Ilioinguinal and iliohypogastric nerves are contained in a well-defined tissue plane between the transversus abdominis and internal oblique muscles. The ability to easily image the musculature of the abdominal wall makes blocking these two nerves much more exact than the “feel-based” blind technique.
Imaging of the abdominal wall medial and superior to the ASIS reveals three muscle layers, separated by hyperechoic fascia: the outermost external oblique (EOM), the internal oblique (IOM), and the transversus abdominis muscles (TAM) (Figures 42.2-1A and B). Immediately below transversus abdominus muscle is the fascia transversalis, located just above the peritoneum and the abdominal cavity below, easily recognized as moving structures due to peristalsis. The hyperechoic osseous prominence of the anterior-superior iliac spine (ASIS) is a useful landmark which can be used as a reference, and is seen on the lateral side of the US image in Figure 42.2-1. The iliohypogastric and ilioinguinal nerves pierce the TAM above the ilium and lie in the plane between the TAM and the IOM. They are often seen side by side or up to 1 cm apart, and they typically appear as hypoechoic ovals. Use of color Doppler may be useful to identify the deep circumflex iliac artery, which lies adjacent to the nerves in the same plane as an additional landmark useful in identifying the nerves.
FIGURE 42.2-1. (A) Ultrasound anatomy of the iliohypogastric and ilioinguinal nerve. (B) Labeled ultrasound anatomy of the iliohypogastric and ilioinguinal nerve, ASIS, anterior superior iliac spine; EOM, external oblique muscle; IOM, internal oblique muscle; TAM, transverse abdominal muscle; IiN, ilioinguinal nerve; IhN, iliohypogastric nerve.
Distribution of Blockade
Block of the iliohypogastric and ilioinguinal nerves results in anesthesia of the hypogastric region, the inguinal crease, the upper medial thigh, the mons pubis, part of the labia, the root of the penis, and the anterior part of the scrotum. There is considerable variation in sensory distribution between individuals.
Equipment needed is as follows:
• Ultrasound machine with linear transducer (6–18 MHz), sterile sleeve, and gel
• Standard nerve block tray
• Syringe(s) with 20 mL of local anesthetic
• 50-100 mm, 21-22 gauge needle
• Sterile gloves
Landmarks and Patient Positioning
The block of the iliohypogastric and ilioinguinal nerves is done in supine position. Palpation of the ASIS provides the initial landmark for transducer placment. This block is often performed under general anesthesia, particularly in pediatric patients.
The goal is to place the needle tip in the plane between the IOM and the TAM, and deposit local anesthetic between the muscle layers.
With the patient supine, the skin is disinfected and the transducer placed medial to the ASIS, oriented on a line joining the ASIS with the umbilicus (Figure 42.2-2). The three muscle layers should be identified. The nerves should appear as hypoechoic ovals between the IOM and TAM muscles. Moving the transducer slightly cephalad or caudad to trace the nerves can be useful. In addition, color Doppler may be used to attempt to visualize the deep circumflex iliac artery. A skin wheal is made on the medial aspect of the transducer, and the needle is inserted in-plane in a medial to lateral orientation, through the subcutaneous tissue, EOM, and IOM, and is advanced toward the ilioinguinal and iliohypogastric nerves (Figure 42.1-1B and Figure 42.2-3). A pop may be felt as the needle tip enters the plane between the muscles. After gentle aspiration, 1 to 2 mL of local anesthetic is injected to confirm the needle tip position (Figure 42.2-4). When injection of the local anesthetic appears to be intramuscular, the needle is advanced or withdrawn carefully 1 to 2 mm and another bolus is administered. This is repeated until the correct needle position is achieved. The block can be done either with in-plane or out-of-plane needle insertion.
FIGURE 42.2-2. Transducer position to image the ilioinguinal (IiN) and iliohypogastric nerves (IhN). The transducer is positioned in the immediate vicinity of the anterior superior iliac spine (ASIS).
FIGURE 42.2-3. Simulated needle path (1) to reach the ilioinguinal (IiN) and iliohypogastric (IhN) nerves.
FIGURE 42.2-4. Simulated needle path (1) and spread of local anesthetic (area shaded in blue) to anesthetize the ilioinguinal and iliohypogastric nerves.
• An out-of-plane technique may be a better option in obese patients. Because the needle tip may not always be seen throughout the procedure, we recommend administering intermittent small boluses (0.5–1 mL) as the needle is advanced through the internal oblique muscle, confirming the position of the needle tip.
In an adult patient, 10 mL of local anesthetic per side is usually sufficient for successful blockade. In children, a volume of 0.15 mL/kg per side (ropivacaine 0.5%) is adequate for effective analgesia when using ultrasound guidance.
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