Thoracolumbar Nerve Blocks - Transabdominis plane, Rectus sheath, and
Ilioinguinal-Iliohypogastric Nerve blocks: A Review of the Anatomy and
Techniques.
1.
The Anatomy of the anterolateral abdominal wall.
The borders of the anterolateral abdominal wall are the costal margin superiorly,
the pubic crests inferiorly and the posterior abdominal wall laterally at the midaxillary lines. The layers of the abdominal wall are: skin and fascia, muscles, fascia
transversalis, extra peritoneal fat and the parietal peritoneum. Towards the midline,
the abdominal wall musculature comprise the paired rectus abdominis muscles
adjacent to the midline linea alba. The anterolateral abdominal wall contains three
muscles, from superficial to deep: the external oblique, internal oblique and
transversus abdominis; each covered within its own fascial sheath. Features such
as the direction of muscle fibres, their origins and insertion points distinguish the
abdominal wall muscles. The three lateral muscles are replaced by an aponeurosis
(the linea semilunaris) at the lateral border of the rectus sheath. At this point the
internal oblique aponeurosis splits into two layers1,2 and is an important sonoanatomical landmark for performing transabdominis plane (TAP) and rectus sheath
blocks. The linea semilunaris comprises the aponeurosis of both the internal and
external oblique muscles and the transversus abdominis muscles and it continues
medially to meet its contralateral counterparts via the linea alba in the midline. The
linea semilunaris extends from the cartilage of rib 9 to the pubic tubercle.
The superficial fascia of the abdominal wall comprises a single layer with variable
amount of adipose tissue. Closer to the groin it is divisible into a superficial and
deep layer. The deeper layer (Scarpa’s fascia) is thinner and more membranous
than the superficial layer and blends with the fascia lata of the thigh below the
inguinal ligament.3
The external oblique muscle is a broad quadrilateral muscle that runs from
superolateral origins (external surface of ribs 5 -12) to inferomedial insertions
(anterior half of the iliac crest).2,4 Its fibres begin from the outer surface of ribs 5-12
and insert into the xiphoid process, the linea alba, the pubic crest, the pubic
tubercle, and the anterior half of the iliac crest. It’s upper and lower four serrations
run under the digitations of serratus anterior and latissimus dorsi respectively. The
muscle’s insertions are by means of a broad aponeurosis that covers the centre of
the abdominal wall. The inferior border of this aponeurosis thickens and forms the
inguinal ligament between the anterior superior iliac spine (ASIS) and the pubic
tubercle.
The internal oblique muscle is immediately deep to the external oblique. It
originates from the lumbar vertebrae (via thoracolumbar fascia), the anterior two
thirds of the iliac crest and the lateral two thirds of the inguinal ligament and inserts
into the lower borders of ribs 10, 11 and 12 and costal margin (upper margin – 10th
costal cartilage),5 the xiphoid process, the linea alba, and the pubic symphysis.2,4,5
The directions of its fibres are perpendicular to those of external oblique, except for
the fibres originating from the inguinal ligament which are parallel to the external
oblique.
The transversus abdominis muscle is deep to the internal oblique muscle and is a
thin muscle layer that runs horizontally across the abdominal wall. It has a broad
origin from the inner surface of the lower six costal cartilages (interdigitating with
the diaphragm), the lumbar vertebrae (via thoracolumbar fascia), the anterior two
thirds of the iliac crest, and the lateral third of the inguinal ligament.2 The fibres
become aponeurotic anteriorly and insert into the xiphoid process and the pubic
symphysis.
The neurovascular plane between the internal oblique and transversus abdominis
muscles is known as the TAP. It contains a fascial layer that is not adherent to the
internal oblique, but binds the intercostals nerves on its deep surface to the
transversus abdominis muscle. This layer is present throughout the TAP and
extends medially to the linea semilunaris.6
The rectus abdominis muscles arises by two tendinous heads from the pubic
symphysis medially and the pubic crest laterally.2 The two muscles lie edge to
edge in the lower part 1 but above the arcuate line they are separated by the linea
alba due to the presence of the posterior layer of the internal oblique fascia. Above
the arcuate line the rectus abdominis becomes broader but thinner. The rectus
abdominis is divided by three tendinous intersections,7,8 strongly attached to the
rectus sheath.
The rectus sheath contains the rectus abdominis muscles, the anterior rami of the
lower sixth thoracic nerves (T7-T12), the superior and inferior epigastric vessels
and lymph vessels. It is formed by the fusion of the aponeuroses of external
oblique, internal oblique and transversus abdominis. Above the costal margin, the
rectus sheath attaches to the 5th – 7th costal cartilages and corresponding
intercostal spaces.1 From the costal margin to the arcuate line, the external oblique
aponeurosis fuses with the anterior layer of the internal oblique aponeurosis to
become the anterior layer of the rectus sheath; whilst the posterior layer of the
internal oblique aponeurosis together with the transversus abdominis aponeurosis
form the posterior layer of the rectus sheath.1,9 Below the arcuate line, the
aponeuroses of all three lateral muscles pass anteriorly; thus the rectus muscle lies
posterior to the rectus sheath against the transversalis fascia. The position of the
arcuate line is variable and is rarely symmetrical.10,11 However some authors doubt
the existence of the arcuate line and report that the posterior layer of the rectus
sheath thins into transversalis fascia, while the anterior layer becomes thickened.12
In contrast, close to the xiphoid process, is the presence of the transversus
abdominis muscle posterior to the rectus sheath and rectus abdominis muscles
providing a potential site of injection of local anesthetic to cover T6 –T7.
The extraperitoneal connective tissue between the parietal peritoneum and the
transversalis fascia comprises an adipose layer and a fibrous or membranous layer
in contact with the transversalis fascia. The extraperitoneal layer adjacent to the
peritoneum can be a very distinct, transparent and fairly tough layer.13
2.
Course of the thoracolumbar nerves (T6 – L1) innervating the
anterolateral abdominal wall.
The posterior branches of the spinal nerves supply the skin of the back and the
back musculature. The intercostal nerves are the anterior primary rami of T1 – T11,
T12 is termed a subcostal nerve because it does not run between two ribs. The
intercostal nerves exit the intervertebral foramen. enter the paravertebral region
and become related to the intercostal muscles posteriorly. The nerves then run
posterolaterally within the grooves of their corresponding ribs between the
innermost and internal intercostal muscles. T12 has a similar relationship except it
is a subcostal nerve. Segmental nerves T6 – T9 emerge from the costal margin to
enter the TAP, between the midline and the anterior axillary line. T6 enters the TAP
just lateral to the linea alba and T7, T8 and T9 emerge from the costal margins at
increasingly lateral positions respectively.6 The T10 segmental nerve is located
caudal to the costal margin (rib 10) and T11, T12, and L1 are located in a caudal
direction, to the level of the iliac crest. The intercostal, subcostal and first lumbar
nerves are located in the TAP as are the blood vessels (deep circumflex iliac,
inferior epigastric, superior epigastric). Proximal to the angle of the ribs T6 – T12
nerves send lateral cutaneous branches to the dermis, which in turn bifurcate into
anterior and posterior cutaneous branches that supply the skin of the lateral torso.
Note that the origins of the lateral cutaneous nerves are at or proximal to the costal
angle. However, the point at which it pierces the muscle layers is at the angle of rib
or mid-axillary line.5 The lateral cutaneous branches are significant because they
send fibres that supply much of the abdominal wall (T6 – T12) and thorax (T1 – T5).
The anterior cutaneous nerve sends twigs to the external oblique muscle as well as
skin to the lateral margin of the rectus abdominis. The posterior cutaneous branch
runs backwards supplying the skin to the lateral border of paravertebral region.
The main intercostal nerve throughout its course supplies adjacent muscle (e.g.
innermost intercostal) in addition to parietal pleura and peritoneum. There is also a
collateral branch formed before the angle of the rib that runs in the intercostal
space before entering the posterior rectus sheath. The collateral branch may join
the main intercostal nerve for a distance and then separate to form an additional
anterior cutaneous branch.14
The iliohypogastric and ilioinguinal nerves pass laterally through the psoas muscle
and then course anterior to quadratus lumborum and then caudally towards the
iliac crest. The nerves continue their course on the inner surface of the transversus
abdominis muscle and then pierce the transversus abdominis muscle at a variable
location and run for a short distance in the TAP. Most of the literature considers
these nerves to be variable in their origin, course and distribution.15-17 While most
texts describe them as originating from L1, there is some contention about their
origin.15 In one cadaver study, both the nerves were located in the TAP 5 cm
cranial and posterior to the ASIS.18 This is consistent with recommendations made
by Jamieson, that the initial point of injection for both nerves be 4 – 6 cm posterior
to the ASIS (along the external lip of the ilium) where the nerves perforate the
transversus abdominis muscle.19 Medial to the anterior superior iliac spine the
ilioinguinal nerve (IIN) pierces the internal oblique close to the inguinal ligament16,20
and it is at this location that landmark techniques usually aim to locate the IIN. The
iliohypogastric nerve (IHN) and the IIN supply the skin and muscles of the pubic
and inguinal region and genitalia. In summary, the IHN and IIN are located in the
TAP close to the iliac crest and anterior superior iliac spine.18 Medial and inferior to
the anterior superior iliac spine the nerves are located between the internal and
external oblique muscles.15
Rozen et. al.6 documented extensive communication between T9 – L1 in the TAP.
For example in one cadaver study both T11 and T12 nerves had communicating
branches to the nerves of segmental origins above and below. Similarly Sakamoto
documented complex distal communications between all thoracolumbar nerves.5
Because of the proximal communications the nerves in a given rectus sheath
‘segment’ correspond to multiple spinal origins.
The intercostal nerves (lower five) pierce the lateral margin of the linea semilunaris
to enter the rectus sheath posterolaterally. The intercostal (T6 – T9), subcostal and
L1 nerves terminate in the rectus abdominis muscle with three patterns: 1.
Terminate simply within the muscle, 2. Supply the muscle and then terminate as a
cutaneous branch, and 3. Pass through the muscle and terminate as a cutaneous
branch.5 Rozen et. al.6 also noted that as the nerves approach the posterior
surface of the rectus abdominis, a longitudinal branch of fibres run craniocaudally
with the deep inferior epigastric artery (DIEA). In addition, the cutaneous branches
were closely related to the perforating musculocutaneous vessels. Together T7 –
L1 supply the skin from the xiphoid sternum (T7 nerve root) to the pubic symphysis
(L1 nerve root), with T10 nerve supplying the umbilical segment. The IHN nerve
supplies the lowermost segment of the rectus abdominis muscle and overlying skin.
3. Transabdominis plane block.
The TAP block is a relatively new regional analgesic technique suitable for use in
abdominal surgery. TAP block involves injection of local anesthetic into the
neurovascular plane (TAP) between the internal oblique and transversus
abdominis muscles. The TAP block was first described by Rafi21 and then subject
to randomized controlled trials by McDonnell.22,23 Their landmark approach utilized
the lumbar triangle of Petit to access the neurovascular plane of the abdomen.
Ultrasound techniques (posterior and subcostal) were then described and
popularized by Hebbard.24,25 TAP block has been combined with multimodal
analgesia for postoperative analgesia following colonic surgery23,
appendicectomy26, cholycystectomy, prostatectomy, caesarean section27,
gynaecological surgery28, hernia repair and other types of abdominal surgery. The
TAP block is well described following paediatric surgery29,30 and suited for
paediatric patients providing analgesia following ambulatory surgery including
hernia repair.31 A systematic review of studies has demonstrated clinically
significant reductions of post-operative opioid requirements and pain32, in addition
to reduced opioid-related side effects.33 Recently, TAP block has been combined
with sedation and utilized for surgical anesthesia for caesarean section.34
Knowledge of the location of the thoracolumbar nerves in the transversus
abdominis plane is the basis for this procedure.
A. Techniques and approaches
1. Triangle of Petit (Landmark technique)
The triangle of Petit is formed by the iliac crest as the base, the external oblique
muscle as the anterior border and the latissimus dorsi muscle as the posterior
border. The floor of the triangle is formed by the fascia of the external and internal
oblique muscles. This is the only location in the abdominal wall where the internal
oblique muscle is readily accessible. The space can be located by palpating from
the anterior superior iliac spine posteriorly along the iliac crest till the finger slips
over the edge of a muscle.21 Using this technique, a short-bevel or blunt needle is
inserted immediately cephalad to the iliac crest through the skin perpendicular into
the triangle with two pops indicating entry into the TAP. Further detail can be found
in the initial description by Rafi21 and also in subsequent work by McDonnell.35
2. Ultrasound guided techniques
Regardless of side of TAP block, the ultrasound machine can be placed on the
opposite side of the patient to where the proceduralist stands, so that he/she
directly faces the screen. An intermediate frequency probe (with a wide field of
view) and a 100 – 150 mm needle are suitable. Although the abdominal wall
sonogram is straightforward, identification of the exact muscular layer in obese
patients can be challenging. Therefore, a useful starting point for both the
subcostal and posterior approaches is the linea semilunaris. This aponeurosis is
readily identified with sonography, separating the rectus abdominis from the three
muscles of the lateral abdominal wall and dynamic scanning from medial to lateral
or vice-versa helps with correct identification of muscle layers. In many patients,
the rectus abdominis muscle may be displaced further away from the midline than
expected. The presenter’s preferred needle trajectory is from medial to
posterolateral using an in-plane technique with the probe initially located medially.
Needle imaging can be improved by separating the initial needle entry point
through the skin from the probe by 2 – 5 cm and heel-toe the probe.
Oblique Subcostal TAP block
It is useful to locate the tranversus abdominis muscle posterior to the rectus
abdominis muscle close to the xyphoid process.25 The segmental nerves are
located in this plane but T6 – T 8 may only have a short course between the
transversus abdominis and the rectus abdominis muscles. The nerves may, after a
short distance beyond the costal margin penetrate the posterior rectus sheath and
rectus muscle, therefore the injection close to the costal margin may improve
anesthesia.36 Beginning at the transversus-rectus abdominis interface medially, an
incremental injection of 3 – 5 mls of local anesthetic can then be extend the
hydrodissection laterally along a line parallel to the costal margin (oblique
subcostal line) so that the number of nerves involved in local anesthetic is
maximized.36
Posterior TAP block
The ultrasound probe is positioned in the mid to anterior axillary line between the
iliac crest and the costal margin. In many patients the distance between these two
landmarks is small, so the exact position (e.g. halfway between the iliac crest and
costal margin) probably does not matter. An in-plane technique with the needle
directed anterior to posterior (towards the midaxillary line) is straightforward.
Incremental injections, hydrodissection then needle advancement is similar to the
subcostal technique.
3. Local anesthetic dosage, volume and spread
There have been inconsistent descriptions of the extent of sensory blockade
achieved with TAP. In one cadaver study a mock ultrasound-guided posterior TAP
block demonstrated that segmental nerves T10, T11, T12, and L1 were involved in
dye in 50%, 100%, 100%, and 93% of the hemi-abdomens respectively.37 This is
consistent with a clinical study where the posterior approach consistently produced
sensory block of three dermatomal segments using a 20 ml injectate, the most
cephalad being T10.38 Confinement of sensory blockade to the lower abdomen
following posterior TAP block is consistent with correspondence from Shibata et.
al.39 who assessed sensory block in 26 patients having laparoscopic
gynaecological surgery and reported sensory block over the T10–L1 dermatomes.
In contrast, a study by McDonnell and colleagues reported a sensory block from T7
to L1 in three volunteers when a landmark technique was used to access the TAP
through the lumbar triangle.40
Sensory blockade following the subcostal approach results in of four segments
blocked, the most cephalad being T8.38 Ninety-eight percent of patients had some
degree of demonstrable sensory block and the dermatomal spread differed
between posterior and subcostal approaches (P <0.001).38 The posterior approach
appears to be appropriate for lower abdominal surgery and the subcostal approach
better suited to upper abdominal surgery. The spread of injectate following TAP
block is limited by the linear semilunaris and tendinous intersections of rectus
abdominis. The posterior rectus sheath, together with the tendinous intersections,
can limit spread of and contain local anaesthetic within the injected rectus sheath
segment. Optimal volumes and concentrations of local anesthetics for use in TAP
block have yet to be determined. It is the presenter’s practice is to use
approximately ropivacaine 3 – 3.5 mg/kg as the total dose received by the patient
diluted to 20 - 30 ml for each side.
4. Rectus sheath block.
The rectus sheath block can provide postoperative analgesia for abdominal
surgery requiring midline incisions around the umbilicus and has been used
effectively following incisional and umbilical hernea repair, caesarian section (when
midline incision used) and laparoscopy. The rectus sheath block aims to block the
terminal branches of segmental nerves, T6 – L1. Anteriorly, the rectus sheath is
tough and fibrous from the xiphoid to the pubis. Posteriorly, it is well defined and
identifiable till the level of the umbilicus and then inferiorly fades into the
transversalis fascia. The posterior rectus sheath is substantial and has been
described as a “backboard” for injecting local anesthetics.12
Techniques and approaches
1. Loss-of-resistance (LOR) technique
This technique relies on the LOR indicating entry of a block needle through the
anterior layer of the rectus sheath into the rectus sheath. Probably, this technique
has been underutilized because of perceived accuracy in needle tip location and
underlying peritoneum. If ultrasound equipment and appropriate skills were
available, then the LOR technique is probably not justified.
2. Ultrasound-guided techniques
Identifying the linea semilunaris on the lateral border of the rectus abdominis
muscle as for the subcostal approach is very useful, then slide the transducer
medially to locate the rectus abdominis. The layers of the rectus sheath are readily
identified with ultrasound with transverse scanning of the rectus abdominis and
either out-of-plane or in-plane needle imaging techniques. In a training study,
ultrasound techniques significantly improved the accuracy of needle placement
compared with a LOR technique.41 Ultrasound-guided rectus sheath catheters
have been recently described using an in-plane technique.42
5. Ilioinguinal-iliohypogastric nerve blocks.
IIN and IHN blocks are commonly performed blocks suitable for
anesthesia/analgesia for procedures such as hernea repair.
Techniques and approaches
1. Loss-of-resistance (LOR) technique
This technique relies on the LOR indicating entry of a block needle through the
external and internal oblique fascial planes usually medial and inferior to the ASIS.
2. Ultrasound-guided techniques
Identification of the TAP is obtained close to the anterior superior iliac spine by
using the same dynamic transducer movements beginning medially to identify the
rectus and then sliding laterally to identify, first the linea semilunaris and then the
three muscle layers of the anterolateral abdominal wall. Because of variability in
where the nerves enter the TAP, it is worthwhile not just relying on injection in the
plane, but actually locate the nerves with sonography.18
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