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- Recent findings emphasize the “two-wound model” after abdominal surgery:
- The somatic wound corresponds to the abdominal wall
- The autonomic wound corresponds to the peritoneal layer and visceral component
- Pain after abdominal surgery arises predominantly from parietal somatic afferents, making parietal blocks very effective
- Peritoneal and visceral sensory innervation is provided by visceral afferents running along the sympathetic nerves and joining the dorsal horn at the higher thoracic levels. However, a significant part of these afferents may reach the central nervous system by the vagus nerve, and will not be blocked even by an epidural block
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- The abdominal wall is innervated by intercostal nerves (arising from T6 to T12) and ilioinguinal/iliohypogastric nerves (arising from L1). These nerves are easily blocked throughout their course between the abdominal muscles
- After emerging from the paravertebral space, intercostal nerves lie between the transversus and the obliquus internus abdominis muscles in the so-called transversus abdominis plane (TAP) (Figure 151-4)
- Approximately at the level of the midaxillary line, intercostal nerves give out perforating branches innervating the lateral abdominal wall (Figures 151-1 and 151-2)
- Segmental nerves T6–T9 emerge from the anterior costal margin between the midline and the anterior axillary line
- At the level of the rectus abdominis muscle, intercostal nerves enter the muscle sheath and give out perforating musculocutaneous branches that provide the sensitive innervation of the anteromedial abdominal wall (Figures 151-1 and 151-2)
- Near the anterior superior iliac spine, ilioinguinal and iliohypogastric nerves, previously located in the TAP, move to the space between obliquus internus and obliquus externus muscles (Figure 151-3). They provide the sensory innervation to the inguinal territory and the area just above the pubic symphysis (where Pfannenstiel incisions are performed)
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