Clinical anatomy

The anterolateral abdominal wall comprises three musculoaponeurotic layers. From deep to superficial these are the tranversus abdominis, internal oblique, and external oblique muscles. The subcostal (T12) nerve is the ventral primary ramus of the T12 spinal nerve. It follows a similar course to other intercostal nerves in the subcostal groove of the rib. The subcostal nerve ends by innervating the upper part of the rectus abdominis muscle and the skin overlying it. Its lateral cutaneous branch innervates the skin of the anterior buttock between the iliac crest and greater trochanter.

The ilioinguinal and iliohypogastric nerves are branches of the primary ventral ramus of L1, which stems from the lumbar plexus and immediately receives a branch from the 12th spinal nerve. They run parallel to the intercostal (T1–T11) and subcostal (T12) nerves, which are located in the intercostal spaces and below the 12th rib respectively. The L1 primary ventral ramus enters the upper part of psoas major where it commonly branches into the ilioinguinal and iliohypogastric nerves which emerge at the lateral border of the psoas major, anterior to the quadratus lumborum and posterior to the kidneys. At the lateral border of the quadratus lumborum, the two nerves pierce the lumbar fascia to reach the plane between the internal oblique and transversus abdominis. They then slope down and around the abdominal wall.

The iliohypogastric nerve is situated cephalad to the ilioinguinal nerve. At the level of the iliac crest, the iliohypogastric nerve divides into two terminal branches, the lateral cutaneous branch and medial cutaneous branches. The lateral cutaneous branch perforates the internal and external oblique and supplies the skin over the ventral part of the buttocks. This innervated area is behind that innervated by the subcostal nerve. The medial cutaneous branch continues ventrally until it pierces the internal oblique muscle above the anterior superior iliac spine, slopes downward between the internal oblique and external oblique muscles (Fig. 35.1), then pierces the external oblique aponeurosis 3 cm above the superficial inguinal ring, and ends by innervating skin over the lower part of the rectus abdominis and front of the pubis.

Figure 35.1 Cadaver structures illustrating anatomy pertinent to the inguinal block technique. 1: anterior superior iliac spine; 2: pubic tubercle; 3: inguinal ligament; 4: external oblique aponeurosis (retracted); 5: internal oblique muscle; 6: iliohypogastric nerve.

The ilioinguinal nerve runs ventrally, caudad to, and in a deeper plane than the iliohypogastric nerve. It perforates the transversus abdominis at the level of the anterior superior iliac spine and continues ventrally deep to the internal oblique (Fig. 35.2). Gradually, it pierces both internal and external oblique to reach the lower border of either the spermatic cord (in males) or the round ligament of the uterus (in females), where it finally reaches the inguinal canal. It contributes fibers to the internal oblique, the skin of the upper medial part of the thigh, and either the skin of the upper part of the scrotum and the root of the penis or the skin covering the labium majus and the mons pubis.

Figure 35.2 The external oblique close to the inguinal ligament is now retracted, illustrating the ilioinguinal nerve. 1: ilioinguinal nerve.

The genitofemoral nerve arises from the first and second lumbar nerves and consists mainly of sensory fibers with a motor component to the cremaster muscle (cremasteric reflex). It lies within the fascial lining of the abdomen by piercing the psoas muscles and psoas fascia near its medial border opposite the third or fourth lumbar vertebra. It descends under the peritoneum on the surface of the psoas major and crosses obliquely behind the ureter. At a variable distance above the inguinal ligament, the nerve divides into the genital (external spermatic) and femoral (lumboinguinal) branches. The femoral branch is the cutaneous nerve to the femoral triangle. Branches of the femoral branch descend laterally to the external iliac artery, behind the inguinal ligament, and through the fascia lata into the femoral sheath. The femoral branch supplies the skin over the upper part of the femoral triangle and communicates with the intermediate cutaneous nerve of the thigh. The genital branch (external spermatic) crosses the lower end of the external iliac artery and enters the inguinal canal through the internal (deep) inguinal ring. It passes through the superficial inguinal canal close to the pubic tubercle. It supplies the cremaster muscle and traverses the inguinal canal in the spermatic cord to the end of the skin of the scrotum (anterolateral aspect). In women, the genital branch accompanies the round ligament of the uterus and ends in the skin of the mons pubis and labium majus.

Great variation of the sensory nerves to the inguinal region is not uncommon, there being communication between branches of the genitofemoral, ilioinguinal, or iliohypogastric nerves.

Ilioinguinal/iliohypogastric nerve block

Surface anatomy

The important bony structure for the ilioinguinal/iliohypogastric nerve block is the anterior superior iliac spine. The needle insertion site for ilioinguinal and iliohypogastric nerve blocks is 1 cm medial and 1 cm inferior to the anterior superior iliac spine (i.e. above the inguinal ligament) (Fig. 35.3).

Figure 35.3 Inguinal block technique: ilioinguinal and iliohypogastric nerves. The needle insertion point is 1 cm medial and 1 cm inferior from the anterior superior iliac spine (above the inguinal ligament). The needle is inserted perpendicular to the skin.

Sonoanatomy

Perform a systematic anatomical survey from the iliac crest to the lower abdomen. The abdominal wall is scanned about 5 cm cranial to the anterior superior iliac spine. A sagittal oblique transducer orientation is used (Fig. 35.4). At this point, all three muscle layers of the abdominal wall can easily be identified by ultrasound and facilitate orientation (Fig. 35.5). The peritoneum and bowel are seen deeper to these (Fig. 35.5). The nerves appear as hypoechoic fascicular structures with hyperechoic rims sandwiched between the layers of muscle (Fig. 35.6). Trace the course of the nerves from above ASIS and distally towards the inguinal region. The iliohypogastric and ilioinguinal nerves consistently lie between the internal oblique and transversus abdominis muscles here. The recommended injection site for landmark-based approaches is situated medial to the anterior superior iliac spine. At this site, both nerves are often penetrating the internal oblique muscle. Performing a ‘blind’ technique here may result in difficulty for the injected local anesthetic to reach both nerves if they are not lying in the same compartment. This is a possible explanation for the high failure rates of 20–30%. It is more likely to reach both nerves with local anesthetic using the landmark-based approach where the nerves are lying in the same layer of the abdominal wall. Small vessels are frequently seen to accompany nerves within the plane.

Figure 35.4 A sagittal oblique transducer orientation is used for the ultrasound-guided ilioinguinal/iliyohypogastric nerve block.