Ultrasound-guided lateral 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 lateral abdominal wall pain that is associated with point tenderness over the affected anterior cutaneous nerve. The pain of lateral cutaneous nerve entrapment syndrome radiates a small distance from the entrapped lateral cutaneous nerve. Sensory abnormalities including allodynia, hypoesthesia, and on occasion hyperesthesia in the area subserved by the lateral cutaneous nerve are often noted. The pinching and lifting of the skin in the affected area will often be perceived by the patient suffering from lateral cutaneous nerve entrapment as significantly more painful than pinching and lifting of the skin in the same area on the contralateral side. The patient can often localize the source of pain quite accurately by pointing to the spot at which the lateral cutaneous branch of the affected intercostal nerve pierces the fascia of the external oblique abdominal muscle. It is at this point that the lateral cutaneous branch of the intercostal nerve turns sharply in an anterior direction to provide innervation to the lateral thorax and abdomen (Fig. 94.1). 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. 94.2). 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 lateral cutaneous nerve. The presence of Carnett sign is considered diagnostic for lateral cutaneous nerve entrapment syndrome (Fig. 94.3). Lateral cutaneous nerve entrapment syndrome occurs most commonly in females.

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 including ultrasound, CT, and MRI of the flank and retroperitoneum should be considered to rule out occult pathology that may be erroneously attributed to lateral cutaneous nerve entrapment (Fig. 94.4). 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.

The intercostal nerves arise from the anterior division of the thoracic paravertebral nerve. A typical intercostal nerve has four 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 branch, which arises in the anterior axillary line
and provides the majority of the cutaneous innervation of the chest and lateral abdominal wall. The lateral cutaneous branch pierces the fascia of the external oblique abdominal muscle (see Fig. 94.1). The nerve turns sharply in an anterior and posterior direction to provide innervation to the lateral thoracic and abdominal 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 (see Fig. 94.2). The fourth branch is the anterior cutaneous branch, which supplies innervation to the midline of the chest and abdominal wall (see Fig. 94.1). The anterior cutaneous branch pierces the fascia of the abdominal wall at the lateral border of the rectus abdominis muscle (see Fig. 94.1). 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 (see Fig. 94.2). 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 12th 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.