Ultrasound-guided anterior cutaneous nerve block is utilized as a diagnostic and therapeutic maneuver in the evaluation and treatment of anterior cutaneous nerve entrapment syndrome. This commonly overlooked cause of abdominal pain presents with a constellation of symptoms including severe, knife-like anterior abdominal wall pain that is associated with point tenderness over the affected anterior cutaneous nerve. The pain of anterior cutaneous nerve entrapment syndrome radiates medially to the linea alba and rarely crossed the midline. This entrapment syndrome occurs most commonly in young females and is often attributed to ovarian pain or mittelschmerz. Often, the patient can accurately localize the site of nerve entrapment, which can be confirmed by the clinician by palpating the spot the patient identifies with a straightened index finger. If the patient confirms that the point being palpated is the nidus of the patient’s pain symptomatology, the patient is then asked to contract the abdominal muscles, which should further exacerbate the pain if the cause is anterior cutaneous nerve entrapment. This increase in pain is thought to be caused by the herniation of small amounts of fat into the fascial ring, which contains the anterior cutaneous nerve as it turns anteriorly along with the epigastric artery and vein to provide sensory innervation to the anterior abdominal wall (Fig. 93.1). Patients suffering from anterior cutaneous nerve entrapment will often attempt to avoid eliciting anterior abdominal wall pain by splinting the affected nerve by keeping the thoracolumbar spine slightly flexed to avoid increasing tension on the abdominal musculature.

Plain radiographs are indicated for all patients who present with pain thought to be emanating from the lower costal cartilage and ribs to rule out occult bony pathology, including rib fracture and tumor. Radiographic evaluation of the gallbladder is indicated if cholelithiasis is suspected (Fig. 93.2). Based on 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 of the abdomen is indicated if intra-abdominal pathology or an occult mass is suspected.

Exiting their respective intervertebral foramen and passing just below the transverse process are the paravertebral nerves. After exiting the intervertebral foramen, the intercostal nerve gives
off a recurrent branch that loops back through the foramen to provide innervation to the spinal ligaments, meninges, and its respective vertebra and can be an important contributor to spinal pain. The paravertebral nerve also provides fibers to the sympathetic nervous system and the thoracic sympathetic chain via the myelinated preganglionic fibers of the white rami communicantes as well as the unmyelinated postganglionic fibers of the gray rami communicantes. The intercostal nerve then divides into a posterior and an anterior primary division (Fig. 93.3). The posterior division courses posteriorly and, along with its branches, provides innervation to the facet joints and the muscles and skin of the back. The larger, anterior division courses laterally to pass into the subcostal groove beneath the rib along with the intercostal vein and artery to become the respective intercostal nerves. The 12th thoracic nerve courses beneath the 12th rib and 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. The intercostal and subcostal nerves provide the innervation to the skin, muscles, ribs, and the parietal pleura and parietal peritoneum. The anatomic basis of the anterior cutaneous nerve block is the fact that the innervation of the anterolateral abdominal wall is provided by the lower six intercostal nerves and the first lumbar nerve. The anterior branches of these nerves pass within a fascial plane between the internal oblique muscle and the transversus abdominis muscle making them easily assessable for blockade with local anesthetic by placing a needle into this fascial plane (see Chapter 99). The anterior cutaneous branch then pierces the fascia of the abdominal
wall at the lateral border of the rectus abdominis muscle (see Fig. 93.3). The nerve turns sharply in an anterior direction to provide innervation to the anterior wall (Fig. 93.4). 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.