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Is the erector spinae plane (ESP) block a sheath block?

2017; Wiley; Volume: 72; Issue: 7 Linguagem: Inglês

10.1111/anae.13912

1365-2044

Duncan Hamilton, Baskar P. Manickam,

Shoulder Injury and Treatment

We commend Chin et al. for important work on the use of erector spinae plane (ESP) block for abdominal analgesia 1. Having successfully used this technique to treat pain associated with multiple rib fractures 2, we have previously suggested ESP blocks provide proxy paravertebral blockade 3. We think the likely anatomical basis for effective blockade occurs when local anaesthetic (LA) is deposited within the erector sheath. Willard et al. have described the anatomy of the paraspinal muscles and their relations in detail 4. It is clear that the erector spinae muscles and their associated sheath exhibit a complex three-dimensional anatomy, a detailed understanding of which may help to explain the mechanism of this block. In their simplest form, the erector spinae muscles can be thought of as paired elliptical cylinders, one on each side of the vertebral column, analogous to the rectus abdominis muscles on either side of the anterior abdominal wall. Each cylinder is surrounded by a retinacular fascial sheath, separating its contents from the other muscle compartments of the thoraco-abdominal cavity. The anterior wall of this fascial sheath is incomplete, that is to say, the sheath has multiple varied apertures or perforations in its substance. In addition, the sheath, which extends from the nuchal fascia cranially to the sacrum caudally, is intermittently tethered anteromedially to bony structures along its course, notably the spinous processes and transverse processes of the vertebrae it crosses 4. Provided LA is deposited within the erector sheath compartment, the block will be successful by distribution of the LA not only cranially and caudally along the sheath, but by subsequently gaining access to the paravertebral space via apertures existing in the anterior sheath wall that act as conduits for injected LA. In our experience to date, such longitudinal spread of LA within the sheath is the end-point for a successful injection. Any injection deeper to the anterior sheath wall does not permit spread of LA beyond one inter-transverse space, due to tethering of the sheath to the transverse processes. We propose the LA spreads within the sheath by this mechanism, blocking the dorsal and ventral rami of thoracic spinal nerves at multiple vertebral levels. This explains why a successful block has been achieved by depositing LA both superficial and deep to the erector spinae muscle 5, as long as the LA is deposited within the erector sheath compartment between the erector spinae muscle and its investing sheath. However, the evidence suggests that targeting the sheath deep to the erector spinae muscle is most effective 1, 2, 5. The contrast study performed by Chin et al. clearly demonstrates the spread of dye injectate medially and on both surfaces of the erector spinae muscles in the cadaver, which further confirms the anatomical basis of this sheath block. To underscore the importance of depositing the LA within these anatomical boundaries, we suggest the ESP block be renamed the erector sheath block (ESB), akin to rectus sheath block in the anterior abdominal wall. Further anatomical and clinical investigation is necessary to elucidate the detailed mechanism and clinical applications of the ESB 6.