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.

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).

The EOM originates as eight fleshy slips from the lower eight ribs (Fig. 4–2). The upper slips of the origin of the EOM interdigitate with that of the serratus anterior muscle, and the lower slips of the EOM interdigitate with that of the latissimus dorsi muscle. The fibers of the muscle run downwards, forward, and medially (Fig. 4–2) to end in a broad aponeurosis (Fig. 4–1), which is inserted (from above downwards) to the xiphoid process, pubic symphysis, pubic crest, and the pectineal line of the pubis. The caudal fibers of the muscle are inserted to the anterior two-thirds of the outer lip of the iliac crest (Fig. 4–2). The caudal end of the external oblique aponeurosis is folded on itself to form the inguinal ligament, and above the pubic tubercle there is a small triangular opening called the superficial inguinal ring. Medial to the lateral edge of the rectus abdominis muscle the external oblique aponeurosis contributes to forming the rectus sheath (Fig. 4–6, see later).

The IOM originates from the lateral two-thirds of the inguinal ligament, anterior two-thirds of the intermediate area of the iliac crest (Fig. 4–5), and the thoracolumbar fascia posteriorly. From its origin the fibers of the IOM run obliquely upwards, forwards, and medially, crossing the fibers of the EOM at right angles (Fig. 4–5), to end in an aponeurosis through which it is attached to the xiphoid process, the seventh to ninth costal cartilage, linea alba, pubic crest, and pectineal line. The IOM aponeurosis also contributes to the formation of the rectus sheath (Fig. 4–4, see later).

The TAM has a fleshy origin from the lateral one-third of the inguinal ligament, anterior two-thirds of the inner lip of the iliac crest, thoracolumbar fascia posteriorly, and the inner surface of the lower six costal cartilages. The fibers of the TAM are directed horizontally forwards (Figs. 4–6 and 4–7) and end in an aponeurosis that is attached to the xiphoid process, linea alba, pubic crest, and pectineal line of the pubis. At the lower part of the TAM the lower fibers of the muscle fuse with the lower fibers of the IOM to form the conjoint tendon. The TAM aponeurosis also takes part in the formation of the rectus sheath (Fig. 4–6, see later). The neurovascular structures of the abdominal wall lie in between the IOM and TAM (Fig. 4–8). This intermuscular plane is also referred to as the transversus abdominis plane (TAP, Figs. 4–9 to 4–11) and is a popular site for ultrasound-guided abdominal wall nerve blocks.

The rectus abdominis muscle (RAM) originates as two heads from the lateral (lateral head) part of the pubic crest and from the anterior pubic ligament (medial head). The fibers of the RAM run vertically upwards to be inserted into the anterior aspect of the chest wall, that is, to the xiphoid process and the fifth to seventh costal cartilages (Fig. 4–12). There are three fibrous bands, also called the tendinous insertions or inscriptions, on the anterior surface of the RAM (Figs. 4–6 and 4–12). The most cephalad tendinous insertion lies opposite the free end of the xiphoid process, the second opposite the umbilicus, and the third approximately midway between the two (Fig. 4–6). This divides the RAM into six or eight bellies (sections), which is also colloquially called the “six-pack” (Fig. 4–4). The tendinous insertions pass transversely or obliquely across the muscle, are adherent to the anterior wall of the rectus sheath, and traverse only the anterior half of the muscle. The RAM is enclosed in a sheath, the rectus sheath (see later, Fig. 4–6), formed by the aponeurosis of the three flat muscles of the abdomen.

Nerves of the Anterior Abdominal Wall

The skin and musculature of the abdominal wall is innervated by the anterior primary rami of the lower six thoracic nerves (T7-T12, Fig. 4–8) and the first lumbar nerve (L1) through its iliohypogastric and ilioinguinal branches (Fig. 4–1). The anterior primary rami of the lower five intercostal nerves (T7-T11) emerge from their respective intercostal spaces and come to lie in a neurovascular plane between the internal oblique and transversus abdominis muscles (Fig. 4–8). This intermuscular plane is also referred to as the transversus abdominis plane (TAP). The segmental nerves travel anteriorly and medially towards the midline in the TAP, giving off their lateral cutaneous branches at the level of the midaxillary line and pierce the posterior lamina of the internal oblique aponeurosis anteriorly to enter the rectus sheath (Fig. 4–8). While within the rectus sheath the nerves pass behind the rectus abdominis muscle and lie in front of the epigastric arteries. They then pierce the rectus muscle and the anterior rectus sheath to emerge anteriorly as the anterior cutaneous branches, which supply the overlying skin (Fig. 4–8). The lateral and anterior cutaneous branches supply the skin of the abdomen from the midline to the anterior axillary line. T7 provides sensory supply to the epigastrium, T10 to the umbilicus, and L1 to the groin.

The subcostal nerve is the anterior primary rami of the 12th thoracic nerve and enters the abdomen posteriorly under the lateral arcuate ligament of the diaphragm. It then passes laterally on the anterior surface of the quadratus lumborum muscle and pierces the transversus abdominis muscle to enter the TAP. The remaining part of the course of the subcostal nerve is similar to that of the other thoracolumbar nerves except that it supplies the pyramidalis muscle, and its lateral cutaneous branch supplies the upper and lateral aspect of the gluteal region (Fig. 4–1).

The first lumbar nerve (L1) divides in front of the quadratus lumborum muscle into the iliohypogastric and ilioinguinal nerves after which they pierce the transversus abdominis muscle to enter the TAP (Figs. 4–13 and 4–14). The iliohypogastric nerve then travels anteriorly in the TAP and pierces the internal oblique muscle about 1 inch in front of the anterior superior iliac spine (Fig. 4–1). It then becomes superficial by piercing the external oblique aponeurosis close to the superficial inguinal ring and supplies the skin over the suprapubic region. The lateral cutaneous branch of the iliohypogastric nerve supplies the upper and lateral aspect of the gluteal region (Fig. 4–1). The ilioinguinal nerve has no lateral cutaneous branch but also pierces the internal oblique muscle. It then traverses the inguinal canal with the spermatic cord or the round ligament of the uterus to emerge through the superficial inguinal ring or through the adjacent external oblique aponeurosis to supply the skin of the upper and medial aspect of the thigh and the genitals.

Gross Anatomy

The lateral (midaxillary) TAP refers to the neurovascular plane between the internal oblique and transversus abdominis muscle along the lateral abdominal wall (Figs. 4–10 and 4–11). The thoracolumbar nerves (T10-L1) traverse through the lateral (midaxillary) TAP.

Computed Tomography Abdomen Showing the Lateral (Midaxillary) Transverse Abdominis Plane

Fig. 4–15

Magnetic Resonance Imaging Abdomen Showing the Lateral (Midaxillary) Transverse Abdominis Plane

Fig. 4–16

Ultrasound Scan Technique

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   Position:

   
   
   
   
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      Patient: Supine with the abdomen exposed between the subcostal margin and the iliac crest.

      
      
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      Operator and ultrasound machine: Right-handed operators who hold the ultrasound transducer with their left hand and carry out needle interventions with their right hand should stand on the right side of the patient and position the ultrasound machine on the contralateral side and directly in front. This is vice versa for left-handed operators.

   
   
   
   
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   Transducer selection: High-frequency (13-8 MHz) linear array transducer.

   
   
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   Scan technique: The ultrasound transducer is placed in the transverse orientation to the lateral abdominal wall in the midaxillary line between the costal margin and the iliac crest (Fig. 4–17). The aim is to identify the three muscular layers of the lateral abdominal wall with the fascial layers that separate them in the sonogram. It may be necessary to gently slide the transducer in a craniocaudal direction or even gently tilt or rotate the transducer to obtain an optimal ultrasound image.

   
   
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   Sonoanatomy: On a transverse sonogram, the EOM, IOM, and TAM are identified as three longitudinal and hypoechoic structures deep to the skin and subcutaneous tissue (Fig. 4–18). A hyperechoic fascial layer (possibly the epimysium of the individual muscle) is seen between the three muscles (Fig. 4–18). The EOM is the outermost (superficial) layer, the IOM the intermediate, and the TAM is the innermost layer. The thickness of the muscles also varies, but the TAM is in general the thinnest and it also appears darkest (hypoechoic) of the three muscles on the sonogram (Fig. 4–18). The TAP is located between the IOM and TAM (Fig. 4–18). Deep to the TAM are the fascia transversalis and the underlying peritoneum, which also appear hyperechoic (Fig. 4–18). It is difficult to differentiate the fascia transversalis from the peritoneum on a sonogram, but the peritoneum can be identified as a hyperechoic layer by observing peristaltic movement of the bowel loops (Fig. 4–18). The segmental thoracolumbar nerves are small terminal branches and are difficult to define within the TAP using ultrasound. Occasionally the terminal nerves may be seen in the TAP as multiple flat, hyperechoic structures (Fig. 4–19). This is best done by locating the nerves distally in the groin (iliohypogastric and ilioinguinal nerve) and then tracing them (trace back technique) back to the TAP.

   
   
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   Clinical Pearls: During a lateral (midaxillary) TAP block with an in-plane needle insertion, the point of needle insertion (ie, how far medial to the transducer) can be determined by noting the depth at which the TAP is located on the ultrasound monitor (depth scale). Normal saline can be used to hydrodissect the TAP to confirm correct needle tip position before the local anesthetic is injected. It is common to see a prominent bulge along the lateral abdominal wall, indicating paralysis of the abdominal muscles, during the postoperative period after a posterior TAP block.

Gross Anatomy

Subcostal TAP refers to the neurovascular plane between the IOM and the TAM that lies just below the costal margin (Fig. 4–9). The terminal branches of the intercostal nerves (T7-T9) emerge from under the costal margin and enter the subcostal TAP. T7 and T8 nerves pass deep to the costal margin and between the digitations of the TAM to enter the TAP, and T9 and T10 nerves exit from their respective intercostal spaces directly into the TAP.

Computed Tomography Abdomen Showing the Subcostal Transverse Abdominis Plane

Fig. 4–20

Magnetic Resonance Imaging Abdomen Showing the Subcostal Transverse Abdominis Plane

Fig. 4–21