Anatomical variations of the thoracolumbar nerves with reference to transverse abdominal plane (TAP) block

Abstract

Introduction The transverse abdominal plane (TAP), formed between the transversus abdominis muscle (TAM) and the internal oblique muscle (IOM), contains the thoracolumbar nerve plexus. The plexus is anaesthetised through a blind or ultrasound-guided TAP block, mainly used for post-operative pain management. Ultrasounds are not always readily available in the public sector, creating a need to improve the blind TAP block. The L1 nerve and its terminal branches, also running in the TAP, can be blocked with a TAP block or separately. By studying the anatomy of the nerve plexus, the TAP block and the iliohypogastric and ilioinguinal blocks could be improved. This study aimed to determine the course and branching patterns of the thoracolumbar nerve plexus, as well as the branching of the L1 nerve in the posterior abdominal wall before entering the TAP. Methods Bilateral dissections were done on 54 embalmed cadavers to examine the TAP by noting the number of nerves at the mid-axillary line (MAL) and at the linea semilunaris. The needle tip position, as well as the general branching patterns were evaluated. For the L1 dissections, the root contributions and branching patterns were evaluated before entering the TAP. Abdominal ultrasounds were taken bilaterally on 43 volunteers to measure depth, individual muscle layer thickness, and subcutaneous fat thickness on a line at the injection point and at the IOM and TAM tendon junction. Differences between sides and the effect of BMI categories were analysed. Results The average number of nerves from the MAL to the linea semilunaris increased by one nerve. The needle was in the correct plane in only 7.6% of cases, with the needle going too deep in 79.3% of cases. The pop method used in the blind TAP block ensures the needle tip is not too superficial, but it is easy to go deeper than required. A nerve was pierced in 6.5% of cases, while the mean distance between the needle tip and the closest nerve (4.56 mm 6.83 mm) indicated the needle tip is generally close enough to the nerves to provide anaesthesia without nerve damage. Nerve interactions observed includes branching or not, merging or not, or any combination thereof. Various variations were seen for the root contributions of the L1 nerve, affecting the innervation of the anterolateral abdominal wall. The fourth lumbar artery accompanied the L1 or its terminal branches to enter the TAP in 40.7% of cases. The IOM and TAM tendon junction appears as a hyperechoic dot on an ultrasound, which can be used as an additional landmark. Measurements revealed the needle should be advanced at least 2 cm and 3 cm in healthy and overweight BMI individuals respectively. Conclusions Notable differences were seen between the current study and studies using a different population, indicating the anatomy of the TAP block is specific to population. The anatomy surrounding the TAP showed significance for a South African population. By analysing the anatomy, this study adds ways to improve the blind and ultrasoundguided TAP blocks.