Peripheral nerve blockade is one of the key strategies for analgesia, improving the quality of postoperative recovery.1 The transversus abdominis plane (TAP) block is a commonly used regional anesthesia technique for the abdominal region. The three primary approaches to the TAP block are subcostal,2 lateral,3,4 and posterior.5 Anesthesiologists select the approach according to the surgical site because subcostal TAP block provides analgesia chiefly to the upper abdomen, whereas lateral TAP and posterior TAP blocks provide analgesia mainly to the lower abdomen.5,6 In surgeries on the lower abdomen, the posterior TAP block is more effective than the lateral TAP block,7,8,9 probably because the local anesthetic injected in the posterior area spreads centrally and affects the lateral cutaneous branches of thoracolumbar nerves before branching or anastomosing and entering the TAP.9,10 The oblique subcostal TAP block11 is a little difficult for beginners; it involves injecting a local anesthetic incrementally in the TAP using a long needle passing along the oblique subcostal line from the xiphoid to the anterior part of the iliac crest, providing a wide anesthetic area. These TAP blocks, excluding the posterior approach, mainly affect the anterior branches, restricting their anesthetic effect to the mid-abdomen.27 indicated in their discussion, the potential residual effect of opioids may affect the results of the pinprick test. Our study in healthy volunteers did not need to consider these confounding factors, and the area of sensory loss was coincident with the anatomical distribution of the thoracoabdominal nerves.

Our study also described novel costal and lateral EXOP blocks. Local anesthetic injection superficial to the external oblique muscle effectively blocked the lateral cutaneous branches of the thoracoabdominal nerves. Interestingly, the costal EXOP injection anesthetized the lateral cutaneous branches of T7–10, whereas the lateral EXOP-ant anesthetized the lateral cutaneous branches of T11–12 (Fig. 3A and B). We suggest that an unknown factor prevented the injectate from spreading over the costal arch. Therefore, both costal and lateral EXOP injections are required to block the lateral cutaneous branches on the entire abdomen. Additionally, since the lateral cutaneous branches depart from their respective anterior rami near the angle of the rib,11,28,29 we tested two approaches for the lateral EXOP block: injection at the level of the anterior or posterior axillary line. The analgesic effect of the lateral EXOP-post injection was unstable and less effective than that of the lateral EXOP-ant injection. This is probably associated with the location of the lateral cutaneous branches that penetrate through the external oblique muscle to supply the skin. The local anesthetic should be administered at the level of the anterior axillary line for an effective lateral EXOP block. We believe that the combined use of the M-TAPA and EXOP blocks to anesthetize the entire abdominal wall may be anatomically plausible (Fig. 4).

Several techniques for blockade of the lateral cutaneous branches have been previously reported. The EOFP21 and EOI22 blocks are similar. The EOFP block involves the injection of the local anesthetic at the T6 level, superficial or deep to the external oblique muscle, around the midclavicular line, while the EOI block involves the injection of the local anesthetic deep to the external oblique muscle, at the level of the sixth rib, just medial to the anterior axillary line. These blocks can anesthetize the lateral cutaneous branches approximately from T6/7 to T10. Intriguingly, the level of the lateral cutaneous branches anesthetized by the costal EXOP injection is almost the same as that of these blocks, but the area of sensory loss is restricted to the abdominal wall. Furthermore, the plane of local anesthetic injection of the EOFP and EOI blocks, deep to the external oblique muscle, is substantially equivalent to that of the I2-TAPA. Nevertheless, the current study showed that I2-TAPA had no analgesic effect. This finding suggests that a local anesthetic administered into the plane deep to the external oblique muscle does not easily spread to the site of the lateral cutaneous branches piercing the intercostal and external oblique muscles when injected at the costal margin. The current study shows that local anesthetic injection superior to the external oblique muscle at the costal margin is more effective for blocking the lateral cutaneous branches in the abdominal area. Moreover, the costal EXOP injection may be a practical procedure because it can be performed with the same probe orientation and motion as the M-TAPA block, but with the needle angled superior to the external oblique muscle. On the other hand, the SII/SIP block23,24,25,26 can also block the lateral cutaneous branches of the thoracoabdominal nerves from T7 to T11. It involves local anesthetic injection into the fascial plane between the serratus anterior and external intercostal muscles on the mid axillary line. The block is achieved by single injection at the level of the 8th rib23 or by multiple injections of a small dose (3 mL) for each target dermatome.24,25,26 These blocks, including the EOFP and EOI blocks, are performed on the chest wall, so the anesthesia extended from the thoracic to the upper-umbilical area. We believed that the costal and lateral EXOP blocks are reasonable procedures for abdominal surgery because the anesthetized area is restricted to the abdominal region after the EXOP blocks.

This study had several limitations. First, it was a pilot study with a small number of healthy volunteers. All participants who volunteered in this study were younger than 50 yr, whereas abdominal surgery is applicable for all age groups. The spread of the local anesthetic may vary with age and body size because of multiple factors, including muscular degeneration and other anatomical changes. Therefore, cadaver and clinical studies are warranted to evaluate the injectate spread and clinical effectiveness of EXOP blocks. Furthermore, we used 20 mL of ropivacaine 0.2% for each injection in this volunteer study. Since 60 mL is required for each side (total of 120 mL) when performing the bilateral M-TAPA and EXOP blocks to anesthetize the entire abdominal wall in accordance with our protocol, attention should be paid to local anesthetic systemic toxicity and the need to reduce the concentration of local anesthetics. More studies evaluating the optimal volume and concentration of local anesthetics will be informative.

In this proof-of-concept pilot study, we showed two things: 1) The M-TAPA block anesthetized only the anterior branches from T6/7 to T11/12, and 2) the costal and lateral EXOP blocks anesthetized the lateral cutaneous branches of T7–10 and T11–12, respectively. This study shows that the combined use of these blocks to anesthetize the entire abdominal wall may be anatomically plausible. Further clinical study is warranted to optimize the volume, concentration, and type of local anesthetic. We believe that blocking the lateral cutaneous branches of thoracoabdominal nerves may offer increased analgesia in abdominal surgeries.