Ultrasound-guided abdominal plane blocks have become essential for early functional recovery after abdominal surgery [
1]. A thorough understanding of the abdominal nerve supply is required for effective use of these blocks. The midline abdomen is innervated by the anterior branches of the thoracoabdominal nerves originating from the T6–T12 intercostal nerves, while the lateral abdomen is innervated by the lateral cutaneous branches (LCBs) [
2]. Therefore, an LCB blockade is required for abdominal surgeries involving lateral incisions. Currently, several block approaches for anesthetizing the LCBs of the abdomen have been reported, including the external oblique fascial plane (EOFP) [
3], external oblique intercostal (EOI) [
4], serratus-intercostal interfascial plane (SIP) [
5], and external oblique muscle plane (EXOP) blocks [
6]. The EOFP and EOI blocks are performed by instilling local anesthetics (LA) deep to the external oblique muscle (EOM) at the sixth intercostal space. The SIP block involves injecting LA into the serratus-intercostal space along the mid-axillary line at the eighth intercostal space. These techniques can anesthetize the LCBs and are applicable to lateral incisions from the thoracic to supraumbilical regions [
3–
6]. Although the recently described costal and lateral EXOP injections can block the LCBs of the entire lateral abdomen when combined, this technique requires multiple injections with large volumes of LA, raising concerns regarding logistics and the risk of LA toxicity [
7]. In this proof-of-concept pilot study, we propose a modified version of the EOI block with a different needle insertion point. This technique involves injecting LA deep into the EOM at the tenth rib along the mid-axillary line followed by continuous caudal hydrodissection. We applied and assessed this approach in three patients who underwent robot-assisted partial nephrectomy (RAPN) and conducted a cadaveric study for further evaluation.
Case Report
This case series involved three patients who received modified EOI blocks for analgesia during RAPN (
Table 1). All patients provided written informed consent for the publication of this study. The Institutional Review Board of Juntendo University Medical School (Tokyo, Japan) waived the review requirements for this case series.
All surgeries were performed under standard general anesthesia induced with remifentanil (0.3–0.5 μg/kg/min), propofol (1.0–1.5 mg/kg), and rocuronium (0.6–0.7 mg/kg) and maintained with sevoflurane (1.5%) and remifentanil (0.05–0.3 μg/kg/min). The patients received 1 g of acetaminophen at the completion of surgery. Intraoperative fentanyl was titrated as needed for visceral pain. The surgeries were performed using the da Vinci Xi system via the retroperitoneal approach with multiple port holes and surgical incisions for tumor extraction extending from the anterior to the posterior axillary line at the level of dermatomes T10–T12 (
Fig. 1).
A modified EOI block was performed with the patient in the supine position. Using a SonoSite S II ultrasound system (Fujifilm), a linear ultrasound probe (15–6 MHz) was placed on the lateral abdomen in the sagittal orientation at the mid-axillary line to visualize the EOM and the cross-section of the tenth rib. At this location, the EOI plane was identified between the fascia of the EOM and the tenth rib/intercostal muscle (
Fig. 2A). Subsequently, a 20 gauge (G), 80-mm Tuohy needle (Hakko Medical) was inserted in-plane in the cephalocaudal direction (
Fig. 2B). The initial tip endpoint was positioned at the superior end of the tenth rib along the mid-axillary line (
Figs. 3A and
B). When the needle came in contact with the rib, LA was injected to continuously hydrodissect the EOI plane, with a further 4–5 cm of needle advancement. Thirty milliliters of 0.25% levobupivacaine was injected, and extensive spreading along the mid-axillary line was visualized on ultrasound in both the EOI and fascial planes between the EOM and internal oblique muscle (
Figs. 2C and
D).
The block procedures were performed immediately after the surgery was completed while the patients were still anesthetized except for Case 3, as this patient had an EOI catheter planned for pain management due to a previous experience of intense pain from a contralateral RAPN using the same surgical approach under general anesthesia alone. A single-shot modified EOI block was performed following induction of general anesthesia, and a catheter was inserted postoperatively to avoid interference with the surgical field. An 18-G Tuohy needle was introduced into the EOI plane at the tenth rib on the mid-axillary line. A catheter with four side holes (Contiplex Ultra 360, B.BRAUN) was then threaded 4–5 cm from the tip after hydrodissecting the EOI plane with normal saline. The catheter was secured to the skin at the ninth rib, ensuring a sufficient distance from the abdominal incision site (
Figs. 2B and
3A). At 12 h and 24 h after catheter insertion, 20 ml of 0.25% levobupivacaine was administered via the catheter. Postoperative pain management included scheduled acetaminophen (1 g) three times a day and fentanyl-based intravenous patient-controlled analgesia (no basal infusion, rescue doses 20–30 μg, lockout 10 min).
The patients received effective postoperative analgesia without opioid use either in the post-anesthesia care unit or in the ward after RAPN. A pinprick test performed 3 h postoperatively revealed consistent sensory changes in the T8–T12 dermatomes along the anterior-to-posterior axillary lines (
Fig. 1). Additional analgesics were not required on the ward except for routine acetaminophen infusions. No block-related complications were observed upon discharge from the ward.
An anatomical evaluation of this technique was also conducted with the approval of the Institutional Ethics Committee of Juntendo University Medical School (approval number: 20-304). Under ultrasound guidance, two EOI injections were performed using the same modified approach described for the case series on a soft-embalmed Thiel cadaver. The injectate comprised 20 ml of 0.1% methylene blue solution. After the injection procedure, an anesthesiologist (T.F.) and an experienced anatomist (H.A.) meticulously dissected the thoracoabdominal wall to avoid the unintended spread of the dye. On anatomical dissection, the injectate was deposited within the EOI plane from the eighth to eleventh ribs and diffused into the fascial plane between the EOM and internal oblique muscle. On both sides of the lateral abdomen, the LCBs T8–T12 were stained before penetrating the EOM (
Figs. 4A and
B).
Discussion
In this case study, we describe a modified EOI block that anesthetizes the entire lateral abdomen and provides effective analgesia after RAPN. This approach consistently demonstrated dermatomal sensory coverage from T8 to T12 along the anterior-to-posterior axillary line, which aligned with the anatomical findings. We believe that this approach could expand the current ultrasound-guided abdominal plane block options for lateral abdominal pain management.
Few studies have focused on regional techniques that provide effective analgesia for the entire lateral abdomen (
Fig. 3B). Hamilton and Manickam [
3] identified the EOI plane as suitable for blocking the T6–T10 LCBs, forming the basis for the EOFP block for upper abdominal analgesia. Elsharkawy et al. [
4] further developed this concept into the EOI block, targeting the anterior and LCBs of the T6–T10 nerves. The SIP block targets the lower intercostal nerves at the mid-axillary line [
5]. Additionally, these techniques have been applied in supraumbilical surgeries [
4,
5,
8,
9].
The thoracoabdominal nerve block through the perichondrial approach (TAPA) has been suggested to anesthetize both the lateral cutaneous and anterior branches of the thoracoabdominal nerves [
10]. However, while superficial TAPA injections target the LCBs by injecting beneath the EOM at the tenth costal cartilage, a recent volunteer and cadaveric study found that superficial injections targeting the same plane as the EOI block failed to anesthetize the lateral abdomen [
6,
11]. The author speculated that injection into the plane underneath the EOM at the ninth–tenth costal cartilages may not effectively reach the LCBs beyond the costal arch because of strong connective tissue binding at the costal arch.
The LCBs originating from the transversus abdominis plane (TAP) along the mid-axillary line pass through the EOM and provide sensory innervation to the lateral abdomen [
2,
7]. Therefore, the mid-axillary line is a logical anatomical target for efficiently blocking the LCBs. In addition, connective tissue binding to the costal arch is not a concern at this location. Therefore, we hypothesized that injecting LA into the EOI plane along the mid-axillary line would enable a more reliable and extensive LCB blockade, and this was supported by the initial successful outcomes in our case series and cadaveric evaluation.
Recently, costal and lateral EXOP blocks have been used to anesthetize the T7–T10 and T11–T12 dermatomes, respectively [
6]. However, the advantage that our approach has over this technique is that only one injection is required to achieve complete anesthesia in the entire lateral abdomen. Incorporating the EOI block using the modified approach alongside regional techniques for the midline, such as the subcostal TAP or rectus sheath block, could efficiently provide anesthesia to the entire abdomen while reducing the risk of LA toxicity and the logistical concerns associated with multiple injections.
Several case reports have demonstrated successful pain control in open surgery using EOI block catheters [
9,
12]. This suggests that a continuous modified EOI block could be applied for analgesia in open surgery, although further evidence is needed to confirm its efficacy in this context.
This study had some limitations. One obvious limitation was its small sample size. However, it was designed as a proof-of-concept pilot study to demonstrate that the modified approach could achieve complete lateral abdominal analgesia. Future research with a larger sample size is required to validate the efficacy of this technique compared with other regional techniques. Second, as catheter placement was not planned in Cases 1 and 2, nerve blocks were performed after surgery to extend the analgesic benefits. However, preoperative blocks may have provided more optimal intraoperative analgesia while minimizing opioid consumption. Third, the administration of fentanyl intraoperatively may have resulted in an underestimation of postoperative pain scores. Finally, this approach was only applied to RAPN procedures involving lateral incisions. However, we chose this group solely as a proof-of-concept and our pilot findings suggest that this approach can also be applied to other surgeries involving lateral incisions such as laparoscopic adrenalectomy.
In summary, the initial successful findings of our case series and anatomical evaluation of the modified version of the EOI block suggest that this technique could be beneficial for treating lateral abdominal pain.
Acknowledgments
We would like to thank Ms. Ayako Ono for the artistic work included in this manuscript.
Fig. 1.
Surgical incision site of robot-assisted partial nephrectomy via the retroperitoneal approach. The shaded area represents the distribution of sensory loss assessed through pinprick testing, and the white asterisk indicates the umbilicus.
Fig. 2.
Ultrasound image, orientation of the ultrasound probe, and needle insertion for the modified EOI block. (A) Sonographic anatomy of the modified approach. (B) Ultrasound probe orientation and needle position for the modified approach to the EOI block along the mid-axillary line. The costal margin is indicated by a dashed line. (C, D) Ultrasound images showing local anesthetic spread following EOI injection using the modified approach. The needle is indicated by the white arrowheads. The local anesthetics diffused extensively into the plane below the EOM in a cephalocaudal direction, extending to the plane between the EOM and ICM. Cep: cephalad, Lat: lateral, EOM: external oblique muscle, ICM: intercostal muscle, IOM: internal oblique muscle, LA: local anesthetics.
Fig. 3.
Schematic representation of the modified approach to the EOI block. (A) Respective dermatomal distribution of the expected sensory block during the modified (pink) and original (blue) EOI approaches. (B) Insertion sites of the corresponding blocks targeting the lateral abdomen. The EOM is depicted in transparency. The needle tip positions for each block are as follows: original EOI: deep to the EOM at the sixth intercostal space, lateral to the mid-clavicular line; modified EOI: deep to the EOM above the tenth rib on the mid-axillary line; costal EXOP: superficial to the EOM at the tenth costal cartilage; lateral EXOP: superficial to the EOM on the anterior axillary line; SIP: deep to the serratus anterior muscle at the eighth intercostal space on the mid-axillary line. EOI: external oblique intercostal, EOM: external oblique muscle, EXOP: external oblique muscle plane, SIP: serratus-intercostal interfascial plane.
Fig. 4.
Cadaveric dissection of the lateral abdomen demonstrating dye spread following EOI injection using the modified approach with 20 ml of methylene blue. Dissection reveals dye staining of the lateral cutaneous branches of T8–T12 on both the left (A) and right (B) sides. The EOM was meticulously dissected from its aponeurosis and inverted laterally to identify the lateral cutaneous branches of the intercostal nerves. Cep: cephalad, Lat: lateral, EOM: external oblique muscle, IOM: internal oblique muscle.
Table 1.
Patient Demographics and Observed Outcomes in the Case Series
|
Case 1 |
Case 2 |
Case 3 |
Patient (sex/age) |
M/50–60 |
F/60–70 |
M/50–60 |
Body mass index (kg/m2) |
28.3 |
35.1 |
30.0 |
ASA physical status |
II |
Ⅲ |
II |
EOI technique |
Single injection |
Single injection |
Single injection and catheter |
Intraoperative fentanyl (μg) |
400 |
300 |
250 |
Duration of surgery (min) |
213 |
186 |
187 |
Dermatomal sensory coverage |
T7–L1 at anterior axillary line T8–T12 at mid to posterior axillary line |
T8–L1 at anterior axillary line T8–T12 at mid to posterior axillary line |
T7–T12 at anterior axillary line T7–T12 at mid to posterior axillary line |
PACU NRS resting |
0 |
0 |
0 |
PACU NRS coughing |
0 |
0 |
0 |
3-h NRS resting |
2 |
0 |
0 |
3-h NRS coughing |
3 |
2 |
0 |
24-h NRS resting |
0 |
0 |
0 |
24-h NRS coughing |
3 |
3 |
0 |
48-h NRS resting |
0 |
0 |
0 |
48-h NRS coughing |
2 |
2 |
0 |
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