Abdominal cutaneous nerve entrapment syndrome (ACNES) is not an uncommon condition. When a patient is seen for abdominal pain without other clinically significant symptoms, ACNES should be high on the list of likely diagnoses.
Early recognition and intervention in cases of ACNES is important if patients are to be spared unnecessary anxiety, pain, loss of time and expense. It may potentially prevent the development of central sensitization and more complex pain. Diagnosis can be made based on history, examination, and the response to local anaesthetic infiltration. When conservative measures fail, the current treatment paradigm predominantly involves minimally invasive injection therapies.
The condition was first described by J P Frank in 1792, who coined the term “peritonitis muscularis.” Abdominal wall pain can often be wrongly attributed to intra-abdominal disorders. This misdirected diagnosis can lead to unnecessary consultation, testing, and even abdominal surgery. All of these can be avoided if the initial examination points to the right diagnosis.
Carnett in the early 20th century, called this syndrome “intercostal neuralgia” and claimed to have seen three patients per week with this diagnosis, including as many as three per day in consultation sessions.
The most common cause of abdominal wall pain is nerve entrapment at the lateral border of the rectus abdominis muscle. Kopell and Thompson stated that peripheral nerve entrapment occurs at anatomic sites where the nerve changes direction to enter a fibrous or osseofibrous tunnel. It may also occur where the nerve passes over a fibrous or muscular band. This entrapment can be at these sites because mechanically induced irritation is most likely to occur at these locations. Mechanical irritation causes localized swelling that may injure the nerve directly or compromise the nerve’s circulation. Tenderness of the main nerve trunk may be found proximal or distal to the affected portion (Valleix phenomenon). Proximal tenderness may result from vascular spasm or from unnatural traction on the nerve trunk against the point of entrapment. In ACNES, all these mechanisms can work together.
The thoracoabdominal nerves, which terminate as the cutaneous nerves, are anchored at six points:
1)the spinal cord; 2) the point at which the posterior branch originates; 3) the point at which the lateral branch originates; 4) the point at which the anterior branch makes a nearly 90° turn to enter the rectus channel; 5) the point from which accessory branches are given off in the rectus channel, and 6) skin.
The most common cause of abdominal wall pain is nerve entrapment at the lateral border of the rectus muscle. In the rectus channel, the nerve and its vessels are surrounded by fat and connective tissue that bind the nerve, artery, and vein into a discrete bundle capable of functioning as a unit independently from surrounding tissue. At a point located about three quarters of the way through the rectus muscle (from back to front), there is a fibrous ring that provides a smooth surface through which the bundle can slide. Positioned anterior to the ring, the rectus aponeurosis provides a hiatus for the exiting bundle.
The hypothesis that nerve ischemia is caused by localized compression of the nerve at the level of the ring is deduced from juxtaposition of the soft bundle to the hard ring. Herniation of the bundle through the ring due to too much pressure from behind or from pulling from in front will compress the bundle’s vessels and the nerve itself. Too much traction on the bundle from behind or from pulling in front will cause the bundle to be “strummed” against the ring, which then causes irritation and swelling even before herniation occurs.
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.
Symptoms of ACNES can be acute or chronic. The acute pain is described as localized, dull, or burning, with a sharp component (usually on one side) radiating horizontally in the upper half of the abdomen and obliquely downward in the lower abdomen. The pain may radiate when the patient twists, bends, or sits up. Lying down may help but sometimes worsens the pain. Younger people, who are usually more physically active than older persons, are more often seen for the first episode of acute pain.
ACNES can become chronic in nature and persist for months to years. Medical history in these patients shows that acute exacerbation of pain may occur over several days or weeks and then disappear for varying lengths of time, sometimes for years.
ACNES-related pain is well-localized and usually affects only one side. However, the pain can occur on both sides at the same level (usually in the lower abdomen), or more than one nerve can be affected on opposite sides and at different levels. Pain radiating into the scrotum or vulva suggests involvement at the T12/L1 level, but inguinal or femoral hernia and pain arising from the adductor muscles of the thigh must be ruled out. Pain and tenderness posterolaterally just below the iliac crest can occur with involvement at the T12/ L1 level. This finding is useful because it is present with abdominal wall pain but is absent if the pain arises from inside the abdomen. Pain radiating from T11 and T12 runs at an oblique angle and follows the course of these nerves. Such pain can suggest urolithiasis; however, patients with urolithiasis are usually seen writhing in pain, whereas patients with ACNES tend to lie quietly on the table with their hand placed over the area of discomfort. T11 involvement on the right side may suggest appendicitis, and involvement on either side may suggest ovarian involvement or spigelian hernia; all these conditions should be identified by proper physical examination. Pain on the right side at the T8 or T9 level may suggest cholecystitis or peptic ulcer. Pain at the T6, T7, or T8 levels can suggest pleurisy, costochondritis, or slipping rib syndrome (which is probably a form of ACNES caused by traction). Pain and numbness laterally in the thigh and hip may be caused by meralgia paresthetica.
Chronic ACNES patients suffer considerable anxiety and worry that they may have some horrible condition as yet undiscovered. As a result, they may be given a psychiatric diagnosis (eg, anxiety, somatization, or depression) and therefore often take antidepressant drugs.
A suggestive medical history should direct the examiner to precisely locate the tender spot by asking the patient to point to the most painful area with one finger. The examiner now must confirm that the point located by the patient is actually a nerve exit. As finger pressure is gradually increased, the patient with ACNES will recoil or grab the examiner’s hand. Hershfield called this the Hover Sign.
To further differentiate the source of the pain, Carnett’s sign should be elicited. While in supine position with the arms crossed over the chest, the patient should be asked to raise his or her head or feet off the table while the examiner pushes on the tender spot. If splinting the muscles in this manner reduces the amount of pain, the source is probably intra-abdominal. If the pain is in the abdominal wall, splinting the muscles will not reduce the pain and may actually increase it.
The “pinch test” can also be used if the examiner is initially unable to identify the side on which the pain originates. This test consists of picking up the patient’s skin and subcutaneous fat with the thumb and index finger, first on one side of the midline of the abdomen and then on the other side. The patient will state whether one side hurts more than the other. Cotton and pinprick technique can be used to check for hypoesthesia or hyperesthesia around the pain site, and Knockhaert notes that electromyelographic studies of the affected nerve show abnormalities in 60% of patients studied. However the sensitivity for EMG studies is low.
Application of heat or cold, use of abdominal binders, and transcutaneous electrical nerve stimulation machine may be useful in managing the acute flare-ups of ACNES. Although non-specific pharmacological therapies such as paracetomol, non-steroid anti-inflammatories, anti-convulsants, anti-depressants, and opioids are commonly used, their efficacy is unclear in established cases of ACNES. Topical treatments such as 5% lidocaine plasters, and capsaicin cream (0.025–0.075%) have also been used in the management. In spite of paucity of evidence for these medications in nerve entrapment syndromes, they are useful in the treatment of some patients.
As ACNES is a peripheral nerve entrapment syndrome, it is mainly diagnosed after the response to local anesthetic infiltration. Ultrasound-guided blockade of the abdominal cutaneous nerve serves as a precision diagnostic tool with potential therapeutic efficacy. Ultrasound guidance is useful to localize the nerve, and to spread the injectate around it. This results in hydro dissection, thereby releasing any entrapment. The ability to visualize the needle’s course and the spread of injectate imparts safety to the procedure.
Local injections can be therapeutic. Kuan et al. report 95% of patients experiencing pain relief after one or two injections. Mcgrady and Marks used a peripheral nerve stimulator to localize the nerve and used 1 ml of 6%
phenol for treatment. They reported that 59% of their patients diagnosed as ACNES got complete pain relief and 36% had a few symptoms but did not require further treatment.
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).