Anterior Cutaneous Nerve Entrapment Syndrome

A chronic neuropathic pain syndrome of the abdominal wall caused by entrapment of anterior cutaneous branches of 7 to 12th intercostal nerves along the lateral border of the anterior rectus abdominis fascia causing severe pain and tenderness of the involved dermatome.

Epidemiology

Around 20% of patients with chronic abdominal pain have abdominal wall pain, which is often caused by ACNES. The prevalence in patients alleged to have functional abdominal pain is estimated to be 3-4%. One out of 50 patients consulting an emergency department for abdominal pain suffers from ACNES. The incidence in general population is probably 1: 4000 to 5000. ACNES, hence may be not so rare as is thought.

Clinical description

ACNES most commonly affects young women (75%) but can occur at any age, also in children. History of trauma, pregnancy and delivery or abdominal surgery is sometimes present. Patients present with sharp stabbing pain emanating from the anterior abdominal wall. Presentation can be acute or chronic. Patients can usually localize the source of pain to the spot where the nerve is entrapped (lateral margin of rectus abdominis muscle), palpation of which triggers a lancinating pain along the distribution of the involved nerve. The most frequent involved pain location is in the right lower quadrant (50%), however, all other quadrants can be involved. Pain is always in the same area and is aggravated by exercise. Most patients also have concomitant various pseudo-visceral complains, anorexia, nausea, bloating, altered defecation, (pseudo visceral complaints). Asking the patient to tense his/her abdominal wall will elicit the pain (positive Carnett's test).

Etiology

ACNES is caused by the entrapment of the cutaneous end branches of the intercostal nerve in the muscular foramen as they pass through the rectus abdominis, which probably leads to ischemic neuropathy. Etiology is unclear, but there seems to be a causative relation with any kind of abdominal surgery, pregnancy and sometimes trauma. However, in more than half of the patients there is a sudden onset of pain, without any specific event.

Diagnostic methods

Imaging and laboratory tests are indicated to rule out potential visceral causes of abdominal pain. The diagnosis is clinical. Physical examination reveals altered skin sensibility in the painful area with one specific painful spot and reproducibility of pain by pinching the affected skin area. Subfascial injection of local anesthetic around the entrapped nerve (the trigger point) serves as both diagnostic and can be therapeutic in about 30% of patients.

Differential diagnosis

Differential diagnoses include hernias, tumors, tears and endometriosis of abdominal wall, radiculopathy (diabetic, traumatic, herpetic), herniated vertebral disc, rib and vertebral column abnormalities.

Management and treatment

Most pain killers have no effect on this type of neuropathic pain. Treatment is by injection of the anterior cutaneous nerve with a local anesthetic at the point at which it pierces the fascia by free hand or under ultrasound guidance. Neuropathic pain killers like pregabaline or amitryptilline may be helpful. In patients in whom pain persists after several injections, surgical exploration and neurectomy is indicated. This strategy of injections followed by neurectomy is curative in 80% of patients.

Prognosis

Diagnostic delay is common; untreated ACNES can lead to functional disability and reduced quality of life resulting in high medical health care costs, highlighting the importance of the awareness of ACNES among medical professionals.