ANTERIOR CUTANEOUS NERVE ENTRAPMENT SYNDROME

Anterior Cutaneous Nerve Entrapment Syndrome (ACNES) is a chronic painful condition of the anterior abdominal wall mimicking intra-abdominal pain. It is caused by the entrapment of anterior cutaneous branches of intercostal nerves (7- 12) along the lateral border of the anterior rectus abdominis fascia. It is a neuropathic pain that causes severe pain and tenderness of the involved dermatome.

ACNES is one of the causes of functional abdominal pain in 3 -4% of population. ACNES is the most common cause of pain in upto 30% of patients with chronic abdominal pain.

Etiopathogenesis:

The sensory branches of the intercostal nerve runs in a plane between internal oblique and transversus abdominis muscles. The thoracic nerves then advance to the posterior wall of the rectus sheath and each enters a neurovascular channel in the rectus muscle which supplies the skin. The neurovascular channel has a fibrous ring, which allows the anterior cutaneous nerve. When the muscular foramen through the rectus abdominis becomes site of nerve compression, ischemic neuropathy occurs, resulting in pain.

Though etiology is unclear, there is a causative relation between pregnancy, laparoscopic surgery, abdominal surgery and trauma. Ongoing pain leads to central sensitization, causing increased sensitivity to touch and pain, because of effect of persistent pain on neuroplasticity of the central nervous system. This leads to chronic abdominal pain, anxiety, distress and depression.

Presentation:

It most commonly affects young women but can occur at any age, even in children. Young women were often referred to psychiatry services for consultation, ACNES being misdiagnosed as functional pain. The most common presentation is the sharp, stabbing pain in the anterior abdominal wall. The pain can be acute or chronic and patients can localize the pain to the area where nerve is entrapped, usually the lateral margin of rectus abdominis muscle.

Most patients will have associated vomiting, nausea, anorexia, bloating, pseudo visceral defecation (pseudo visceral complaints). Positive Carnett’s test – eliciting pain when abdominal muscles are contracted is a clinical sign that can be elicited in most patients.

Valleix phenomenon, the retrograde radiation of pain from an entrapment neuropathy may mimic thoracic radiculopathy.

Radiation of pain can occur with contraction of muscles suggesting entrapment occurs within muscles. This can be tested by eliciting Carnett’s test where in pain is elicited by contracting abdominal muscles. Pressure over the nerve passage in the rectus sheath will cause pain and this is called positive Hover sign.

Diagnosis:

ACNES can be diagnosed by local anesthetic injection into the neurovascular channel in the rectus muscle. The local anesthetic solution not only blocks the sensory transmission thus alleviating pain, but also reduces compressive herniation through fibrous channel by hydro dissection of tissue causing nerve entrapment.

The local anesthetic solution can be injected by identifying oval shaped depressions, palpated on the lateral edge of the rectus muscle corresponding to the aponeurotic openings of the thoracic nerves. The needle is advanced through the skin, subcutaneous tissue and aponeurosis into the fatty plug surrounding the nerve. A nerve stimulator can also be useful to identify the nerve.

Alternatively, ultrasound can be utilized to identify the entrapment and inject local anesthetic accurately, giving minimal discomfort to the patient.

Management:

Management of ACNES involves multi-disciplinary approach with education, rectus muscle stretch exercise and reduction of precipitating factors.

Acute flare up of ACNES can be managed with ice application, abdominal binders and TENS (Transcutaneous electrical nerve stimulation). Since this is a neuropathic pain, most patients benefit with use of co analgesics like paracetamol, pregabalin, gabapentin or amitriptyline.

Topical application of capsaicin cream and 5% lignocaine plasters are also useful if available.

Minimally invasive techniques such as nerve stimulator or ultrasound guided injection of local anesthetics and corticosteroids will help in pain relief for 10 – 12 weeks.

Neuromodulation with pulse radiofrequency lesions have been used with variable success.

The injection of these drugs has a significant risk of secondary nerve injury resulting in iatrogenic neuropathic pain on long term.

In cases of refractory pain, surgical neurectomy of the affected nerve has also been tried to treat ACNES.

Intraperitoneal on lay mesh reinforcement may prevent the entrapment of anterior cutaneous nerves and has been proposed as a treatment option in patients with refractory ACNES with varying results.

REFERENCES

  1. Thompson C, Goodman R, Rowe WA, et al. Abdominal wall syndrome: A costly diagnosis of exclusion (abstract). Gastroenterology 2001; 120:A637.
  2. Srinivasan R, Greenbaum DS. Chronic abdominal wall pain: a frequently overlooked problem. Practical approach to diagnosis and management. Am J Gastroenterol 2002; 97:824.
  3. McGrady EM, Marks RL. Treatment of abdominal nerve entrapment syndrome using a nerve stimulator. Ann R Coll Surg Engl 1988; 70:120.
  4. van Assen T, Brouns JA, Scheltinga MR, Roumen RM. Incidence of abdominal pain due to the anterior cutaneous nerve entrapment syndrome in an emergency department. Scand J Trauma Resusc Emerg Med 2015; 23:19.
  5. Boelens OB, Scheltinga MR, Houterman S, Roumen RM. Randomized clinical trial of trigger point infiltration with lidocaine to diagnose anterior cutaneous nerve entrapment syndrome. Br J Surg 2013; 100:217.
  6. Oor JE, Ünlü Ç, Hazebroek EJ. A systematic review of the treatment for abdominal cutaneous nerve entrapment syndrome. Am J Surg 2016; 212:165.
  7. van Assen T, Boelens OB, van Eerten PV, et al. Long-term success rates after an anterior neurectomy in patients with an abdominal cutaneous nerve entrapment syndrome. Surgery 2015; 157:137.
  8. Applegate WVAbdominal cutaneous nerve entrapment syndrome (ACNES): a commonly overlooked cause of abdominal pain Perm J, 6 (2002), pp. 20-27