The anatomy of the anterior abdominal wall has been inconsistently described since the early 1900’s (1). This corresponds with various approaches to regional anesthesia of the abdomen, and wide variability in reporting the sensory blockade achieved with each of these approaches.
The anterolateral abdominal wall is innervated by the intercostal nerves (T6-‐T11), the subcostal nerve (T12) and ilioinguinal/iliohypogastric nerves (L1). These all ultimately branch from their respective anterior rami of the T6-‐L1 spinal nerves, which explains the segmental cutaneous supply of the abdominal wall as shown in Figure 1.
Figure 1: The approximate segmental distribution of the cutaneous nerve on the anterior trunk. Courtesy of Williams (2)
The intercostal and subcostal nerves enter the abdominal wall between the interdigitations of the diaphragm and transversus abdominis. They travel anteriorly in the neurovascular transversus abdominis plane (TAP) between the internal oblique and transversus abdominis muscles, as shown in Figure 2 (3). In a cadaver study Rozen et al highlight that the nerves within this plane are deep to a distinct thin fascial sheet within the TAP that is not adherent to the internal oblique muscle and extends medially from the linea semilunaris, as shown in Figure 3. Identifying injection of local anaesthetic within the TAP deep to this fascia and on top of the transversus abdominalis muscle is described as essential to ensure appropriate spread and clinical effect (1, 4).
Figure 2: The path of the intercostal nerve from the spine, through the transversus plane, and posterior rectus sheath to form the anterior cutaneous nerves. Courtesy of Ellis et al (3) and Williams (2)
At the mid-‐axillary line the lateral cutaneous nerve branches off the intercostal nerve through the intercostal and external oblique muscles. These branches provide innervation to the external oblique muscle, and cutaneous supply from the edge of the rectus anteriorly to the erector spinae posteriorly. The lateral branches of the subcostal and iliohypgastric nerves develop very proximally in the TAP, and provide sensory innervation to the skin over the upper lateral aspect of the buttock to the level of the greater trochanter (3, 5, 6).
The segmental T6-‐T8 branches emerge from the costal margin and enter TAP between the midline and the anterior axillary line. Branches from T9 may enter medially or laterally to the anterior axillary line. Nerves located at the anterior axillary line between the costal margin and the inguinal ligament will have variable segmental origin from T9-‐L1 only, with no contribution from levels more cranial than this (1).
The intercostal and subcostal nerves continue anteriorly in the TAP to pierce the posterior rectus sheath and form the anterior cutaneous nerves that provide innervation to the rectus muscles and overlying skin (7-‐9).
Figure 3: (Left) Dissection of the right anterolateral abdominal wall revealing the nerves of the TAP and rectus sheath. The relationship of the nerves to the DCIA is tracked by the colored pins in the lower half of image, and to the DIEA in the upper half of image. (Right) Schematic representation of the same. Courtesy of Rozen et al (1) and Gray (10) KEY: White flag #1 = level of umbilicus. White flag #2 = anterior superior iliac spine. Red flag = pubic symphysis. Pins: purple = L1, yellow = T12, blue = T11, pink = T10, orange = T9, green = T8. RA = rectus abdominis muscle. LS = linea semilunaris. TA = transversus abdominis muscle. IO = internal oblique muscle. EO = external oblique muscle. F = fascial layer deep to internal oblique.
All thoracolumbar nerves that innervate the anterior abdominal wall travel as multiple mixed segmental nerves. Extensive branching and communication of the segmental nerves occurs at various locations, with various plexus being identified (1):
• Intercostal plexus – relates to large branch communications in the anterolateral TAP, sometimes referred to as the ‘upper TAP’ plexus (11).
• TAP plexus – associated with the deep circumflex iliac artery (DCIA) within the TAP, sometimes referred to as the ‘lower TAP’ plexus (11).
• Rectus sheath plexus – associated with the deep inferior epigastric artery (DIEA) within the rectus sheath.
In progressing laterally to medially in the TAP to rectus sheath, the extent of this branching phenomenon increases. In recognising that every segmental origin contributes to at least two nerves in the anteromedial TAP (12), significant cutaneous anaesthesia is likely to require anaesthesia of at least two sequential nerves.
The clinical application of this information is illustrated in a study by Stoving et al. The authors measured the sensory cutaneous block area and motor block effects following an ultrasound guided posterior TAP block using 20ml of 0.75% ropivacaine on one side, and water as a control on the other (4). As shown in Figure 4, the sensory block after 90 minutes represented a non-‐dermatomal pattern, with a caudal-‐lateral dominance to the coverage. A motor block was consistently measurable with a significant effect noted on all three muscle layers of the lateral abdominal wall.
Figure 4: The cutaneous sensory block area 90 minutes after an unilateral ultrasound guided posterior TAP block in 20 study volunteers. Courtesy of Stoving et al (4)
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