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To the Editor, The lateral cutaneous branches (LCB) of the thoracoabdominal nerves (T6 to L1) arise proximal to the angle of the rib, run a short distance with the main nerve, and emerge obliquely through the overlying muscles in the midaxillary line. They pass superficially to supply the skin of the lateral thorax, the abdomen, the iliac crest, and the upper thigh as far as the greater trochanter of the femur. As previously described, it is rare to produce block of the LCB of the subcostal (T12) and iliohypogastric (L1) nerves when performing ultrasound-guided posterior transversus abdominis plane (TAP) block. The subcostal and iliohypogastric nerves normally send out their LCB preceding entry or very proximal in the TAP. The LCB leave the TAP in a more posterior position than the local anesthetic of the ultrasound-guided posterior TAP block, which appears on imaging as being restricted from spreading posterior to the extent of the muscle belly. The subcostal and iliohypogastric nerves pass deep over the anterior surface of the quadratus lumborum muscle, which extends from the 12th rib to the iliac crest. The subcostal nerve then continues a short distance deep to the aponeurotic posterior extension of the transversus abdominis muscle before passing through the aponeurosis into the TAP. The iliohypogastric nerve continues deep to the transversus muscle aponeurosis and belly to penetrate the transversus in a more anterior and highly variable position. Local anesthetic injected between the transversus abdominis muscle and its deep investing transversalis fascia will spread over the inner surface of the quadratus lumborum muscle and block the proximal portions of the T12 and L1 nerves. This will produce block of both the anterior and the lateral branches of these nerves. This transversalis fascia block (TFP) targets these nerves anatomically between the lumbar plexus block and the TAP block. With the patient in a supine position, the needle is advanced from the anterior using an in-plane technique. A linear or curvilinear ultrasound probe is orientated transversely over the lateral abdomen between the iliac crest and the costal margin. The external oblique, internal oblique, and transversus abdominis muscles are imaged, and the more posterior transversus aponeurosis is distinguished. The reflection of the peritoneum curving away from the muscles from anterior to posterior is identified, and the perinephric fat, which lies behind the peritoneum and deep to the transversalis fascia, identified. The perinephric fat is generally more prominent closer to the iliac crest. The quadratus lumborum is identified medial to the aponeurosis of the transversus abdominis. It may be confused with the partly overlying erector spinae muscle, which is more superficial and often more prominent on ultrasound (Fig. 1). To minimize the risk of peritoneal penetration or liver trauma, the block area should be sufficiently posterior so that the perinephric fat, rather than the peritoneum and liver, underlies the transversalis fascia. To enhance needle visibility, the needle insertion point is selected such that a 100–150 mm needle is introduced relatively perpendicular to the ultrasound beam, and the probe is slid anteriorly to image the needle throughout its course. The end point is more visible if the needle is passed through the posterior ‘‘tail’’ of the transversus muscle, as the transversus aponeurosis is thinner and less distinct as a separate layer. P. D. Hebbard, FANZCA (&) Anaesthesia and Pain Management Unit, University of Melbourne, Melbourne, VIC, Australia e-mail: p.hebbard@bigpond.com

Transversalis fascia plane block, a novel ultrasound-guided abdominal wall nerve block