Review of Transversus Abdominis Plane Blocks and Their Application to Chronic Abdominal Pain
The transversus abdominis plane (TAP) nerve block was first described almost 20 years ago as a landmark-based technique for performing abdominal field blocks via the triangle of Petit approach. The block entails depositing local anesthetic in the transversus abdominis plane, located superficial to the transversus abdominis and deep to the internal oblique, in the anterolateral abdomen (see Figure 1). Its use in the setting of acute perioperative pain has been reviewed elsewhere,[2-4] with applications in bariatric, gynecologic, colorectal, urologic, and caesarean section surgeries, among others. This article reviews the analgesic coverage associated with various techniques of TAP blocks and introduces its utility in the diagnosis and treatment of chronic abdominal pain.
Figure 1: Ultrasound visualization of the TAP compartment
Ultrasound anatomy of the TAP compartment is depicted on the left, untouched, and on the right, with annotations of the three muscle layers of the anterolateral abdominal wall. Ventral rami of T6-L1 thoracolumbar nerves lie in the plane between the internal oblique and transversus abdominis.
Functional Neuroanatomy of TAP
The abdominal wall consists of three muscle layers—the external oblique, the internal oblique, and the transversus abdominis—and their associated fascial sheaths. Those muscles are mainly innervated via the ipsilateral ventral rami of the T6–L1 thoracolumbar nerves. After emerging through the intervertebral foramina, they follow a curvilinear course anteriorly in the intercostal spaces toward the body’s midline. Along that course, they enter a fascial plane between the transversus abdominis and the internal oblique muscles, accompanied by blood vessels in what is known as the TAP compartment. They further divide to the lateral cutaneous and anterior cutaneous branches corresponding to specific dermatomes. The anterior cutaneous branches innervate from the midline to the midclavicular line, whereas the lateral cutaneous branches innervate from the midclavicular line to as far posterior as the latissimus dorsi.
Pain relief following a TAP block can assist in confirming a somatosensory origin of symptoms—that is, relating to superficial tissues or the parietal peritoneum—and can help guide a comprehensive management strategy that includes pharmacologic, psychologic, and physical therapies.
Although the blockade distribution may vary based on patient anatomy, TAP blocks usually provide coverage in the area of the T10–L1 dermatomes ipsilateral to the intervention and are therefore most useful for surgery below the umbilicus. TAP plexuses are highly anastomotic, however, and the precise boundaries of cutaneous anesthesia are often not inclusive of the entire dermatome. Using a posterior approach to deposit injectate near the transition of the transversus abdominis to the aponeurosis can extend coverage superiorly to include T9. It also has a greater chance of providing coverage to the lateral abdominal wall because of the increased likelihood of lateral cutaneous branch coverage as compared with the lateral approach at the midaxillary line. Coverage can be further extended to T6 by using an oblique subcostal approach, wherein local anesthetic is used to hydrodissect the TAP on a line connecting the xiphoid to the anterior iliac crest.
Application to Chronic Pain States
Figure 2: Ultrasound visualization of nerve catheter placement in TAP compartment. The hyperechoic nerve catheter is visualized appropriately positioned in the TAP compartment, in the plane just superficial to transversus abdominis. Ext. oblique = external oblique, Int. oblique = internal oblique, transversus. abd. = transversus abdominis
TAP blocks have potential for both diagnostic and therapeutic use in the chronic pain setting, especially as it relates to chronic postsurgical pain: pain that persists for at least 2 months following surgery and is attributed to the surgery itself. For patients with chronic pain following abdominal surgery, determining whether the pain is visceral or somatosensory in origin can be challenging. Pain relief following a TAP block can assist in confirming a somatosensory origin of symptoms—that is, relating to superficial tissues or the parietal peritoneum—and can help guide a comprehensive management strategy that includes pharmacologic, psychologic, and physical therapies. A positive response to a diagnostic TAP block would also prevent the treating clinician from pursuing a course targeting visceral pain, which may include celiac plexus or splanchnic nerve blocks, both of which carry risk profiles.
Once patients are diagnosed with chronic pain of somatosensory origin, TAP blocks can provide useful therapeutic effect as well. Although the typical duration of effect is 8–12 hours when used for acute perioperative pain, it can be extended by including a corticosteroid in the injectate, or even continuous infusion through a catheter (see Figure 2). TAP blocks can provide value as part of a multimodal strategy with the goal of minimizing or even eliminating chronic opioid therapy for patients with chronic abdominal pain.
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