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.