Phrenic nerve considerations in continuous brachial plexus blocks for shoulder surgery: a reply

We thank Mistry and Nair [1] for their thoughtful comments on our systematic review and meta-analysis [2]. As a pairwise systematic review and meta-analysis, our conclusions were necessarily constrained by the interventions compared and the outcomes measured and reported consistently in the available randomised trials. Our review was intentionally conducted within a patient-reported outcome framework, with multi-dimensional pain intensity as the primary focus. Subjective patient-reported dyspnoea was assessed inconsistently in the included studies [3]; we also acknowledge that dyspnoea is a non-specific symptom that may be influenced by numerous peri-operative factors. Consequently, such patient-reported respiratory outcomes were not amenable to synthesis within our framework. We agree with Mistry and Nair that these outcomes are important, and we encourage clinical researchers to include standardised assessments of them so that they can be incorporated into future systematic reviews and meta-analyses. Assessing phrenic nerve paresis using diaphragmatic ultrasound, with standardised reporting of the severity of hemidiaphragmatic paresis relative to baseline, has been used as a surrogate outcome for clinical respiratory events, with some important limitations. We explored this issue recently in a narrative review synthesising 10 randomised, controlled trials in which phrenic nerve paresis was the primary outcome, highlighting the inconsistencies in diaphragmatic ultrasound methodology, outcome definitions and reporting, and we proposed a pragmatic framework for balancing analgesic efficacy against diaphragmatic preservation [4]. Our search strategy was not restricted to interscalene brachial plexus block techniques. Specifically, we sought randomised studies evaluating alternative shoulder regional anaesthetic techniques for both single-injection and continuous analgesia; however, only anterior suprascapular nerve block was identified as a published alternative continuous technique. This reflects the existing literature, in which continuous catheter techniques have remained largely centred on interscalene approaches. There is also inconsistent nomenclature between publications, as the ‘superior trunk block’ has been described previously as a ‘low interscalene’ brachial plexus block. Balancing target-specific analgesia against avoidance of phrenic nerve paresis will always be an issue when managing peri-operative pain after shoulder surgery, and data support the titratability and extended duration of regional analgesia provided by continuous techniques [5]. How continuous techniques influence the incidence, duration and clinical significance of phrenic nerve paresis, and the extent to which these are determined by different approaches and infusion regimens, is unlikely to be resolved using the current literature alone. Rather, it will require future randomised studies comparing these techniques with standardised serial assessments of diaphragmatic function using objective ultrasound, and correlation with clinical respiratory outcomes. Ultimately, the objective for shoulder nerve blocks should not be phrenic nerve sparing in isolation. Rather, the challenge is to optimise the trade-off between analgesic efficacy, preservation of diaphragmatic function and meaningful patient-reported recovery according to the clinical context. We enjoyed this thoughtful correspondence and the discussion it has advanced in this evolving area. We look forward to future contributions from Mistry and Nair as the evidence base for selective shoulder regional anaesthesia continues to grow.