Tuyen Tran¹, Warren Rozen², Mark Ashton², Michael Barrington¹, Jason Ivanusic³, Geoffrey Taylor²

¹Department of Anaesthesia, St Vincent's Hospital Melbourne, Fitzroy, Victoria, Australia.
²Jack Brockhoff Reconstructive Plastic Surgery Research Unit, Department of Anatomy and Cell Biology, The University of Melbourne, Parkville, Victoria, Australia.
³Department of Anatomy and Cell Biology, The University of Melbourne, Parkville, Victoria, Australia

Background: The Transversus Abdominis Plane (TAP) block is an increasingly utilised technique for postoperative analgesia in a range of abdominal surgical procedures. The block involves injection of local anaesthetic into the neurovascular TAP plane, between the internal oblique and transversus abdominis muscles, with or without ultrasound guidance. Although block efficacy has been reported, descriptions of nerve involvement or regions of sensory blockade remain unclear. The current study comprises a thorough cadaveric study and literature review to comprehensively describe the course of anterior abdominal wall nerves. This improved understanding may enhance the approach and effectiveness of TAP blocks.

Methodology: Twenty cadaveric hemi-abdominal walls were dissected from the anterior axillary line to the midline, tracing the course of the thoracolumbar nerves in the TAP. In five specimens, branches were traced extensively to their origins in their intercostal or subcostal spaces.

Findings: All thoracolumbar nerves innervating the anterior abdominal wall comprise mixed segmental nerves, which branch and communicate widely within the TAP. This communication occurs at multiple locations, including large branch communications anterolaterally (Intercostal Plexus), and plexuses running with the Deep Circumflex Iliac Artery (TAP Plexus) and the Inferior Epigastric Artery (Rectus Sheath Plexus). These are newly described sites for this communication. Rectus abdominis is innervated by segments T6-L1, with a constant branch from L1. The umbilicus is always innervated by T10. All nerves located in the anterior axillary line, between costal margin and inguinal ligament, comprise mixed branches of T9-L1.

Conclusion: Individual nerves in the TAP or rectus sheath do not involve a single segmental nerve. The common injection site for TAP block, between the costal margin and iliac crest, may only cover branches of T9-L1, limiting its use to only lower abdominal surgery or suggesting that multiple injections may be required.