Abdominal Cutaneous Nerve Block
Chronic abdominal wall pain occurs in about 10–30% of patients presenting with chronic abdominal pain. A variety of causes such as myofascial pain, thoracic radiculopathy, abdominal wall lesions including nerve entrapment and referred pain secondary to neural convergence from abdominal or thoracic viscera have been mentioned in literature. Of these, abdominal cutaneous nerve entrapment syndrome (ACNES) is a well-recognized entity. Entrapment of abdominal cutaneous nerves at the lateral border of the rectus abdominis muscle has been attributed as a cause of abdominal wall pain.
The thoracoabdominal (T8–12) intercostal nerves run in the muscle plane between internal oblique and transverse abdominis to reach the lateral border of rectus abdominis, where they turn 90 degrees to enter the rectus channel. The nerve and its vessels form a distinct bundle and traverse the rectus channel to continue as anterior cutaneous nerves of the abdomen. There are five such channels on each side of the lateral border of the rectus abdominis. The nerves can be viewed emerging from these foramina under ultrasound.
Although any main branch of the nerve may become symptomatic, the anterior branches are most likely to be affected, because stretching of the nerve is greatest at the point most distant from its origin (ie, the spinal cord). Because the anterior branches enter the back of the muscle at nearly a right angle, they are more susceptible to mechanical irritation than are the posterior and lateral branches, which enter the muscle at a more oblique angle. Lateral branches are affected by lateral bending and twisting of the trunk; posterior branches are affected by bending, lifting, and twisting.