Anterior Cutaneous Nerve Block

Indications and Clinical Considerations

Anterior cutaneous nerve entrapment syndrome is a constellation of symptoms, including severe, knifelike pain emanating from the anterior abdominal wall associated with 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 females. The patient can often localize the source of pain quite 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 rectus abdominis muscle. It is at this point that the anterior cutaneous branch of the intercostal nerve turns sharply in an anterior direction to provide innervation to the anterior wall ( Fig. 113.1 ). The nerve passes through a firm, fibrous ring as it pierces the fascia, and it is at this point that the nerve is subject to entrapment ( Fig. 113.2 ). The nerve is accompanied through the fascia by an epigastric artery and vein. There is the potential for small amounts of abdominal fat to herniate through this fascial ring and become incarcerated, which results in further entrapment of the nerve. Contraction of the abdominal muscles puts additional pressure on the nerve and may elicit sudden, sharp, lancinating pain in the distribution of the affected anterior cutaneous nerve.

Physical examination reveals that the patient will attempt to splint the affected nerve by keeping the thoracolumbar spine slightly flexed to avoid increasing tension on the abdominal musculature. Pain is reproduced with pressure on the anterior cutaneous branch of the affected intercostal nerve at the point at which the nerve pierces the fascia of the abdominal wall at the lateral border of the rectus abdominis muscle. Having the patient do a sit-up often reproduces the pain, as will a Valsalva maneuver ( Fig. 113.3 ). The presence of Carnett sign is considered diagnostic for anterior cutaneous nerve entrapment syndrome ( Fig. 113.4 ).

Plain radiographs are indicated for all patients with pain thought to be emanating from the lower costal cartilage and ribs to rule out occult bony disease, including rib fracture and tumor. Radiographic evaluation of the gallbladder is indicated if cholelithiasis is suspected. On the basis of the patient’s clinical presentation, additional testing may be indicated, including complete blood cell count, rectal examination with stool guaiac, sedimentation rate, and antinuclear antibody testing. Computed tomography and ultrasound imaging of the abdomen are indicated if intra-abdominal disease or an occult mass is suspected. The injection technique described later serves as both a diagnostic and a therapeutic maneuver. Table 113.1 presents a differential diagnosis for patients suspected of suffering from entrapment of the anterior cutaneous nerve.

Clinically Relevant Anatomy

The intercostal nerves arise from the anterior division of the thoracic paravertebral nerve. A typical intercostal nerve has 4 major branches. The first branch is the unmyelinated postganglionic fibers of the gray rami communicantes, which interface with the sympathetic chain. The second branch is the posterior cutaneous branch, which innervates the muscles and skin of the paraspinal area. The third branch is the lateral cutaneous division, which arises in the anterior axillary line and provides the majority of the cutaneous innervation of the chest and abdominal wall. The fourth branch is the anterior cutaneous branch, which supplies innervation to the midline of the chest and abdominal wall (see Fig. 113.1 ). The anterior cutaneous branch pierces the fascia of the abdominal wall at the lateral border of the rectus abdominis muscle ( Fig. 113.5 ). The 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 it is at this point that the nerve is subject to entrapment. The nerve is accompanied through the fascia by an epigastric artery and vein. Occasionally, the terminal branches of a given intercostal nerve may actually cross the midline to provide sensory innervation to the contralateral chest and abdominal wall. The twelfth nerve is called the subcostal nerve and is unique in that it gives off a branch to the first lumbar nerve, thus contributing to the lumbar plexus.