Ultrasound Blocks for the Anterior Abdominal Wall

18 | Ultrasound Blocks for the Anterior Abdominal Wall
intercostal plexus. The nerves from T9 to L1 contribute to a
longitudinal nerve plexus, named the transverse abdominal
muscle plexus, that lies alongside and lateral to the ascending
branch of the deep circumflex iliac artery (Rozen 2008).
The nerves from T6 to L1 form a further plexus into the rectus
sheath named the rectus sheath plexus. This plexus runs
cranial-caudally and laterally to the lateral branch of the deep
inferior epigastric artery (Rozen 2008). A branch of T10
innervates the umbilicus.
Iliohypogastric and Ilioinguinal Nerves
The lumbar plexus, formed by the ventral branches of the
spinal nerves from L1 to L4, projects laterally and caudally from
the intervertebral foramina. Its roots innervate the lower part of
the anterior abdominal wall, the inguinal field, through the
iliohypogastric nerve (IHN-greater abdominogenital nerve), the
ilioinguinal nerve (IIN-minor abdominogenital nerve) and the
genitofemoral nerve (GFN) (Horowitz 1939).
A communication branch from T12 that is called the subcostal
nerve may join in 50 to 60% of cases the anterior primary
division of L1. More rarely a branch of T11 may also join L1.
The IIH and IIN pass obliquely through or behind the psoas
major muscle and emerge from the upper lateral border of the
psoas major muscle at the L2 to L3 level (Mirilas 2010). The IHN
nerve, the first of the lumbar plexus, and the IIN may be found as
a single or divided trunk in the retroperitoneal space. They cross
obliquely parallel to the intercostal nerves and behind the lower
pole of the kidney towards the iliac crest (which explains the
referred pain to genitalia in kidney and ureter affections)
(Anloague 2009).
Above the iliac crest, the IHN and IIN pierce the posterior
surface of the TAM and run between this muscle and the IOM
toward the inguinal region (Jamieson 1952, Mirilas 2010).