INNERVATION OF ANTERIOR ABDOMINAL WALL
The somatic innervation of anterior abdominal wall is from thoracoabdominal nerves (T7-L1).These includes anterior rami of lower thoracic nerves which are continuation of intercostal nerves and run in transversus abdominis plane and ilioinguinal, iliohypogastric nerves from first lumbar nerve. Each of the thoraco abdominal nerve gives off branch as posterior, lateral and anterior cutaneous branch to supply the skin of abdomen and back. Lateral cutaneous nerve supplies lateral abdominal wall after passing through TAP and terminal branches pierces the rectus sheath and rectus muscle to continue as anterior cutaneous nerves. Aberrant nerves may arise from lateral aspect of linea semilunaris and may cause block sparing. Ilioinguinal and iliohypogastric(L1) may also receive contributions from T12 and L2-L3. These nerve passes through anterior inferior abdominal wall into the TAP and have indistinct course which explains patchy block.
The TAP block is an alternative technique for abdominal wall analgesia. The distribution of sensory blockade is different to that seen with rectus sheath. Multiple approaches have been described – namely subcostal, posterior, or lateral. Niraj [[i]] demonstrated adequate analgesia on auditing 4 quadrant TAP blocks for abdominal surgery after single shot levobupivacaine followed by catheters placed either subcostally or posteriorly. Only 10% were deemed to have therapeutic failure i.e. necessitated a rescue PCA. Maeda [[ii]] inserted bilateral subcostal TAP multiorifice catheters for analgesia after living liver donation demonstrating reduction in both opioid consumption and nausea and vomiting. Interestingly, the catheters were inserted following serial hydro-dissection of 10-15cm along the TAP plane. On cold touch testing for this technique, only 3 dermatomes were reliably covered.
[i] Niraj et al. Four quadrant Transversus Abdominis plane block and continuous Transversus Abdominis plane analgesia: a 3-year prospective audit in 124 patients. Journal of Clinical Anesthesia (2015) 27, 579-584
[ii] Maeda et al. The efficacy of continuous subcostal transversus abdominis plane block for analgesia after living liver donation: a retrospective study. J Anesth (2016) 30:39-46