TAP block: Anatomy

Innervation of the Abdominal Wall

The anterior rami of spinal nerves T7-L1 innervate the anterolateral abdominal wall. The anterior divisions of the intercostal nerves (T7-11) enter the abdominal wall between the internal oblique and transversus abdominis muscles; they continue in this space anteriorly until they pierce and innervate the rectus abdominis and end as anterior cutaneous branches, which innervate the skin on the anterior abdomen. Midway in their course, the intercostal nerves pierce the external oblique muscle to give off lateral cutaneous branches, which divide into anterior and posterior branches that innervate the external oblique muscle and latissimus dorsi, respectively. The anterior division of the subcostal nerve (T12) communicates with the iliohypogastric nerve and gives a branch to the pyramidalis muscle. Its lateral cutaneous branch perforates the internal and external oblique muscles and descends over the iliac crest to innervate part of the gluteal region. The iliohypogastric nerve (L1) divides into lateral and anterior cutaneous branches near the iliac crest. The lateral cutaneous branch innervates the skin of the gluteal region, and the anterior cutaneous branch innervates the hypogastric region. The ilioinguinal nerve (L1) communicates with the iliohypogastric nerve between the internal oblique and transversus abdominis near the anterior part of the iliac crest. It innervates the upper and medial part of the thigh and part of the skin covering the genitalia.

TAP block technique

The goal of the TAP block is to inject local anesthetic in the plane between the internal oblique and transversus abdominis muscles. This will interrupt innervation to the abdominal skin, muscles, and parietal peritoneum; however, it will not block visceral pain. The TAP block can be performed using a blind approach or with ultrasound guidance.

Blind approach

The point of entry is the lumbar triangle of Petit. It is bound inferiorly by the iliac crest (IC), anteriorly by the external oblique muscle, and posteriorly by the latissimus dorsi. The costal margin (CM) is slightly superior to the triangle of Petit. The provider will feel a “double pop” as the needle traverses the fascial extensions of the external oblique and the internal oblique muscles.

Ultrasound-guided approach

The ultrasound probe is placed in a transverse plane to the lateral abdominal wall in the midaxillary line, between the lower costal margin and iliac crest. This allows for more accurate deposition of the local anesthetic in the correct neurovascular plane (as compared to the blind technique).