Erector Spinae Plane Block

A. Essentially all into the erector spinae plane, but only about 57% into the paravertebral space

Spread reached the intercostal space in about half and the epidural space in about 38%. The block consistently fills the compartment you inject into and reaches the paravertebral space only about half the time, which is why the reviewers say paravertebral spread cannot be taken as the established mechanism of action. Keep the boundary in view: this is an anatomical review measuring where injectate goes, not a comparison of how the block performs, so it constrains the proposed mechanism rather than showing anything about clinical reliability.

Methylene blue spread variably and extensively deep to the fascia of the erector spinae muscles, but involvement of the spinal rami and the paravertebral space was uncommon and level-dependent: it occurred in 1 of 11 cadavers when injected at T8 and in 4 of 11 at T2, and there was crossover to the contralateral side of the spine. The authors state the clinical implication as a possibility rather than a demonstration: in cadavers the block follows the fascial planes with unpredictable spread, which might explain its varying clinical efficacy. Hold it at that strength — this study dissected dye, not analgesia, so it offers an explanation for variable performance rather than evidence that performance varies.

An injection that follows fascia goes where the fascia allows, not where you intend. Contralateral crossover in a cadaver is also a reminder that a bilateral block is not necessarily two independent blocks.

The erector spinae plane block failed to meet its non-inferiority criterion: 75.2% of the erector spinae plane group needed morphine within two hours against 50.3% of the paravertebral group. Pain scores were higher with the erector spinae plane block, particularly on movement, and the required dermatomal area was not covered in 55.9% of erector spinae plane patients against 20.4% of paravertebral patients. Total morphine consumption and patient satisfaction were similar and no major complications were observed in either arm. The authors' conclusion is that for major breast surgery the thoracic paravertebral block remains the preferred technique. What follows from this trial is that the erector spinae plane block is not an equivalent swap; where it does belong — as a fallback when the paravertebral block is contraindicated or unavailable — is a question the trial did not put to the test.

The dermatomal coverage figure explains the analgesic one. A block that misses the required area in more than half of patients cannot be non-inferior, and this is the anatomical variability of the cadaver studies showing up as a clinical endpoint.