Anterior Cutaneous Nerve Entrapment

Anterior Cutaneous Nerve Entrapment

Anterior cutaneous nerve entrapment is an uncommon cause of anterior abdominal wall pain that is a frequently overlooked clinical diagnosis. Anterior cutaneous nerve entrapment syndrome is a constellation of symptoms consisting of severe knife-like pain emanating from the anterior abdominal wall, associated with the physical finding of point tenderness over the affected anterior cutaneous nerve. The pain radiates medially to the linea alba but in almost all cases does not cross the midline. Anterior cutaneous nerve entrapment syndrome occurs most commonly in young women. The patient can often localize the source of pain accurately by pointing to the spot at which the anterior cutaneous branch of the affected intercostal nerve pierces the fascia of the abdominal wall at the lateral border of the abdominus rectus muscle. At this point, the anterior cutaneous branch of the intercostal nerve turns sharply in an anterior direction to provide innervation to the anterior wall. The nerve passes through a firm fibrous ring as it pierces the fascia, and at this point the nerve becomes subject to entrapment. The nerve is accompanied through the fascia by an epigastric artery and vein. The potential exists for small amounts of abdominal fat to herniate through this fascial ring and become incarcerated, which results in further entrapment of the nerve. The pain of anterior cutaneous nerve entrapment is moderate to severe in intensity.

Signs and Symptoms

As mentioned earlier, the patient often can point to the exact spot that the anterior cutaneous nerve is entrapped. Palpation of this point often elicits sudden sharp, lancinating pain in the distribution of the affected anterior cutaneous nerve. Voluntary contraction of the abdominal muscles puts additional pressure on the nerve and may elicit the pain. The patient attempts to splint the affected nerve by keeping the thoracolumbar spine slightly flexed to avoid increasing tension on the abdominal musculature. Having the patient do a sit-up often reproduces the pain, as does a Valsalva maneuver. Patients with anterior cutaneous nerve entrapment will also exhibit a positive Carnett test when the patient is asked to tense his or her abdominal musculature, which is indicative of abdominal wall pain rather than pain with an intra-abdominal nidus.

Testing

Plain radiographs are indicated in all patients with pain thought to be emanating from the lower costal cartilage and ribs to rule out occult bony pathological conditions, including rib fracture and tumor. Radiographic evaluation of the gallbladder is indicated if cholelithiasis is suspected. Based on the patient’s clinical presentation, additional tests, including complete blood cell count, rectal examination with stool guaiac, erythrocyte sedimentation rate, and antinuclear antibody testing, may be indicated. Ultrasonography and computed tomography (CT) scan of the abdomen are indicated if intra-abdominal pathological process or occult mass is suspected. Injection of the anterior cutaneous nerve with or without ultrasound guidance at the point at which it pierces the fascia serves as a diagnostic and therapeutic maneuver.

Differential Diagnosis

The differential diagnosis of anterior cutaneous nerve entrapment syndrome should consider ventral hernia, peptic ulcer disease, cholecystitis, intermittent bowel obstruction, renal calculi, angina, mesenteric vascular insufficiency, diabetic polyneuropathy, and pneumonia.

Rarely the collagen-vascular diseases, including systemic lupus erythematosus and polyarteritis nodosa, may cause intermittent abdominal pain; porphyria also may cause intermittent abdominal pain. Because the pain of acute herpes zoster may precede the rash by 24 to 72 hours, the pain may be attributed erroneously to anterior cutaneous nerve entrapment.

The Differential Diagnosis of Anterior Cutaneous Nerve Entrapment Syndrome

Differential Diagnosis	Investigations and Characteristics
Anterior cutaneous nerve entrapment syndrome	Carnett test, injection of local anesthetics
Thoracic lateral cutaneous nerve entrapment	History of previous surgery, clinical examination
Ilioinguinal or iliohypogastric nerve entrapment	History of previous groin surgery, clinical examination, injection of local anesthetics
Endometriosis	History of cyclic abdominal pain, laparoscopy
Myofascial pain syndrome	Clinical examination, myofascial strain
Slipping rib syndrome	Hypermobile, luxating eighth to tenth ribs, clinical examination
Diabetic radiculopathy	Paraspinal EMG, patient with diabetes mellitus
Abdominal wall tear	History of acute pain related to lifting or stretching, athletes
Abdominal wall or rectus sheath hematoma	Abdominal ultrasound or CT scan, after laparoscopy, after coughing in anticoagulated patient
Herpes zoster	History and clinical examination, dermatomal
Abdominal wall tumor (lipoma, desmoid, metastasis)	History and clinical examination, abdominal CT scan
Spinal nerve irritation	Referred pain by thoracic spine pathological condition
Hernia	Abdominal ultrasound, clinical examination
Traction symphysitis or pubalgia	Athletes, positive findings on MRI or scintigraphy

CT , Computed tomography; EMG, electromyography; MRI, magnetic resonance imaging.

Treatment

Initial treatment of the pain and functional disability associated with anterior cutaneous entrapment syndrome should include a combination of nonsteroidal antiinflammatory drugs (NSAIDs) or the cyclooxygenase-2 (COX-2) inhibitors and physical therapy. Local application of heat and cold may be beneficial. The repetitive movements that incite the syndrome should be avoided. For patients who do not respond to these treatment modalities, injection of the anterior cutaneous nerve at the point at which the nerve pierces the fascia with a local anesthetic and steroid may be a reasonable next step. If the symptoms of anterior cutaneous entrapment syndrome persist, surgical exploration and decompression of the anterior cutaneous nerve are indicated.

Complications

The major complications associated with anterior cutaneous entrapment syndrome fall into two categories: (1) iatrogenically induced complications secondary to incorrect diagnosis and (2) failure of the clinician to recognize that a hernia coexists with the nerve entrapment until bowel ischemia occurs.

Clinical Pearls

Patients with pain from anterior cutaneous nerve entrapment syndrome often attribute their pain symptoms to a gallbladder attack or ulcer disease. Reassurance is required, although it should be remembered that this musculoskeletal pain syndrome and intra-abdominal pathological conditions can coexist.

The use of physical modalities, including local heat and gentle range-of-motion exercises, should be introduced several days after the patient undergoes this injection technique for anterior cutaneous nerve entrapment syndrome. Vigorous exercises should be avoided because they would exacerbate the symptoms.

Simple analgesics and NSAIDs may be used concurrently with the aforementioned injection technique. Radiographic evaluation for intra-abdominal pathological conditions is indicated in patients with anterior abdominal pain of unclear origin.