Quadratus Lumborum Spares Paravertebral Space in Fresh Cadaver Injection

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We read the article by Dam et al1 on the spread of injectate with transmuscular quadratus lumborum (QL) blocks with great enthusiasm. The authors provide tremendous images and a rigorous discussion. We, too, have been curious regarding the mechanism for somatic and visceral analgesia after QL block. We recently compared the spread of paravertebral blocks (PVB) and QL blocks in a fresh tissue cadaver. With the cadaver in the lateral position, a 5-MHz curvilinear transducer (BK Medical Inc, Peabody, MA) was used to guide a 21-gauge, 100-mm needle (Sonoplex, Pajunk, Germany) into the space between the QL and psoas muscles via the transverse oblique paramedian approach at L3. Contrast (Isovue®-300; Bracco Diagnostics, Milan, Italy) was injected in 10-mL incremental volumes up to 40 mL with continuous fluoroscopic imaging (OEC 9800 Plus; GE Healthcare, Milwaukee, WI). The contrast spread after QL block (Figure, A) appears distinct and notably spares the paravertebral space. We then performed a PVB on the contralateral side of the cadaver at T10 with incremental injection. The PVB achieves 5 segments of spread with a cephalo-caudal streak that appears completely distinct from that of QL (Figure, B). This direct comparison of QL versus PVB in the same cadaver highlights the different distribution of dye spread after QL and PVB nerve blocks.
We are enthusiastic about the block, but we do not agree that the mechanism is due to paravertebral spread. Our images demonstrate distinct sparing of the paravertebral space after QL injection. Furthermore, previously published images of PVB spread have never demonstrated reverse flow from the PVB space to the QL region.2 There cannot be a 1-way valve separating the 2 spaces. Direct injection within the paravertebral space can be unreliable, and it would be surprising if spread from a more distal QL injection could spread so diffusely to the PVB space.2
The exact mechanism for the visceral coverage of the QL block is yet to be determined. We query if the injectate may spread to the celiac ganglion or sympathetic chain, as previously described with PVB via the splanchnic nerves.
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