A sono-anatomical and cadaveric study of ultrasound-guided retrolaminar block

Aim: Retrolaminar block (RB) is known to confer chest wall analgesia but, its mechanism has not been established. Our primary objective was to determine if the spread of injectate following RB extends to the paravertebral space (PVS). Second-ary objectives were to determine the predefined anatomical areas and nervous tissues contacted by injectate; the effect of volume on spread; and the extent to which experts can predict PVS spread based on examination of US videos of the injection. Material and methods: US-guided RB was performed on cadavers using a single injection technique of 10, 20 or 30 ml dye. Anatomical dissection was performed to identify the extent of spread of injectate to the retrolaminar, intercostal and PVS. Ultrasound recordings of the injection were independently evaluated by experts in US-guided regional anaesthesia. Results: Spread of injectate to the ipsilateral PVS was identified in 6/10 dissected regions (0/1, 1/3 and 5/6 when injectate volumes of 10, 20 and 30ml were administered respectively). The extent of cephalad-caudad spread within the PVS varied from 1 to 3 levels. Expert interpretation of ultrasound images regarding spread to the PVS demonstrated poor correlation with dye staining observed on dissection. Conclusions: Injectate spread following RB demonstrated substantial variability. Inconsistent spread to the ipsilateral PVS may account for clinically occurring incomplete blocks. The likelihood of spread to the ipsilateral PVS was greater when a larger volume was injected. Expert evaluation of the dynamic ultrasound images obtained at injection can-not reliably predict spread to the PVS.