Anterior Suprascapular Nerve Block or Low-Volume Supraclavicular Nerve Block?
We read with interest the recent randomized controlled trial by Wiegel et al1 comparing anterior suprascapular nerve block (SSNB) with interscalene brachial plexus block (ISB) for shoulder surgery. We commend their efforts in seeking a viable alternative to ISB that aims to avoid respiratory compromise and motor blockade of the arm. However, we wish to identify a number of concerns we have with this study.
While we acknowledge the important contribution made by the suprascapular nerve to shoulder joint innervation, it is important to remember the elaborate nature of the nerve supply to the shoulder. The shoulder joint has a range of motion extending far beyond any other joint in the body, which requires the complex interplay of various muscles with diverse innervation.
Anterior shoulder joint innervation comes from the subscapular and axillary nerves, both branches of the posterior cord of the brachial plexus, in addition to the lateral pectoral nerve, which arises from the lateral cord at a distance considerably distal to the origin of the suprascapular nerve.2 The posterior shoulder joint capsule is innervated by the suprascapular nerve and small branches of the axillary nerve. It is surprising therefore that Wiegel et al1 report that patients in the SSNB group had a significantly lower mean pain level than did patients in the ISB group. What accounts for the fact that blockade of 1 nerve of the 5 that innervate the shoulder joint provides better analgesia than the more complete blockade to be expected with ISB? We postulate that given the high volume and high concentration of local anesthetic (10 mL ropivacaine 1%) used for SSNB, cephalad spread to the interscalene brachial plexus is occurring deep to the omohyoid muscle. Alternatively, as pointed out by Siegenthaler et al3 in the original description of this technique, the median distance from the suprascapular nerve to the supraclavicular brachial plexus was 8 mm in cadavers, and anterior spread of dye was detected with a volume as low as 0.1 mL. Wiegel et al1 cite unchanging grip strength as evidence of lack of spread to the interscalene brachial plexus. Sensory-motor assessment after the block by a blinded observer may have delineated whether spread of local anesthetic to other nerves of the brachial plexus had occurred. We posit that further cadaveric studies with an equivalent volume of dye are warranted before a definitive statement can be made in this regard.
A further concern is the apparent modification of an a priori protocol after study completion and submission for publication, which led to a reassignment of methodology for a primary outcome from superiority to noninferiority. Was this decision independent of the data collected to that point? Had the data or interim data been reviewed, and did the decision makers have knowledge of the end-point results? This requires further clarity in order to rule out the possibility of operational bias.
Finally, the use of numeric rating scale (NRS) to define pain severity and block success seems somewhat arbitrary and contradictory in nature. In the present study, considerable pain is defined as an NRS of greater than 3, whereas block failure is designated to be NRS of greater than 6.