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.
Abdominal cutaneous nerve block should be considered in all patients with localized chronic abdominal pain to rule out abdominal wall pain as a possible cause. Ultrasound can reliably be used for infiltration of the abdominal cutaneous nerves and this improves the safety as well as diagnostic utility of the procedure. Abdominal cutaneous nerve ablation should be considered for sustained pain relief in patients with a positive response to the diagnostic injection.
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.
The procedure should be considered in patients with history of chronic abdominal pain where abdominal wall pain is suspected. The diagnosis should be suspected from history and physical findings. A positive carnett’s test provides further substantiation, though the test has only 78% sensitivity and 88% specificity.
The procedure is usually done on an outpatient basis. Ultrasound can reliably be used for infiltration of the abdominal cutaneous nerves. This will improve the safety as well as diagnostic utility of the procedure. The medial border of the rectus muscle and the linea alba are visualized. The rectus muscle can be identified as a hypoechoic area contained in hyperechoic rectus fascia. The transducer is moved laterally to obtain a view of the lateral end of the rectus muscle and the linea semilunaris. Both the linea alba and the semilunaris appear as hyperechoic bands. Approximately 0.5–1.0 cm medial to the linea semilunaris, the abdominal cutaneous nerve can be seen as a “hyperechoic dot” within the muscle bulk. This usually corresponds to the maximal tender point marked previously.
Local anaesthetic and steroid can be injected under real-time ultrasound guidance targeting the abdominal cutaneous nerve at the point of maximum tenderness. A positive response confirms the diagnosis of abdominal cutaneous nerve entrapment and can also provide therapeutic benefit lasting weeks to months.
Pulsed radiofrequency treatment can be applied to the abdominal cutaneous nerves for patients who get a positive response to local anaesthetic blocks. Pulsed radiofrequency can provide sustained pain relief in patients with chronic abdominal wall pain or abdominal cutaneous nerve entrapment syndrome (ACNES).
Injections are generally avoided in patients with systemic infection or skin infection over puncture site, bleeding disorders or coagulopathy, allergy to local anaesthetics or any of the medications to be administered.
Complications are rare, particularly if injections are performed under ultrasound guidance. Severe allergic reactions to local anaesthetics are uncommon. Post-procedural pain flare-up can occur in some patients especially if steroids are used in the injection.