Anterior cutaneous nerve entrapment syndrome

INTRODUCTION — 

Chronic pain emanating from the abdominal wall is frequently unrecognized or confused with visceral pain, often leading to extensive diagnostic testing before an accurate diagnosis is established [1-4]. Anterior cutaneous nerve entrapment syndrome (ACNES) is one of the most frequent causes of chronic abdominal wall pain. The diagnosis is suspected based on history and physical examination. Injection of a local anesthetic agent with or without a long-acting corticosteroid is effective for most patients and can help to confirm the diagnosis.

This topic review will review the clinical manifestations, diagnosis, and management of ACNES. Other causes of abdominal pain and the evaluation of patients with abdominal pain are discussed in detail, separately. (See "Causes of abdominal pain in adults" and "Evaluation of the adult with abdominal pain".)

EPIDEMIOLOGY — 

The estimated incidence of abdominal wall pain is 1 in 1800 individuals [5]. In one retrospective study, 2 percent of patients who presented to the emergency department for evaluation of acute abdominal wall pain had ACNES [5]. Among patients with abdominal pain and a negative prior diagnostic evaluation, the prevalence of abdominal wall pain ranges from 15 to 30 percent [2,6]. Females appear to be four times more likely to have ACNES as compared with males. Two peak incidences have been reported, between the ages of 15 to 20 and 35 to 45, although cases have been reported in children and older adults [7].

PATHOGENESIS — 

ACNES is caused by entrapment of the cutaneous branches of sensory nerves supplying the abdominal wall (figure 1) [8]. The cutaneous branches of sensory nerves arising from T7 to T12 make a 90-degree angle as they progress anteriorly through the posterior rectus sheath, passing through a fibrous ring within the lateral border of the rectus abdominis medial to the linea semilunaris. Once the nerves reach the overlying aponeurosis, the nerves divide again at 90-degree angles beneath the skin. Normally, fat in the neurovascular bundle permits the nerve to slide unimpeded within the fibrous ring [1]. Entrapment of the nerve can be caused by intra- or extra-abdominal pressure, ischemia, compression by herniation of the fat pad that normally protects it into the fibrous canal surrounding the nerve, or localized scarring. Other mechanical causes of nerve compression such as obesity and tight clothing may also be important in individual cases. Oral contraceptives and pregnancy have been associated with exacerbation of entrapment syndromes, possibly due to tissue edema from estrogen and progesterone [9,10]. (See "Anatomy of the abdominal wall".)

Pain can usually be localized to a highly discrete region of the abdomen, and patients can usually point to the location of the pain with one finger. This can be explained by the characteristics of the nerves causing the pain. There are two kinds of pain receptors: A-delta and C nociceptors. The A-delta nociceptors comprise up to 25 percent of nociceptors and are found in skin and muscle. They mediate the sharp, sudden pain that is associated with injury such as a cut, trauma, or pain in the abdominal wall.

By contrast, the C-type nociceptor innervates periosteum, parietal peritoneum, and viscera and mediates the dull, difficult-to-localize pain of intraperitoneal disease. With most causes of intra-abdominal pain, localization is therefore difficult, and the patient often waves the hand over a relatively wide area of the abdomen. (See "Causes of abdominal pain in adults".)

Similar pain syndromes due to nerve entrapment can result in pain in the back (posterior cutaneous nerve entrapment syndrome or POCNES) and flank (lateral cutaneous nerve entrapment syndrome or LACNES) [11,12].

CLINICAL FEATURES — 

Patients with ACNES present with pain and tenderness that characteristically exhibits maximal tenderness over a small area of the abdominal wall (less than 2 cm in diameter). Although the pain is typically chronic, patients can present acutely in the emergency department. The pain is usually located along the lateral edge of the rectus abdominis muscle sheath and may occur on either side of the abdomen. However, the pain may be located anywhere on the abdomen and in more than one location, occasionally even bilateral. In some patients, the pain is somewhat more diffuse, with radiation throughout the affected dermatome. The pain may be sharp, dull, or burning. Aggravating factors include tensing of abdominal musculature (eg, standing, walking, stretching, laughing, coughing, and sneezing). The pain is positional. It is least in the supine position and exacerbated by sitting or by lying on the affected side.

DIAGNOSTIC APPROACH

Overview — Initial evaluation consists of a history and physical examination that focuses on assessing for "red flags" that suggest alternative causes of abdominal pain and eliciting characteristic signs on physical examination that point to the diagnosis (eg, Carnett's sign) (algorithm 1). Immediate relief of abdominal pain with trigger point injection of local anesthetic agents establishes the diagnosis. (See 'Trigger point injection' below.)

Physical examination — Patients with ACNES can usually point with one finger to the area of maximal tenderness. Patients may demonstrate voluntary guarding of the affected area on palpation (hover sign) [8]. Up to 75 percent of patients experience hyperesthesia, hyperalgesia, or heightened sensitivity to normally nonpainful tactile stimulation (allodynia) of the surrounding skin [13]. An altered perception of cold may also be present. Pinching of the affected area may be extremely painful (pinch test) [1,14,15].

Increased local tenderness during muscle tensing (Carnett's sign) is a distinctive feature of the examination (video) [15,16]. To elicit Carnett's sign, the patient performs a straight-leg-raising maneuver (raising both legs off the table at the same time while supine) while the examiner's finger is on the painful site. Raising only the head while in the supine position can serve the same purpose. These maneuvers tighten the rectus abdominis muscles, increasing the pain from the entrapped nerve. If the pain is increased or the same, the source of the patient's symptoms is most likely the abdominal wall, and Carnett's sign should be considered positive. If the pain is decreased, the origin of pain is likely from an intra-abdominal organ, as the tensed abdominal wall muscles protect the viscera. Carnett's sign may not be interpretable in patients who cannot comply adequately with leg- or head-raising maneuvers. False positive results may occur from visceral causes of pain that involve the local parietal peritoneum. Other conditions resulting in a positive Carnett's sign are abdominal hernias and slipping rib syndrome.

Red flags for additional evaluation — The presence of "red flags" (potentially worrisome features) increases the suspicion for other organic causes of abdominal pain and decreases the likelihood of ACNES. Potentially worrisome features include, but are not limited to:

●Gastrointestinal bleeding

●Abnormal laboratory studies (eg, unexplained elevation in liver tests, amylase, lipase, tissue transglutaminase antibody, leukocytosis, or iron deficiency)

●Change in bowel habits

●Malnutrition

●Abdominal mass

●Signs or symptoms of systemic illness (eg, fever, chills, weight loss)

The presence of one or more of these features should prompt evaluation for other causes of abdominal pain with appropriate studies. (See "Evaluation of the adult with abdominal pain", section on 'Diagnostic approach to chronic abdominal pain'.)

Laboratory testing — Laboratory tests are normal in patients with ACNES and are not necessary to establish the diagnosis.

Trigger point injection — Injection of local anesthetic agents provides immediate relief of symptoms in 83 to 91 percent of patients, and helps secure the diagnosis [13,17-23]. We perform an initial trigger point injection with combined local anesthetic and glucocorticoids. A change in diagnosis is rare in patients who respond to local injection of anesthetic agents that can be performed under ultrasound guidance [2,4,6,18,24-26]. (See 'Anesthetic, glucocorticoid injection' below and "Evaluation of the adult with abdominal pain".)

DIAGNOSIS — 

The diagnosis of ACNES is based on the presence of all of the following:

●Well-localized abdominal pain.

●Increase in tenderness to palpation during muscle tensing on physical examination (positive Carnett's sign) [15,27].

●Response to trigger point injection of a local anesthetic agent. (See 'Treatment' below.)

Somatosensory disturbance of the surrounding skin (eg, hypoesthesia, hyperesthesia, hyperalgesia, allodynia, or altered cold perception) further supports the diagnosis.

DIFFERENTIAL DIAGNOSIS — 

The differential diagnosis of ACNES includes thoracic nerve radiculopathy and pain generated from the ribcage or chest wall. The differential diagnosis of abdominal pain is discussed separately (see "Causes of abdominal pain in adults"). In the operated abdomen, the edges of surgical scars and laparoscopic insertion points may also cause localized abdominal pain.

●Abdominal wall hernias – Hernias of the abdominal wall (eg, epigastric and Spigelian) can cause abdominal pain [8]. Epigastric hernias occur in the midline (except at the umbilicus). Patients may note a small, slightly uncomfortable lump between the umbilicus and the xiphoid. Up to 20 percent of epigastric hernias are multiple. Pain may increase when lying down, and a small, subcutaneous mass (tag of omentum) may be felt in the linea alba in addition to a defect(s) in the midline. Spigelian hernia occurs along the semilunar line, which is the caudal most extent of the posterior rectus sheath. (See "Overview of abdominal wall hernias in adults".)

●Abdominal wall endometriosis – The most common extrapelvic location of endometriosis is the abdominal wall, particularly in surgical scars related to gynecologic or obstetric surgery [28]. Females with abdominal wall endometriosis have abdominal wall pain and a tender, palpable mass. (See "Endometriosis in adults: Pathogenesis, epidemiology, and clinical impact", section on 'Anatomy and staging'.)

●Thoracic nerve radiculopathy – Spinal and paraspinal diseases (eg, herniated thoracic disc, spinal cord tumor) that affect the T7 to T12 nerve roots may cause pain referred to the abdomen [8,29]. For this reason, careful examination of the spine and back should be performed and imaging studies obtained if appropriate. Patients with diabetes may have chronic and/or recurring abdominal pain similar to acute painful neuropathy that is often localized to the thighs but may also affect the abdomen [8,30-32]. The pain may last for months, but spontaneous recovery is often complete within a year, and the condition does not necessarily progress to polyneuropathy [31]. (See "Polyradiculopathy: Spinal stenosis, infectious, carcinomatous, and inflammatory nerve root syndromes", section on 'Diabetic thoracic radiculopathy'.)

●Xiphoidalgia – Xiphodynia (xiphoid cartilage syndrome or the hypersensitive xiphoid) can cause epigastric pain and sometimes nausea and vomiting [33,34]. Xiphodynia occurs more often in females, 90 percent of whom are in the third to sixth decades of life. The diagnosis is made by the reproduction of pain by moderate pressure on the xiphoid process and its adjacent structures [35]. (See "Major causes of musculoskeletal chest pain in adults", section on 'Xiphoidalgia'.)

●Lower rib pain syndromes – Pain syndromes involving the lower ribs are characterized by pain in the lower chest or upper abdomen, with a tender spot on the costal margin, and reproduction of the pain by pressing on the spot.

•Slipping rib syndrome – The slipping rib syndrome is rare and usually associated with unilateral, sharp, stabbing pain localized just beneath the ribs. Pain may occur at rest and be increased by movement, especially twisting or turning. The diagnosis is made when the examiner hooks the fingers beneath the lowest rib and the rib is moved anteriorly, reproducing the patient's symptoms [36]. (See "Major causes of musculoskeletal chest pain in adults", section on 'Lower rib pain syndromes'.)

•Ribs on pelvis syndrome – The ribs on pelvis syndrome typically affects postmenopausal females with loss of vertebral height and kyphosis secondary to osteoporosis [37]. Forward angulation caused by anterior compression fractures pushes the ribs onto the pelvis, causing chest and abdominal pain. The pain is usually worse later in the day, as physical activity while upright increases the impingement. On physical examination, the space between the distal end of the 11^th or 12^th rib and the iliac crest is reduced to less than two fingerbreadths.

TREATMENT — 

ACNES is a nonprogressive, albeit painful, condition that is typically self-limited. The goal of treatment is to relieve symptoms.

Overall approach

●Initial approach – Our initial approach combines a trigger point injection that is both diagnostic and therapeutic with activity modification (algorithm 1). (See 'Activity modification' below.)

●Recurrent or refractory symptoms – In patients with partial improvement or recurrent pain episodes after complete pain relief, we repeat combined anesthetic and glucocorticoid injections one month after the prior injection (see 'Anesthetic, glucocorticoid injection' below). In patients with recurrent pain after three reinjections of combined local anesthetic and glucocorticoid in the same site within a year, we suggest chemical neurolysis (see 'Chemical neurolysis (preferred)' below). Other treatment options for those with recurrent or refractory symptoms include pulsed radiofrequency, surgical release, and medications used to treat musculoskeletal or neuropathic pain.

●When to reassess – We reassess patients for other causes of abdominal pain who do not respond to repeat injection after an initial response or who have recurrent symptoms that are dissimilar from the initial presentation. Reassessment should pay specific attention to the type of ongoing symptoms, the degree to which symptoms have worsened, and adherence to conservative therapy. (See "Evaluation of the adult with abdominal pain".)

Initial treatment

Anesthetic, glucocorticoid injection — For the initial management of individuals with ACNES, we suggest a trigger point injection that contains a local anesthetic, with or without a glucocorticoid.

●Technique – Trigger point injections can be performed with a local anesthetic alone or combined with a glucocorticoid [2]. Ultrasound guidance is not required [38]. To administer a trigger point injection, we pass a 1.5-inch 26-gauge needle perpendicularly into a lightly pinched skin and subcutaneous tissue fold at the affected site. Insertion of the needle becomes painful when reaching the tender area. We inject 1 to 3 mL of a 1 percent solution of lidocaine into the tender spot and follow it with 1 mL (40 mg) of a long-acting glucocorticoid (triamcinolone). Pain should resolve within five minutes if the diagnosis is correct and if the medication was placed in the proper location. It may return in one to two hours once the anesthetic wears off but often does not when the glucocorticoid is used. Several variations on the technique and agents used for local injection have also been described [2,39].

●Efficacy – Injection of an anesthetic alone results in immediate relief of symptoms in 83 to 91 percent of patients, but they often recur after two to three hours and may be more intense [13,17-23]. Pain relief with injection of local anesthetic is sustained in 20 percent of patients [40]. However, the majority require repeated injections, which lead to lasting relief in 40 to 50 percent of patients [18,41].

Glucocorticoids, with their reported membrane stabilizing effects, may enhance the anesthetic effect. Experimental studies on neuromas suggest that they may also reduce spontaneous, ectopic discharges, possibly also helping to explain their pain-relieving effect [42]. In one study, 79 patients with abdominal wall pain were treated with an injection of 2 mL of 0.25% bupivacaine and 20 to 40 mg of triamcinolone or a comparable agent [18]. Pain relief was observed within 72 hours. A repeat injection (days to months later) was necessary in approximately 30 percent of patients. Some patients required the injection of multiple sites, in which case the total volume injected was limited to 10 mL to avoid systemic effects. A long-term response was noted in 78 percent of patients at a mean follow-up of 13.8 months. However, conflicting data suggest a lack of benefit of additional corticosteroids when combined with local anesthetic [43].

Activity modification — After the initial trigger point injection, we counsel patients about the importance of activity modification. Activity modifications include discontinuing exercises that involve tension of the abdominal muscles (eg, abdominal crunches) and decreasing vigorous exercise if that exacerbates the pain.

Treatment of persistent or recurrent symptoms

Chemical neurolysis (preferred) — In patients who require more than three injections in the same site within one year, we suggest neurolysis with 5 to 6% phenol or absolute alcohol [19,44]. In one study, 44 patients with ACNES were treated with injection of 1 mL of 6% aqueous phenol. Prior to injection the nerve was localized with electrical stimulation using a needle placed into the anterior rectus sheath [19]. Patients with refractory symptoms underwent a repeat injection at one month. A total of 28 patients (64 percent) required two or more treatments. At follow-up, which ranged from six months to four years, complete, partial, and no pain relief were noted in 54, 40, and 6 percent of patients, respectively. Pain due to chemical irritation of the nerve was noted in four patients.

Other treatments — Alternative treatments for patients with recurrent or persistent symptoms of ACNES include surgical release, pulsed radiofrequency, and oral medications.

●Surgical release – Surgical release of entrapped nerves has also been described [8,26,40,45-48]. In one randomized trial, 44 patients with ACNES were assigned to anterior neurectomy or sham surgery. Six weeks after treatment, a significantly higher proportion of patients treated with neurectomy experienced a 50 percent reduction in pain score as compared with the sham surgery group (73 versus 18 percent) [48]. The response to anterior neurectomy also appears to be sustained. In a retrospective study of 154 patients who underwent primary anterior neurectomy, a 50 percent reduction in pain intensity was noted in 61 percent of patients at a mean follow-up of 32 months [47].

●Pulsed radiofrequency – Preliminary data suggest that pulsed radiofrequency may have efficacy for treating ACNES [49].

●Lidocaine patches – Lidocaine patches may offer transient pain relief in individuals with ACNES who do not want injections or surgical intervention. In a small case series, lidocaine patches led to partial or complete relief in 8 of 12 children with ACNES [50].

●Oral medications – Other medication options include nonsteroidal anti-inflammatory drugs, gabapentin, and tricyclic antidepressants. The rationale for using these medications stems from their efficacy in treating musculoskeletal or neuropathic pain; however, their efficacy in ACNES has not been established. Intraperitoneal onlay mesh reinforcement has been used to treat ACNES, but additional studies are needed to confirm its efficacy [26].

SUMMARY AND RECOMMENDATIONS

●Epidemiology – Anterior cutaneous nerve entrapment syndrome (ACNES) is one of the most frequent causes of chronic abdominal wall pain, with an estimated incidence of 15 to 30 percent in individuals with chronic abdominal pain and a negative initial diagnostic workup. (See 'Epidemiology' above.)

●Pathogenesis – ACNES is caused by entrapment of the cutaneous branches of sensory nerves supplying the abdominal wall by intra- or extra-abdominal pressure, ischemia, compression by fat herniation, or localized scarring. (See 'Pathogenesis' above.)

●Clinical features – Pain associated with ACNES is characteristically maximal in an area less than 2 cm in diameter and often along the lateral aspect of the rectus abdominis muscle sheath. Patients also have increased local tenderness during muscle tensing (positive Carnett's sign). (See 'Clinical features' above.)

●Evaluation – Diagnostic evaluation in patients with suspected ACNES aims to establish the diagnosis and identify patients who should be assessed for other causes of abdominal pain. Initial evaluation should begin with history and physical examination (algorithm 1).

We evaluate individuals with "red flags" for other causes of abdominal pain. Red flags include:

•Gastrointestinal bleeding

•Abnormal laboratory studies (eg, unexplained elevation in liver tests, amylase, lipase, tissue transglutaminase antibody, leukocytosis, or iron deficiency)

•Change in bowel habits

•Malnutrition

•Abdominal mass

•Signs or symptoms of systemic illness (eg, fever, chills, weight loss)

We perform a diagnostic trigger point injection with a local anesthetic, with or without glucocorticoids. Relief of pain confirms the diagnosis. Lack of pain improvement with a trigger point injection should prompt additional evaluation for other etiologies of abdominal pain. (See 'Diagnostic approach' above and 'Overview' above.)

●Diagnosis – The diagnosis of ACNES is based on the presence of all of the following (see 'Diagnosis' above):

•Well-localized abdominal pain

•Increase in tenderness to palpation during muscle tensing on physical examination (Carnett's sign)

•Response to trigger point injection of a local anesthetic agent

Somatosensory disturbance of the surrounding skin (eg, hypoesthesia, hyperesthesia, hyperalgesia, allodynia, or altered cold perception) further supports the diagnosis.

●Initial management – Management of patients with ACNES follows a stepwise approach (algorithm 1). For initial management, we suggest trigger point injection with a local anesthetic (eg, lidocaine), with or without a glucocorticoid (eg, triamcinolone) (algorithm 1) (Grade 2C). We also advise activity modification to avoid exercises that tense the abdominal muscles. (See 'Initial treatment' above.)

In patients with partial improvement or recurrent pain after complete pain relief after a diagnostic trigger point injection, we repeat glucocorticoid injections one month after the prior injection. (See 'Anesthetic, glucocorticoid injection' above.)

●Management of persistent or recurrent symptoms

•Reassessment – We reassess patients for other causes of abdominal pain who do not respond to repeat injection after an initial response or who have recurrent symptoms that are dissimilar from the initial presentation. (See 'Overall approach' above and "Evaluation of the adult with abdominal pain".)

•Frequent recurrences – In patients with recurrent pain after three reinjections of combined local anesthetic and glucocorticoid in the same site within a year, we suggest neurolysis (Grade 2C). (See 'Chemical neurolysis (preferred)' above.)

We reserve surgical neurectomy for patients with ACNES whose symptoms are refractory to noninvasive approaches. (See 'Other treatments' above.)