Discussion
This prospective study show that both short- and long-axis injections were beneficial for mild-to-moderate CTS. Furthermore, the short-axis group exhibited a notable reduction in symptoms and disability at 1 month post-injection compared to the long-axis group. Although large improvements in SSS and FSS scores and tendency towards improvement in CSA at most follow-up time points between both groups (short-axis > long-axis group), the difference of BCTQ was not greater than the MCID value and the proportion of patients who met the MCID value of BCTQ between groups; moreover, this difference was not statistically significant. Moreover, the significant improvement from baseline for SNCV and DML was only observed in the long-axis group, and the 2-way ANOVA showed that this difference was significant for DML (p=0.049). Thus, their clinical significance remains uncertain, further studies with a larger sample size are therefore needed to obtain conclusive results.
Studies reported that elevated pressure resulting from an inflamed swollen FR and SSCT could cause MN compression and impaired nerve conduction function.30-32 Even without a substantiated mechanism of HD, the release produced by HD could unleash the trapped nerve and improve gliding resistance. MN remobilization could initiate nerve kinematics rejuvenation, blood flow reperfusion, and nerve re-conduction with the possible downstream effect of nerve regeneration.5-7 Indeed, published research revealed that single HD with 5 mL NS could induce a therapeutic effect for at least 3 to 6 months for mild-to-moderate CTS, which may result from an initial mechanical HD effect with the following possible effect of nerve regeneration.9 In our study, we only recorded the CSA of MN using ultrasonography without measuring other parameters such as, enlarged fascicles, echogenicity of the fascicular pattern, or hyperemia using Doppler ultrasound. Therefore, future studies evaluating above ultrasonographic parameters to further understand the mechanism and therapeutic effects of HD are encouraged.
Although various ultrasound guided-injection techniques for CTS have been advancing for decades, earlier studies have shown inconclusive results for their comparative effectiveness.12, 13; 16-18Smith et al15 announced that the in-plane short-axis injection combines the benefits of viewing the entire MN and needle presentation with better precision and neurovascular injury prevention. Lee et al12 revealed the in-plane short-axis approach above and below MN was better compared to the out-of-plane short-axis approach only above MN. Rayegani et al17 demonstrated that the in-plane long-axis approach merely above the MN showed a slightly greater decline in CSA than the in-plane short-axis approach merely below the MN, although no significant intergroup difference was observed.
The possible reasons for the divergence of effectiveness in the aforementioned studies are outlined below. These studies used corticosteroid ± lidocaine injection, which has a strong anti-inflammatory effect and reduces the pressure of the carpal tunnel for symptom relief. The pharmacological effect of the corticosteroid would have a greater impact on the results than the effect of HD, regardless of the method of injection because these studies only used 1-2 mL of injectate, which may have been insufficient to induce HD effect.33 In contrast to the above studies, this study only used NS, so that the mechanical effects of HD alone could be assessed, without any additional pharmacological effects. Hence, the different HD methods in our study are the cause of the different outcomes.
Compared to the short-axis approach, the long-axis injection barely contributes to decreasing adhesion and gliding resistance between the SSCT and MN, although it is supposed to increase the contact area between the FR and the MN via HD. Even though Nwawka et al34 showed that the injectate reached 50% and 100% of the MN’s circumferential coverage when dissected below and above the MN, respectively, we found more volume distribution between the SSCT and MN (short-axis > long-axis group) (Figure 4). We hypothesize that the intergroup difference might be a result of a greater HD effect between the SSCT and MN, because the adhesion and gliding resistance in these areas contribute to the prominent symptoms of CTS. Although insufficient HD from the proximal-to-distal carpal tunnel via a short-axis injection may be concerning, the following ultrasonography showed complete HD throughout the proximal-to-distal carpal tunnel when using 5 ml of NS (Figure 4c).
The short duration difference (BCTQ scores at 1 month post-injection) between groups may be due to only a single prescribed HD of 5 mL NS. Wu et al9 used 5 mL NS single HD similar to our short-axis approach and observed a significant improvement in the SSS on the 2nd and 3rd months and CSA through the 1stto 6th months compared with the placebo group for mild-to-moderate CTS. Compared to Wu’s study, both groups of our study received the exact HD with the same injection volume. Hence, the short-duration difference between the groups did not extend beyond our prediction. Further studies are encouraged to compare the short- and long-axis approaches with multiple injections as we believe that the intergroup difference would extend for a longer duration. On the contrary, only the long-axis group showed significant improvement of SNCV and DML. These findings could be partially explained because some studies have shown that electrophysiological assessment has limitations in predicting CTS outcomes.19, 35-37 Although only one-month therapeutic difference was observed after single HD, which may limit its clinical applicability, this study is the first trial to investigate the two different techniques of HD without additional pharmacological effect. Our results make it worthwhile to conduct further research to understand the effects of short and long- axis injections.
Other effectiveness, safety concerns should also be discussed. Previous research advocated that a short-axis scan is superior to a long-axis scan considering that the ultrasound image may be confused as swollen nerve fascicles, muscles, and inflamed tendons in the same plane of the long-axis scan; raising concerns of nerve trauma due to long-axis injection (Figure 1e).38 Furthermore, the short-axis approach benefits from faster learning with better accuracy of the injection because the operator has better flexible control of the needle from the initial penetration site to the MN which could contribute to injection precision as compared to the long-axis approach (Figure 1).12, 15, 39Hence, it offers a lower risk of nerve injury with a parallel needle approach to the oval-shaped MN that clearly visualizes the whole needle and neurovascular tissue. As both short- and long-axis HD were effective based on our results, the intervention choice would depend on the operator’s preference. We advocate performing an in-plane short-axis intervention above and below the MN, especially for beginners, as this has the advantage of being safer, easier to learn, and potentially more effective for HD compared to the long-axis approach. There are still some concerns regarding a priori bias towards the short-axis approach and further studies are needed to survey this issue.
Our research has a few limitations. First, studies should assess larger patient population to validate our results. Second, our study does not address the needle placebo effect due to the lack of a sham group; hence, the true effect of HD maybe overestimated. Third, this study did not completely exclude patients with a possible double crush syndrome that could contribute to median neuropathy at the carpal tunnel, and therefore, may have undermined the effect of HD. Fourth, a 6-month follow-up is relatively short and inadequate for comparison with other proven treatment options for CTS. At least a one-year follow-up would be desirable in future research. Final, although no significant intergroup difference was found in the mean value of DML at each time point, the 2-way ANOVA showed that the DML was significantly higher in the long-axis group compared to the short-axis group. Between-group comparison of the mean value may have a larger standard error than comparison of mean change from the baseline; hence, a larger standard error would result in an insignificant difference. Further studies are recommended to compare the mean difference between the groups.