In Response

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We read with great interest the letter to the editor by Kumar et al1 on continuous fluoroscopic imaging of contrast injected with the ultrasound-guided transmuscular quadratus lumborum (TQL) block in 1 fresh tissue cadaver. The authors provide excellent novel imaging showing radiopaque dye spread following a single TQL block with the transducer in the transverse oblique paramedian position at L3.1 Spread of dye is compared with a paravertebral block at T10 on the contralateral side.
The contrast spread after transverse oblique paramedian TQL block appeared distinct and notably spared the paravertebral space (PVS).1 This is in direct contradiction with our recent cadaveric results showing consistent spread of dye cephalad into the PVS to surround the ventral rami T9-12 and the thoracic sympathetic trunk.2 The pathway of injectate spread was posterior to the lateral and medial arcuate ligaments and occasionally via the splanchnic nerve diaphragmatic openings.2
We beg to differ with Kumar et al1 when they claim that previously published images of paravertebral spread have never demonstrated reverse flow from the PVS to the quadratus lumborum region. This statement is clearly not correct. Kumar et al1 claim that there cannot be a 1-way valve separating the 2 spaces. We have never claimed that there is such a 1-way valve. However, in accordance with previous important findings by Saito et al3 and Karmakar et al,4 we are in agreement that a passageway exists between the thoracic PVS and the intermuscular plane between the psoas major and the quadratus lumborum muscles in the lumbar paravertebral region (Figure).
Without ultrasound imaging of the needle tip in relation to the quadratus lumborum muscle, we cannot explain the sparing of the PVS reported by Kumar et al.1 However, if the injection was performed too anterolateral into the pararenal fat compartment, it becomes a variant of a fascia transversalis plane block, which has a different pattern of spread compared to an injection into the plane between the quadratus lumborum and the psoas major muscles.2 An injection into the pararenal fat compartment would have a pattern of spread similar to the radiopaque dye spread observed by Kumar et al.1
To confirm correct cephalad spread of the injectate, we always perform an early intermediate scan during injection, where the transducer is rotated from the transverse position 90° into the longitudinal plane. This technique seems to ensure a high degree of block success, since you can quickly adjust the needle tip, if the correct cephalad spread is not achieved and if the quadratus lumborum and psoas major muscles are not clearly separated by the injectate.
In conclusion, our own cadaveric study has clearly shown consistent spread of dye cephalad into the PVS to surround the ventral rami T9-12 and the thoracic sympathetic trunk, thereby offering a possible explanation regarding the efficacy of the TQL block to alleviate somatic and visceral pain.2 We do agree that further clinical randomized trials must be performed to assess TQL block efficacy.
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