We appreciate the insightful comments (1) from Lam et al. regarding our study titled “Ultrasound-Guided Triamcinolone Acetonide Hydrodissection for Carpal Tunnel Syndrome: A Randomized Controlled Trial” (2) published in Frontiers in Medicine. The dose of triamcinolone acetonide for both groups should be 10 mg in accordance with the statement in ClinicalTrials.gov (Identifier: NCT04346030). We apologize for the misplacement of 40 mg/ml in the abstract.

Regarding the question, could the concentration of corticosteroid play a role in the clinical effect of hydro-dissection as the amount of the injectate was different between Group 1 and 2? Based on our antecedent randomized controlled trial (3), ultrasound-guided injection with 10 or 40 mg triamcinolone acetonide yielded similar improvements for patients with carpal tunnel syndrome. In the aforementioned study (3), the total volume of injectate was 2 ml in either the 10- or 40-mg group. Obviously, the 40-mg group received a higher concentration (and dose) of perineural corticosteroid than the 10-mg group. However, no additional benefit was observed in the 40-mg group, which might partially resolve the query from Dr. Lam et al.

Regarding the effect of hydrodissection, we are not sure whether the factor that Dr. Lam mentioned really contributed to the equal effectiveness in both groups. The formation of a halo can be derived from two clinical scenarios of perineural injections. First, the needle is introduced into the epineurium to let the fluid confine inside the nerve sheath like the common approach for sciatic nerve block. Second, if the target is the subsynovial connective tissue in the carpal tunnel, a circumferential shape of fluid accumulation can be formed by dissecting the upper and lower surfaces of the nerve with more fluid applied over the ulnar aspect. In Figure 1B of the commented article, there are some hyperechoic substances over the radial aspect of the median nerve. According to the anatomy and ultrasound appearance, we believe it to be the subsynovial connective tissues. Another point worth mentioning is that Figure 1B in the commented article was taken during the injection but not after the completion of the injection. Even so, if we carefully look at Figure 1B in the commented article, there is an area with mixed echogenicity radial to the median nerve. As the gain had been adjusted higher for counteracting the anisotropy (4) due to the wrist position, we believed that area had been infiltrated by the corticosteroid-containing injectate. Furthermore, the pathogenesis of carpal tunnel syndrome is complex and adhesion is only one of the proposed mechanisms. The aforementioned viewpoint is supported by our previous observational study (5), showing that the mobility of the median nerve was insignificantly affected by either corticosteroid injections or surgery although the substantial improvement was observed after both treatments. In this sense, more randomized controlled trials are needed to investigate the add-on effect of hydro-dissection considering the variation in the pharmacological effects of different regimens (6).