Anatomical Considerations

The key to understanding nerve blocks of the abdominal wall is knowledge and application of the anatomy. There are three muscle layers within the abdominal wall, each with an associated fascial sheath. From superficial to deep, these are the external oblique, internal oblique, and transversus abdominis. In addition, the paired rectus abdominis muscles form a muscle layer either side of the midline.

The skin and fascia of the anterior abdominal wall overlie the four muscles that help support the abdominal contents and the trunk, the main nerve supply lying in a plane between the internal oblique and transversus abdominis. Between the internal oblique and transversus abdominis muscles lies a plane that corresponds to a similar plane in the intercostal spaces. It contains the anterior rami of the lower six thoracic nerves (T7–T12) and first lumbar nerve (L1), supplying the skin, muscles, and parietal peritoneum. ¹ At the costal margins, thoracic nerves T7 to T11 enter this neurovascular plane of the abdominal wall, traveling along it to pierce the posterior wall of the rectus sheath as anterior cutaneous branches supplying the overlying skin. The nerves T7 to T9 emerge to supply the skin superior to the umbilicus. The T10 nerve supplies the umbilicus, whereas T11, the cutaneous branch of the subcostal T12, the iliohypogastric nerve, and the ilioinguinal nerve supply the skin inferior to the umbilicus.

The iliohypogastric nerve originates primarily from the L1 nerve root and supplies the sensory innervations to the skin over the inguinal region (▶ Fig. 24.1). It runs in the plane between the internal oblique and transversus abdominis muscles and later pierces the internal oblique to lie between this muscle and the external oblique before giving off cutaneous branches. The ilioinguinal nerve (▶ Fig. 24.1) also originates from the L1 nerve root and is found inferior to the iliohypogastric nerve, perforating the transversus abdominis muscle at the level of the iliac crest running medially in a deeper plane than the iliohypogastric nerve. The ilioinguinal nerve innervates the inguinal hernia sac and medial aspect of the thigh and anterior scrotum and labia. ²

24.2 Variations

Al-Dabbagh et al performed a consecutive series of 110 primary hernia patients by mesh repair, and were particular about identifying and following the course of both the ilioinguinal nerve and iliohypogastric nerves and preserving them. ¹⁶ In 46 of the 110 patients, the course was consistent with anatomical texts, but in the remaining 64 it was variant. These variations included (1) acute inferolateral angulation of the IIN at its exit behind the superficial inguinal ring (SIR) fibers in 20 cases; (2) similar direction of the IIN but in a plane superficial to the external oblique aponeurosis (EOA) and proximal to the SIR in 18 cases; (3) a single stem for both nerves over the spermatic cord in 24 cases, with variation in the subsequent course; (4) absence of one or both nerves in eight cases; (5) accessory IIN or IHN in three cases; and (6) aberrant origin of the IIN from the genitofemoral nerve (GFN) in two cases. They also observed that none of patients had any sensory disturbances or pain in a dermatomal distribution.

Klaassen et al ² dissected and analyzed 100 fixed cadavers. All the nerves were documented where they entered the abdominal wall with the point measured in relation to the anterior superior iliac spine (ASIS). The course was followed and the lateral distance from the midline at termination was measured. The ilioinguinal nerve originated from L1 in 130 specimens (65%), from T12 and L1 in 28 (14%), from L1 and L2 in 22 (11%), and from L2 and L3 in 20 (10%). The nerve entered the abdominal wall 2.8 ± 1.1 cm medial and 4 ± 1.2 cm inferior to the ASIS and terminated 3 ± 0.5 cm lateral to midline.

The iliohypogastric nerve originated from T12 on 14 sides (7%), from T12 and L1 in 28 (14%), from L1 in 20%, and from T11 and T12 in 12 (6%). The nerve entered the abdominal wall 2.8 ± 1.3 cm medial and 1.4 ± 1.2 cm inferior to the ASIS, and terminated 4.0 ± 1.3 cm lateral to the midline. For both nerves, the distance between the ASIS and the midline was 12.2 ± 1.1 cm.

The linear course of each nerve was followed, and its lateral distance from the midline at termination was measured.

24.3 Procedure

24.3.1 Ilioinguinal and Iliohypogastric Nerve Blocks

Inguinal herniorrhaphy pain can be significant and difficult to treat without opioid analgesics, but blocking the iliohypogastric and ilioinguinal nerves can provide good analgesia for most operations in the inguinal region. These blocks can be very effective in reducing the need for opioids, and in pediatric patients they have been found as effective as caudal blocks, albeit with a higher failure rate. ²

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