As discussed above, there is a high degree of anatomic variability in not only the course of the nerves but also their branching patterns, areas of penetration of the fascial layers, and dominance patterns [2]. By far, the most consistent location of the II and IH nerves is lateral and superior to the ASIS, where the nerves are found between the transversus abdominis and internal oblique muscular layers in nearly 90 % of the cases [16, 17]. While most of the anatomic variations occur medial to the ASIS, virtually all landmark-guided injection techniques described in the literature are performed in the anterior abdominal wall [2, 3, 18].

Because the existing landmark-based techniques rely on blind infiltration of local anesthetic through different layers, the risk of complications are well understandable: these include inadvertent femoral nerve block [14, 19], colonic puncture [20, 21], and vascular injury [22]. Nevertheless, block failure is still an important matter of concern, and failure rate of the blind technique lies in the range of 10–40 % [14, 23]. This is attributed to the potential for injecting local anesthetics into the wrong abdominal plane [23], and in some situation, it leads to injection inside the peritoneum [24].

As well, the conventional description of the blockade of the genital branch of the GF nerve is mainly a “blind” technique [25, 26] and relies on the pubic tubercle, inguinal ligament, inguinal crease, and femoral artery as landmarks. One involves infiltration of local anesthetic immediately lateral to the pubic tubercle caudal to the inguinal ligament [25]. In another method, a needle is inserted into the inguinal canal to block the genital branch [26]. The blind techniques described are essentially infiltration techniques and rely on high volumes of local anesthetic for consistent results. Although the basis of this landmark is not clear, the needle is likely directed toward the spermatic cord, and important structures of the spermatic cord (testicular artery and vas deferens) or the peritoneum are at risk.

Advantages of ultrasound guidance include the identification of muscular and fascial layers (including structures lying deeper to the peritoneum), the visualization of needle’s trajectory and spread of injectate, and the detection of vascular structures. Ultrasound helps physicians to overcome the wide anatomic variability associated with the structures in this area, helping to identify the nerves themselves, or the plane they lie within. Ultrasound guidance has demonstrated to improve accuracy in the injection of local anesthetics, with less risk of inadvertent injections in the wrong fascial plane [24], reduced amount of local anesthetics used for anesthesia, and lower consumption of systemic pain medications compared to landmark techniques [23, 27].

The IH and II nerves are superficial nerves, best viewed with a linear probe of high frequency (6–13 MHz). In very obese patient, a linear medium-frequency probe may be useful. Most of clinical studies or reports used ASIS as landmark, and the position of the probe was above ASIS and moved along the line joining the ASIS and umbilicus. This approach carries some limitation: when the probe is placed too near to ASIS, the external oblique muscle layer may be missed; when the probe is too far away from the ASIS (the “bone shadow of iliac crest” not in the scan), the nerve seen in between the transversus abdominis and internal oblique muscular layers is likely the 12th intercostal nerve or subcostal nerve instead.

The recommended area for ultrasound scanning of II and IH nerves is lateral and superior to the anterior superior iliac spine (Fig. 54.3), because the two nerves are detectable in this area in 90 % of the cases. The probe is placed cranial and three fingerbreadths lateral to the ASIS with the transducer perpendicular to the inguinal ligament and its lateral end in contact with the iliac crest (Fig. 54.3). At this position, the iliac crest will appear as a hyperechoic structure adjacent to which will appear the three muscular layers of the abdominal wall (Figs. 54.4 and 54.5). Below the transversus abdominis, peristaltic movements of the bowel may be detected. Once the muscular layers are identified, the II and IH nerves will be found in the split fascial plane between the internal oblique and transversus abdominis muscle layers. Both nerves should be within 1.5 cm of the iliac crest at this site, with the II nerve closer to the iliac crest [28]. The nerves are usually in close proximity to each other [9] and located on the “upsloping” split fascia close to the iliac crest. In some cases, the nerves may run approximately 1 cm apart. The deep circumflex iliac artery which is close to the two nerves in the same fascial layer can be revealed with the use of color Doppler (Fig. 54.6). A neural structure within the fascial split may also be seen medial and on the flat part of the internal oblique and transversus abdominis muscle junction: this is the subcostal nerve, and if mistaken for the II and IH nerves, the nerve blockade will result in aberrant distribution of anesthesia.

In some patients (those with previous surgery, multiple childbirth) or when the procedure is performed by inexperienced practitioners, either the II and IH or the fascia split is difficult to visualize; in these cases, the target will be the fascial plane between the internal oblique and transverse abdominal muscles [29].

Both out-of-plane and in-plane techniques can be used and have been described [28, 30]. As the two nerves run very closely within the fascial plane (sometimes as a common trunk) and frequently overlap in sensory innervation, a selective block of one of the two nerves has recently been demonstrated to be very difficult to realize, even in experienced hands and injecting minimal volumes of local anesthetics [31].

Ultrasound-guided blockade of the genital branch of the GF nerve has been described in several review articles [2, 3, 18, 32]. The genital branch is difficult to visualize directly unless a high-frequency probe (up to 18 MHz) is used. Furthermore, the GF nerve can be found within or outside the spermatic cord. Thus, the most reliable method is to infiltrate the inguinal canal and deposit the injectate both inside and outside the spermatic cord [2, 3]. The patient is placed in a supine position, and surface anatomy of the ASIS, inguinal ligament, and femoral artery is established. A linear ultrasound transducer of high frequency (6–13 MHz) is placed over the femoral artery along its long axis (Fig. 54.2). The transducer is moved cephalad over the artery, which starts to descend at a steep angle toward the inguinal ligament. At this point, the femoral artery transitions to become the external iliac artery and runs in a retroperitoneal plane (Fig. 54.7). Once the external iliac artery is identified, the inguinal canal can be viewed superficial to the vessel as an oval, soft tissue structure (Fig. 54.7). This will contain the spermatic cord in men and the round ligament in women. The canal is then scanned medially, so that the final ultrasound probe position is approximately one fingerbreadth away from the pubic tubercle. In males, testicular arteries may be identified in the spermatic cord as pulsatile structures and confirmed with the use of color Doppler (Fig. 54.8). Vessels may be accentuated by asking the patient to cough or perform a Valsalva maneuver, which increases blood flow through the pampiniformis plexus. The vas deferens may also be identified as a thick, tubular structure.