CHAPTER 4: Sonoanatomy Relevant for Ultrasound-Guided Abdominal Wall Nerve Blocks

INTRODUCTION

Ultrasound-guided abdominal wall blocks are a recent innovation of the traditional landmark-based techniques of performing abdominal wall field blocks. ¹ These blocks include the transverse abdominis plane (TAP) block (lateral/midaxillary and subcostal), ^1–8 rectus sheath block, iliohypogastric and ilioinguinal nerve block, ⁸ and the quadratus lumborum block (QLB). ^8–12 They are fairly simple to perform, largely devoid of complications, and produce sensory and motor blockade of the abdominal wall.

GROSS ANATOMY

Muscles of the Anterior Abdominal Wall

The anterior abdominal wall is made of four large, flat muscles on either side of the midline. They are the external oblique muscle (EOM, Figs. 4–1 to 4–3), internal oblique muscle (IOM, Figs. 4–3 to 4–5), transversus abdominis muscle (TAM, Figs. 4–3, 4–6, and 4–7), and the rectus abdominis muscle (RAM, Figs. 4–3 and 4–6). Two other smaller muscles, the cremaster and the pyrimidalis, are also present. The EOM, IOM, and the TAM each end in a fibrous aponeurosis that extends up to the midline (Figs. 4–1, 4–4, and 4–6). The aponeuroses on either side fuse in the midline to form a median band called the linea alba. The RAM is longitudinal in shape, runs vertically on either side of the linea alba (Fig. 4–6), and is enclosed in a fibrous sheath called the “rectus sheath” (see later, Fig. 4–4).