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 [1-7]. Anterior cutaneous nerve entrapment syndrome 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 anterior cutaneous nerve entrapment syndrome. 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 [8]. In one retrospective study, 2 percent of patients who presented to the emergency room for evaluation of acute abdominal wall pain had anterior cutaneous nerve entrapment syndrome [8]. Among patients with abdominal pain and a negative prior diagnostic evaluation, the prevalence of abdominal wall pain ranges from 15 to 30 percent [5,9]. Women appear to be four times more likely to have anterior cutaneous nerve entrapment syndrome as compared with men. Two peak incidences have been reported, between the ages 15 to 20 and 35 to 45, although cases have been reported in children and older adults [10].

ETIOPATHOGENESIS — Anterior cutaneous nerve entrapment syndrome is caused by entrapment of the cutaneous branches of sensory nerves supplying the abdominal wall [2]. 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 [3]. 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 [11,12]. (See "Anatomy of the abdominal wall".)

Pain can usually be localized to a highly discrete region of the abdomen. 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, comprising up to 25 percent of nociceptors, are found in skin and muscle, and mediate the sharp, sudden pain that is associated with injury such as a cut, trauma, or pain in the abdominal wall. The C type nociceptor (approximately 50 percent of nociceptors) innervates periosteum, parietal peritoneum, and viscera and mediates the dull, difficult to localize pain of intraperitoneal disease. With most causes of intraabdominal pain, localization is therefore difficult and the patient often waves the hand over a relatively wide area of the abdomen. In contrast, when the pain is in the abdominal wall and, therefore, mediated by A-delta nociceptors, the patient usually points to the location with one finger. (See "Causes of abdominal pain in adults".)

Similar pain syndromes due to nerve entrapment resulting in pain in the back (Posterior Cutaneous Nerve Entrapment Syndrome or POCNES) and flank (Lateral Cutaneous Nerve Entrapment Syndrome or LACNES) have been described [13,14].

CLINICAL FEATURES — Patients with anterior cutaneous nerve entrapment syndrome present with chronic pain and characteristically maximal tenderness over a small area of the abdominal wall (less than 2 cm in diameter). The pain is usually located along the lateral edge of the rectus abdominis muscle sheath and is predominantly located on the right 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 should begin with history and physical examination (algorithm 1). Patients with anterior cutaneous nerve entrapment syndrome have characteristically well-localized abdominal pain with an increase in tenderness to palpation during muscle tensing (Carnett's sign). In general, patients with anterior cutaneous nerve entrapment syndrome have normal laboratory tests. Immediate relief of abdominal pain with trigger point injection of local anesthetic agents establishes the diagnosis of anterior cutaneous nerve entrapment syndrome. We perform an initial trigger point injection combined with local anesthetic and glucocorticoids. (See 'Trigger point injection' below.)

Patients with "red flags" (potentially worrisome features) should be evaluated for other causes of abdominal pain. The lack of improvement with trigger point injection of local anesthetic agents should also prompt evaluation for other etiologies of abdominal pain. The diagnostic approach to abdominal pain is discussed in detail elsewhere. (See 'Red flags for additional evaluation' below and "Evaluation of the adult with abdominal pain", section on 'Diagnostic approach to chronic abdominal pain'.)

Physical examination — Patients with anterior cutaneous nerve entrapment syndrome are usually able to point with one finger to the area of maximal tenderness. Voluntary guarding of the affected area may be present on palpation (Hover sign) [2]. Hyperesthesia, hyperalgesia, or heightened sensitivity to normally non-painful tactile stimulation (allodynia) of the surrounding skin may be present in up to 75 percent of patients [15]. Altered perception of cold may be seen. Pinching of the affected area may be extremely painful [3,16,17].

Patients have increased local tenderness during muscle tensing (Carnett's sign) [17,18]. To elicit Carnett's sign, the patient is asked to perform 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 decrease the likelihood of anterior cutaneous nerve entrapment syndrome. 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'.)

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 [1,4,15,19-23]. 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 [4,5,7,9,24-26]. (See 'Anesthetic, glucocorticoid injection' below and "Evaluation of the adult with abdominal pain".)

DIAGNOSIS — The diagnosis of anterior cutaneous nerve entrapment syndrome 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) [17,27].

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

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

DIFFERENTIAL DIAGNOSIS — The differential diagnosis of anterior cutaneous nerve entrapment syndrome includes thoracic nerve radiculopathy, and pain generated from the ribcage or chest wall. The differential diagnosis of abdominal pain is discussed in detail, 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 [2]. 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]. Women 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 [2,29]. For this reason, careful examination of the spine and back should be performed and imaging studies obtained if appropriate. Diabetic patients 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 [2,30-32]. 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 women, 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 is usually associated with unilateral, sharp, stabbing pain localized just beneath the ribs. This may occur at rest and may 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 women 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 the 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 — Anterior cutaneous nerve entrapment syndrome is a nonprogressive, albeit painful condition that is typically self-limited. The goal of treatment is to relieve symptoms.

Overall approach — Our approach is to start with conservative therapy following an initial trigger point injection that is both diagnostic and therapeutic (algorithm 1). Conservative therapy includes activity modification and physical therapy (see 'Conservative therapy' below). Gabapentin or tricyclics may be tried (see below).

In patients with partial improvement or recurrent pain after complete pain relief, we repeat combined anesthetic and glucocorticoid injections one month after the prior injection. (See 'Anesthetic, glucocorticoid injection' below.)

Patients who do not respond to repeat injection after an initial response should be carefully reassessed for other causes of abdominal pain, paying specific attention to the type of ongoing symptoms, the degree to which symptoms have worsened, and compliance with conservative therapy. Reassessment is also indicated if a patient has a recurrence of symptoms after initial improvement that does not mimic the initial presentation. (See "Evaluation of the adult with abdominal pain".)

In patients with recurrent pain after three re-injections of combined local anesthetic and glucocorticoid in the same site within a year, we may recommend chemical neurolysis. (See 'Chemical neurolysis' below.)

Conservative therapy — Patients should be advised to discontinue exercises that involve tension of the abdominal muscles (eg, abdominal crunches). Vigorous exercise may exacerbate the pain.

Anesthetic, glucocorticoid injection — Trigger point injections can be performed with a local anesthetic alone or combined with a glucocorticoid [5]. Ultrasound guidance is not required [38]. Injection of an anesthetic alone results in immediate relief of symptoms in 83 to 91 percent of patients, but it often recurs after two to three hours and may be more intense [1,4,15,19-23]. Pain relief with injection of local anesthetic is sustained in 20 percent of patients [39]. However, the majority require repeated injections, which lead to lasting relief in 40 to 50 percent of patients [4,40].

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 [41]. In one study, 79 patients with abdominal wall pain were treated with an injection of 2 mL of 0.25 percent bupivacaine and 20 to 40 mg of triamcinolone or a comparable agent [4]. 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 [42].

Technique — 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 steroid (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 steroid is used. Several variations on the technique and agents used for local injection have also been described [5,43].

Chemical neurolysis — Neurolysis with 5 to 6 percent phenol or absolute alcohol can be considered in responding patients requiring four injections in the same site within a year [19,44]. In one study, 44 patients with anterior cutaneous nerve entrapment syndrome were treated with injection of 1 mL of 6 percent 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.

Pulsed radiofrequency — Pulsed radiofrequency is being evaluated before surgical treatment is considered; preliminary data suggest that this may be beneficial to many patients [45].

Surgical treatment — Surgical release of entrapped nerves has also been described [2,26,39,46-49]. In one randomized trial, 44 patients with anterior cutaneous nerve entrapment syndrome 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) [49]. 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 pain reduction in pain intensity was noted in 61 percent of patients at a mean follow-up of 32 months [48].

Other therapies — Nonsteroidal anti-inflammatory drugs and tricyclic antidepressants have been used for in the treatment of musculoskeletal pain but their efficacy in anterior cutaneous nerve entrapment syndrome has not been established. Intraperitoneal onlay mesh reinforcement has been used to treat anterior cutaneous nerve entrapment syndrome but additional studies are needed to confirm its efficacy [26].

SUMMARY AND RECOMMENDATIONS

●Epidemiology – Anterior cutaneous nerve entrapment syndrome is one of the most frequent causes of chronic abdominal wall pain.

Women appear to be four times more likely to have anterior cutaneous nerve entrapment syndrome as compared with men. The peak incidence is between the ages of 30 to 50 years, although cases have been reported in children and older adults. (See 'Epidemiology' above.)

●Pathogenesis – Anterior cutaneous nerve entrapment syndrome is caused by entrapment of the cutaneous branches of sensory nerves supplying the abdominal wall. 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. (See 'Etiopathogenesis' above.)

●Clinical features – Pain associated with anterior cutaneous nerve entrapment syndrome 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 anterior cutaneous nerve entrapment syndrome aims to establish the diagnosis and to identify patients who should be assessed for causes of abdominal pain. Initial evaluation should begin with history and physical examination (algorithm 1).

Patients with the "red flags" (potentially worrisome features) should be evaluated for other causes of abdominal pain. Red flags for additional evaluation 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)

In general, patients with anterior cutaneous nerve entrapment syndrome have normal laboratory tests and do not have worrisome features on initial evaluation. Immediate relief of abdominal pain with trigger point injection of a local anesthetic agent establishes the diagnosis of anterior cutaneous nerve entrapment syndrome. We perform an initial trigger point injection with combined local anesthetic and glucocorticoids. Lack of improvement in pain with a trigger point injection should also prompt reevaluation of abdominal pain. (See 'Diagnostic approach' above and 'Overview' above.)

●Diagnosis – The diagnosis of anterior cutaneous nerve entrapment syndrome 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)

•Somatosensory disturbance of the surrounding skin (eg, hypoesthesia, hyperesthesia, hyperalgesia, allodynia, or altered cold perception)

•Positive Pinch test

•Response to trigger point injection of a local anesthetic agent

●Management

•Conservative therapy with trigger point injection – In patients with anterior cutaneous nerve entrapment syndrome, we suggest initial conservative therapy following a diagnostic trigger point injection (algorithm 1) (Grade 2C). Conservative therapy includes activity modification to avoid exercises that involve tension of the abdominal muscles. (See 'Conservative therapy' above.)

•Partial improvement or recurrent symptoms

-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.)

-Patients with continued pain despite three re-injections should be carefully reassessed for other causes of abdominal pain, paying specific attention to the type of ongoing symptoms, the degree to which symptoms have improved or worsened, and compliance with conservative therapy. Reassessment is also indicated if a patient has a recurrence of symptoms after initial improvement that does not mimic the initial presentation. (See "Evaluation of the adult with abdominal pain".)

-In patients with recurrent pain after three re-injections of combined local anesthetic and glucocorticoid in the same site within a year, we perform neurolysis. Surgical neurectomy is reserved for ACNES refractory to noninvasive approaches. (See 'Surgical treatment' above and 'Chemical neurolysis' above.)