change (Hall et al., 2010) reaching the clinically relevant improvement for patients with CEH (Hall et al., 2004), in right FRT at T1 unlike left FRT (whose increase in FRT was more reduced from an almost normal FRT at baseline (37.2 ± 8.90)). At T1, both right and left FRT achieved a ROM in the considered values as normal for asymptomatic subjects (39º-45º) (Amiri et al., 2003; Hall y Robinson, 2004; Ogince et al., 2007). At T2, UC-TSM group showed a statistically significant increase in FRT mobility compared to the control group, however not reaching the MDC. The improvement of FRT mobility obtained in the present study, applying UC-TSM with the cervical spine in neutral position, are comparable to those of previous studies using different cervical manual techniques applied at the end of the cervical rotation, in asymptomatic subjects (Clements et al., 2001) and patients with neck pain (Dunning et al., 2012) or CEH (Hall et al., 2007). These findings support the efficacy of UC-TSM to increase upper cervical mobility, suggested as a technique in neutral cervical position meeting the international recommendations (Rushton et al., 2014). Based on the available evidence, these results can be explained by a model in which a mechanical input generated by the UC-TSM triggers a cascade of biomechanical and neurophysiological events, leading to an increase of cervical mobility (Bialosky et al., 2009).
Evidence for hypoalgesic changes following cervical manual therapy interventions
The present study showed that UC-TSM group exhibited statistically significant increases in PPT but with small size effects in T1 and these differences were not maintained in T2. These results are similar to studies using UC-TSM in patients with cervical43 and craniofacial pain44. On the other hand, control group demonstrated a statistically significant reduction of cervical PPT in right thenar eminence in T1. One potential explanation for the reduction in the control group is the possibility that evaluation tests used (especially the use of algometry for PPTs) may have irritated participants (Mansilla-Ferragut et al., 2009), increasing their pain and reducing their cervical mobility.
Current evidence suggest that immediate hypoalgesic effects of manual therapy are possibly due to neurophysiological mechanisms activated, in this case, by the mechanical stimulus of the UC-TSM (Schmid et al., 2010). Possible neurophysiological mechanisms include the activation of descendent pain inhibitory systems via corticospinal projections from the periacueductal gray matter (PAG) (Paungmali et al., 2003; Skyba et al., 2003). Further studies are needed to determine the mechanisms of hypoalgesic effects of manual therapy interventions in CEH patients.
Limitations
Although a potential strength of the current controlled clinical trial was the inclusion of a control group without receiving any intervention, we should recognize potential limitations that should be considered. First, headache intensity during the procedure was low in both groups (VAS = 1.31 and 1.58), hindering to make meaningful interpretations of headache intensity results because of the occurrence of a floor effect. For this reason, headache intensity was not used as a main study variable. Additionally, this study presents immediate post-treatment and 1 month follow-up effects of UC-TSM, so mid and long term effects should not be inferred. Third, control group did not receive any type of intervention, so placebo effect cannot be ruled. On the other hand, one therapist provided the treatment in the current study, which may limit the generalization of the results. Finally, CEH subject selection was based on clinical criteria; however anaesthetic blockades were not used as a criterion. Further studies should address these issues.