Distal middle scalene catheter analgesia: separating infusion effect from catheter‐tip position

We read with interest the randomised trial by Nakazawa et al., which evaluated a continuous middle scalene muscle catheter positioned approximately 25 mm from the interscalene groove after shoulder surgery [1]. The lower pain scores the following morning and at discharge, together with reduced rescue analgesia, provide useful evidence that this technically distinct catheter strategy can sustain postoperative analgesia. We believe, however, that several design and mechanistic considerations are important before the findings are interpreted as showing that proximity of the catheter tip to the interscalene groove is unnecessary. First, the randomised comparison does not isolate catheter-tip position. Patients in the catheter group received continued levobupivacaine infusion, whereas the control group received only the initial single injection. Thus, catheter location; duration of local anaesthetic exposure; and cumulative dose all changed simultaneously. Contemporary evidence also illustrates that the analgesic contribution of continuous infusion is context dependent. In a randomised trial conducted with comprehensive multimodal analgesia, adding continuous interscalene infusion to a single bolus did not reduce 24-h morphine consumption [2]. Regardless of the direction of that effect, infusion remains an independent treatment component and cannot serve as a surrogate for catheter-tip location. A more direct anatomical test would allocate patients randomly to groove-adjacent and distal catheters while keeping the initial bolus, infusion regimen and multimodal analgesia identical. Second, distal catheter placement does not necessarily establish distal drug action. The catheter was advanced through the prevertebral fascial plane and fixed approximately 25 mm from the groove, but the study does not describe post-placement confirmation of the tip or dynamic assessment of infusate spread. Local anaesthetic could track proximally along fascial planes and still reach the brachial plexus region. This distinction matters because catheter-tip location can alter respiratory effects without materially changing analgesia: an earlier randomised trial of continuous interscalene blockade found less hemidiaphragmatic paresis with extrafascial than with intrafascial tip placement, while preserving pain control [3]. Ultrasound confirmation of catheter position and of injectate spread would therefore strengthen mechanistic interpretation. Third, respiratory safety warrants objective assessment. Nakazawa et al. did not include patients with chronic respiratory disease, did not assess diaphragmatic function with ultrasound and used an initial 20-ml bolus near the interscalene groove. Recent evidence suggests that both volume and anatomical level materially influence phrenic involvement. Renard et al. [4] showed that reducing an extrafascial interscalene injection from 20 ml to 10 ml decreased hemidiaphragmatic paresis substantially, albeit with a trade-off in analgesic duration. Similarly, a randomised trial in 2024 found that continuous proximal longitudinal oblique supraclavicular blockade using a low-volume initial injection preserved diaphragmatic function better than continuous interscalene blockade, while providing similar analgesia [5]. These findings support incorporating diaphragm ultrasound and prespecified respiratory outcomes when evaluating a distal middle scalene catheter strategy. Nakazawa et al. provide an important proof of clinical feasibility. A head-to-head trial that isolates catheter-tip position; confirms tip location and local anaesthetic spread; and measures diaphragmatic function objectively would clarify whether distal placement offers not only effective analgesia but also a distinct anatomical and respiratory advantage.