Anatomical insights into injectate spread after thoracic erector spinae plane block: A systematic review

Background and aims: The efficacy of the erector spinae plane block (ESPB) has been demonstrated in several meta-analyses and it is increasingly being used in clinical practice. However, its mechanism of action is still not fully elucidated. Although initial anatomical studies have suggested the spread of the injectate into the paravertebral space as the main mechanism of action, more recent studies have not consistently demonstrated this. This systematic review was conducted to determine the overall proportion and extent of injectate spread following a thoracic ESPB.Methods: PubMed, Scopus and EMBASE were searched. All studies that examined the injectate spread after a thoracic ESPB either through dissection or imaging were included. Excluded were all reviews, studies performed in paediatric patients and non-thoracic ESPBs. The primary outcome was the proportion of subjects with injectate spread in the erector spinae plane (ESP), paravertebral space (PVS), intercostal space (ICS) and epidural space (ES).Results: This review included 29 studies involving 113 cadavers and 79 volunteers. The proportion of subjects with injectate spread in the ESP, ICS, ES and PVS was 1 (95% confidence interval, 0.97-1), 0.51 (95% CI, 0.38-0.64), 0.38 (95% CI, 0.28-0.5) and 0.57 (95% CI, 0.49-0.64) respectively. The mean spread of injectate in the ESP, ICS, ES and PVS were 9.1 (95% CI, 8.0-10.3), 5.6 (95% CI, 4.0-7.3), 3.1 (95% CI, 1.0-5.3) and 3.5 (95% CI, 2.4-4.6) spinal levels respectively. Conclusion: Based on this study, the thoracic ESPB consistently led to injectate spread into the ESP compartment but less reliable spread into the PVS, ES and ICS compartments. There is also preliminary evidence that an increased time to assessment may be associated with an increased spread of injectate into the PVS.