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A 15-year-old girl presented to the emergency department (ED) after 3 days of pain in her lower left abdomen, with no precipitating factors or visceral symptoms, reported Kazuki Iio, MD, and Yu Ishida, MD, of the Tokyo Medical University Hachioji Medical Center, in a case report published in .

She told clinicians the pain had developed slowly over the previous 3 days, but otherwise she felt fine. Her appetite was not affected, she said, nor did she have any nausea, bloating, diarrhea, or blood in her stool.

On questioning, she noted that over the past 2 years, she had periodically experienced the same symptoms, which lasted about 1 week each time.

Asked about her medical history, she reported taking an oral contraceptive. She had never been pregnant, undergone abdominal surgery, or experienced any abdominal injuries.

Findings of a medical workup from 1 year before presentation, including laboratory tests, upper endoscopy, colonoscopy, and an abdominal CT scan, were all normal.

The ED intake physical showed normal vital signs and body mass index. Abdominal examination showed no rigidity or guarding; however, clinicians identified a 1-cm point of significant tenderness in the left lower abdomen.

On palpation, the patient noted worsening pain in this small spot when she raised her head and tightened her stomach muscles. She experienced loss of sensation in the affected area when clinicians applied an alcohol gauze pad, and pinching the skin caused her severe pain.

Clinicians considered several diagnoses, including post-herpetic neuralgia, celiac artery compression syndrome, and acute intermittent porphyria, before concluding that the symptoms were due to anterior cutaneous nerve entrapment syndrome (ACNES).

Iio and Ishida explained that post-herpetic neuralgia was unlikely given the absence of a rash or "dermatomal distribution of abdominal pain." The patient's abdominal pain did not suggest celiac artery compression syndrome symptoms, which tend to be epigastric and worsen after eating, they noted. "And acute intermittent porphyria typically causes diffuse abdominal pain."

"The key to the correct diagnosis was recognition that the 1-cm point of maximal tenderness was localized to the outer rim of the rectus abdominis muscle," they wrote.

The patient was given a subfascial injection of 5 mL of 1% lidocaine under ultrasound guidance into the 1-cm area of maximal abdominal tenderness. "One hour later, the patient reported a decrease in pain from 9 to 0 on a numeric pain rating scale that ranges from 0 (no pain) to 10 (worst pain imaginable)," the authors reported. On follow-up 9 months later, the patient said she remained free of any recurrences of abdominal pain.

Discussion

ACNES is an abdominal wall disorder that is four times more common in females than males, and is usually diagnosed at a median age of 40 to 50 years, Iio and Ishida noted. However, it can occur at any age, they added, "and the incidence increases during adolescence."

ACNES patients typically experience abdominal pain below the level of the umbilicus, often on the right side. A of over 1,100 patients showed that up to 50% of patients with ACNES reported visceral symptoms such as nausea, bloating, abdominal swelling, and loss of appetite.

The condition is widely presumed to be "related to compression and irritation of the intercostal nerve endings that pass through the rectus abdominis muscle and innervate the oblique muscles," Iio and Ishida explained. "Abdominal muscle spasm results in progressive nerve entrapment, which perpetuates the pain."

Approximately one in three patients report a prior event thought to give rise to the condition, including surgery, trauma, or injury; pregnancy; and influenza infection; however, it occurs without provocation in 75% of adolescents affected. Interestingly, oral contraceptive use has also been implicated in ACNES, according to at least .

The estimated incidence of ACNES in the general population is one in individuals, and the condition accounts for 2% of ED presentations for acute abdominal pain, the authors noted.

Still, diagnosis takes time, they said, with a median of 18 months after symptom onset. Other data suggest that diagnosis can take more than 5 years in about 10% of patients with ACNES, since patients are often misdiagnosed with irritable bowel syndrome, fibromyalgia, complex regional pain syndrome, or rheumatoid arthritis.

ACNES diagnosis is "based on history and characteristic clinical findings in patients who have abdominal pain in an area of 1 cm or less at the lateral border of the rectus abdominis muscle," Iio and Ishida noted. The only role of blood tests and imaging is to help exclude other reasons for the abdominal pain.

As in this patient's case, "a positive Carnett test, defined as increased pain with head lifting and tensing of the abdominal muscles while the point of maximal tenderness is palpated, was reported in 87% of 1,116 patients with ACNES," they wrote.

Other research suggests that if the cause of the pain is visceral, such as cholecystitis, pain may decrease with this maneuver.

Physical exam findings characteristic of ACNES include:

• Sensory disturbances at the painful abdominal area (78%)
• Positive pinch sign (78%)
• Positive Carnett sign (87%)
• Positive response to a modified rectus sheath block (>50% pain reduction; 81%)

Diagnosis if pain is reduced by at least half after 5 to 10 mL of local anesthetic (lidocaine) is injected (with or without ultrasound guidance) into the affected area, Iio and Ishida noted.

A single injection of lidocaine achieves immediate in 86% of patients, they added, citing other data showing that a single injection was associated with persistent pain relief in 10% to 20% of patients. Recurrent abdominal pain affects most patients, however, and is generally managed with "up to 2 additional injections of lidocaine."

Another study suggested that about half of patients with ACNES had a decrease in abdominal pain following pulsed radiofrequency therapy, with 8% remaining pain-free at 15-month follow-up.

Finally, patients whose pain persists in spite of multiple lidocaine injections may get relief with neurectomy -- the surgical removal of intercostal nerve branches, the authors noted.

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