The anterolateral abdominal wall muscles (external oblique, internal oblique, and transversus abdominis) are replaced by a well-defined aponeurosis, the linea semilunaris, at the lateral border of the rectus sheath (Fig. 53.2). This is an important sonoanatomical landmark for performing TAP and rectus sheath blocks [8, 9]. The rectus sheath contains the rectus abdominis muscles, the anterior rami of the lower sixth thoracic nerves (T7–T12), the superior and inferior epigastric vessels, and lymph vessels. It is formed by the fusion of the aponeuroses of the three anterolateral abdominal wall muscles. The abdominal wall is innervated by the T6–T11 intercostal and T12 subcostal nerves (Fig. 53.3). The intercostal nerves exit the intervertebral foramina and enter the paravertebral region related to the intercostal muscles posteriorly. Between the midline and the anterior axillary line, segmental nerves T6–T9 emerge from the costal margin to enter the TAP. The T10 segmental nerve is located caudal to the costal margin (rib 10) and T11, T12, and L1 are located in a caudal direction, toward the iliac crest. The TAP contains intercostal, subcostal, and first lumbar (L1) nerves and blood vessels (deep circumflex iliac, inferior epigastric, superior epigastric arteries) (Fig. 53.3). The lateral cutaneous branch of the intercostal nerves divides into anterior and posterior cutaneous branches. The origins of the lateral cutaneous nerves are proximal close to the costal angle. However, the point at which lateral cutaneous nerves pierces muscle layers is more anterior at the angle of rib or midaxillary line [10]. The lateral cutaneous branches are significant because they innervate much of the abdominal wall (T6–T12) and thorax (T1–T5). The anterior cutaneous nerve sends twigs to the external oblique muscle as well as skin to the lateral margin of the rectus abdominis. The posterior branch runs backward supplying the paravertebral region as well as the erector spinae.

The iliohypogastric nerve (IHN) and ilioinguinal nerve (IHN) are branches of L1 and these pass laterally through the psoas muscle, course anterior to the quadratus lumborum and travel caudally toward the iliac crest on the inner surface of the transversus abdominis muscle. The nerves pierce the transversus abdominis muscle to enter the TAP plane at variable locations (Fig. 53.4). They are located in the TAP for a short distance only. Medial to the anterior superior iliac spine, the IIN passes through internal oblique close to the inguinal ligament [11, 12]. It is at this location that landmark techniques usually aim to locate the IIN. The IHN is usually above and medial to it [13]. The IIN and IHN supply the skin and muscles of the pubic and inguinal region and genitalia. In summary, the IHN and IIN are located in the TAP close to the iliac crest and anterior superior iliac spine [14]. Medial and inferior to the anterior superior iliac spine, the nerves are located between the internal and external oblique muscles [15].

The lower five intercostal nerves pierce the lateral margin of the linea semilunaris to enter the rectus sheath posterolaterally. The intercostal (T6–T9), subcostal, and L1 nerves terminate in the rectus abdominis muscle with three patterns: (1) terminate simply within the muscle, (2) supply the muscle and then terminate as a cutaneous branch, or (3) pass through the muscle and terminate as a cutaneous branch [10]. Rozen et al. [16] noted that as the nerves approach the posterior surface of the rectus abdominis, a longitudinal branch of fibers run craniocaudally with the deep inferior epigastric artery. In addition, the cutaneous branches were closely related to the perforating musculocutaneous vessels. Together T7–L1 supply the skin from the xiphoid sternum (T7 nerve root) to the pubic symphysis (L1 nerve root), with T10 nerve supplying the umbilical segment. The IHN nerve supplies the lowermost segment of the rectus abdominis muscle and overlying skin.

The posterior abdominal wall is composed of muscles bound by the thoracolumbar fascia (Fig. 53.5). The thoracolumbar fascia is an extensive tough membranous sheet that envelops the muscles of the posterior abdominal wall, dividing these into anterior, middle, and posterior layers. Key paired muscles of the posterior abdominal wall are psoas major, iliacus, and quadratus lumborum [17].

Infections and skin diseases in the injection area

Surgical dressings obstructing access

Patient refusal

TAP blocks reduce side effects associated with epidural or opioid analgesia failure rate and need for re-siting of block or catheter.

Inadequate analgesic coverage for visceral pain.