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 mid-

axillary 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 sono-

anatomical 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 – 10 th

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

References

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