<scp>MTP</scp> block anatomy and paravertebral spread – a reply

We thank Hamilton, Manickam, Philip and Kaushik for their comments. While we have not thoroughly explored the possibility of multilevel spread with the mid-point transverse process to pleura (MTP) block 1, we have achieved spread to adjacent levels with alternating level injections in one of the cadavers examined. This begs the question of whether multilevel spread is dependent on the volume used. That said, it is important to note that the pattern of injectate spread does not necessarily correlate with the extent of somatic blockade, as noted by Marhofer et al. in their paravertebral block volunteer study 2. Therefore, this question is best addressed in future clinical trials. We agree that a multilevel unilateral block with a single injection site is desirable, but note that Hamilton and Manickam have presented a speculative comparison between the erector spinae plane 3 and MTP blocks. While interesting, these remain unsupported opinions. At the moment, such comparisons remain premature, as conclusions regarding safety and analgesic efficacy require additional evidence from clinical practice and head-to-head comparisons. It may not be easy to tease out the effect of spread into the paraspinal fascial planes from that resulting from spread into the paravertebral space. The spread of solution on ultrasound with the MTP block sometimes demonstrates an erector spinae plane block 3 pattern of spread, and our proposed potential mechanism of spread of solution from the injection posterior to the superior costotransverse ligament may partially account for the efficacy of the erector spinae plane block. It remains unclear whether the erector spinae plane block exerts its effect via spread to the intercostal nerves, or alternatively, via spread to the nerve roots in the traditionally defined paravertebral space 4, 5. The exact mechanism underlying the MTP block is deferred to future studies. The caudad-to-cephalad needle direction is our standard approach for ultrasound-guided paravertebral blocks using a parasagittal scan, as this direction targets the widest part of the paravertebral space. If the imaging is less than ideal, clinicians can easily switch from a paravertebral block to an MTP block without the need to withdraw or redirect the block needle. As the endpoint is the mid-point between transverse process and pleura, the insertion site does not seem to make much difference, and we have also performed the MTP block with needle insertion cephalad to the probe, as well as with an out-of-plane needle insertion. We have not tried inserting a catheter using the MTP approach; the feasibility of continuous MTP block is one of the questions that future research may clarify.

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