Anterior Suprascapular Nerve Block Versus Interscalene Brachial Plexus Block for Shoulder Surgery in the Outpatient Setting

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To the Editor:
We read with interest the study by Wiegel et al1 regarding anterior suprascapular nerve block versus interscalene brachial plexus block (ISB) for shoulder surgery in the outpatient setting. In their effort to determine the best block for outpatient shoulder surgery, the authors propose that the suprascapular nerve block (SSNB) is favored if it provides noninferior analgesia while preserving grip strength. Unfortunately, we think that the trial design imposes significant limitations on both of these outcomes: limited validity for the former and lack of clinical importance and equipoise for the latter.
For the comparison of analgesic efficacy, pain is measured as an area under the curve (AUC). This measure should reflect the patient's overall postoperative analgesic experience. However, in this study, AUC poorly describes this experience, because it measures the combination of both preoperative and postoperative pain. By including block procedural pain and considering that the detected difference in AUC between the 2 blocks is quite small, we cannot help but wonder if claiming noninferiority, and subsequently superiority, of SSNB is actually a result of differences in procedural and not postoperative pain. In addition, the validity of the AUC measurements is further undermined by the relative low frequency of pain severity measurements. In this case, there were no data for pain severity between 6 and 24 hours postoperatively; thus, a straight line connecting these time points is used to extrapolate the AUC. This approach is not accurate and misses significant portions of the study period, especially when patients may experience rebound pain as ISB wears off.2
For the comparison of grip strength, the study design lacks equipoise as it inherently favors the SSNB right from the outset. Clinically significant differences in local anesthetic volume/dose between the SSNB and interscalene block groups have implications on the spread to nerve roots that innervate the hand. Moreover, using 20 mL for ISB for the purposes of analgesia is arguably overkill, when volumes less than 5 mL can guarantee a successful block.3,4 Furthermore, the choice of grip strength as the primary outcome measure is curious, given that most shoulder patients are immobilized in slings for several weeks postoperatively. Preservation of diaphragmatic or respiratory function, for example, would have been more clinically important outcomes to examine.
Consequently, the reader is left wondering whether noninferiority for an outcome of questionable validity combined with superiority for another of questionable clinical importance in a comparison that lacks equipoise truly provides a definitive answer for the comparison in question. While we believe that SSNB may provide certain benefits compared with ISB, we are left, as a matter of fact, with the conclusion that the jury is still out for this research question. Future studies should examine clinically important outcomes and should include more frequent measurements of pain severity if an AUC is contemplated.

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