Determination of spread of injectate after ultrasound-guided transversus abdominis plane block: a cadaveric study

Background: The transversus abdominis plane (TAP) block is a new regionalanaesthesia technique that provides analgesia after abdominalsurgery. It involves injection of local anaesthetic into theplane between the transversus abdominis and the internal obliquemuscles. The TAP block can be performed using a landmark techniquethrough the lumbar triangle or with ultrasound guidance. Thegoal of this anatomical study with dye injection into the TAPand subsequent cadaver dissections was to establish the likelyspread of local anaesthesia in vivo and the segmental nerveinvolvement resulting from ultrasound-guided TAP block. Methods: An ultrasound-guided injection of aniline dye into the TAP wasperformed for each hemi-abdominal wall of 10 unembalmed humancadavers and this was followed by dissection to determine theextent of dye spread and nerve involvement in the dye injection. Results: After excluding one pilot specimen and one with advanced tissuedecomposition, 16 hemi-abdominal walls were successfully injectedand dissected. The lower thoracic nerves (T10–T12) andfirst lumbar nerve (L1) were found emerging from posterior toanterior between the costal margin and the iliac crest. Segmentalnerves T10, T11, T12, and L1 were involved in the dye in 50%,100%, 100%, and 93% of cases, respectively. Conclusions: This anatomical study shows that an ultrasound-guided TAP injectioncephalad to the iliac crest is likely to involve the T10–L1nerve roots, and implies that the technique may be limited touse in lower abdominal surgery.