Anterior Cutaneous Nerve Entrapment (ACNES)

Anterior cutaneous nerve entrapment syndrome (ACNES) is a recognised but severely underdiagnosed cause of chronic abdominal pain. The cutaneous branches of the lower thoracoabdominal intercostal nerves pass through a small muscular foramen at the lateral border of the rectus abdominis muscle. When these branches become entrapped at this point, blood flow to the nerve is compromised, producing ischaemic neuropathy and chronic pain in the distribution of one or more abdominal dermatomes.

Prevalence is estimated at 15–30% of chronic abdominal pain presentations; in adolescents, ACNES accounts for 1 in 8 cases of chronic abdominal pain.

**Symptoms:** - Severe, often refractory, chronic pain just lateral to the abdominal midline — right or left side - Pain typically extends from the lower rib border down to just above the groin - Pain can be pinpointed to a precise spot on the abdominal wall - May radiate around to the back or up and down the abdominal wall - Not accompanied by gastrointestinal symptoms (diarrhoea, constipation, altered bowel habit, weight loss)

**Carnett's test:** When the patient tenses the abdominal muscles (asked to "sit up"), pain worsens if the source is in the abdominal wall (positive test — consistent with ACNES). If a visceral cause is present, tensing the muscles usually diminishes pain by protecting the internal organs (negative test).

**Causes:** The exact aetiology is unclear. Predisposing factors include previous abdominal surgery, pregnancy, and trauma. However, in more than half of patients the pain begins suddenly with no identifiable precipitating event.

Diagnosis requires excluding underlying gastrointestinal or abdominal pathology first. A diagnostic local anaesthetic injection at the point of maximum tenderness both confirms the diagnosis and provides temporary relief.