++
Traditionally, lower limb PNBs have not been popular for use in
children; caudal blocks are more frequently used.1
Nevertheless, selective blockade of children's lower limbs can be very
useful in a number of clinical scenarios. Because the available data
on lower limb blocks were collected mostly in adults, the volumes of local
anesthetic used in children frequently were derived from adult studies. The
insertion techniques used in children are typically extrapolated from the data in
adults as well. All available techniques that rely on ultrasound guidance
for lower limb nerve blockade in children are described in the section that
follows.
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Psoas Compartment Block
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Some 30 years ago, Chayen and coworkers20 and Winnie
and colleagues21 described a posterior approach to the
lumbar plexus in adults. The clinical term psoas compartment block was coined by the Chayen group
at that time.20 Although it is normally inappropriate by
both physiologic and anatomic criteria to think of children as “small
adults,” the lumbar plexus is usually characterized by almost identical
anatomic and topographic conditions, except that it is naturally less deep
in children. Using either the technique described by Winnie or one modified
from Chayen, Dalens and coworkers22 investigated two
posterior approaches to the lumbar plexus in children undergoing surgical
procedures of the hip and femur and found a considerable age-dependent
variability. More recently, Dalens provided data on the depth of the lumbar
plexus based on body weight.23
++
Dadure and investigators used CT scanning in an attempt to measure the
depth of the lumbar plexus, but their attempts to visualize neural
structures failed in most children.24 Therefore, they had
no choice but to take an educated guess as to the depth of the plexus inside
the posterior segment of the psoas major muscle. With the help of
ultrasonography, we were able to visualize the lumbar plexus and its
surrounding structures, and to measure its distance, in an accurate
manner.25 The fact that the neural structures can be
visualized in children while they cannot be visualized in adults is due to
the ability to use higher resolution probes because of the shallower depth
of the plexus. Note that the lumbar plexus is located in the transition zone
between the posterior and medal thirds of the psoas major muscle both in
adults and in children.
++
++
Although peripheral nerves are usually visualized with linear
transducers, better results were obtained with convex transducers in the
lumbar paravertebral region, since the convex geometry of the ultrasound
field allows for better imaging of the paravertebral structures. In newborns
and infants, however, linear transducers with a small array surface are
preferable.
++
The first step toward visualizing the lumbar plexus by ultrasonography
is to locate the access level (usually L4 or L5). In a paravertebral
longitudinal section, the costal processes of the lumbar vertebrae are
visualized and counted one by one in a caudocranial direction, beginning at
the dorsal ultrasound reflection of the sacrum. The transducer is then
gradually shifted parallel to the spinous processes in a cranial direction.
Once the L4-5 intervertebral space is reached, the transducer is turned by
90 degrees to a transversal plane (Figure 52–23). Based on this
configuration, the lumbar plexus can be targeted inside the posterior
segment of the psoas major muscle. In infants and small children, a
perpendicular needle orientation transverse to the transducer should be
selected because of the limited space and short distance to the plexus (only
2–3 cm; Figure 52–24). By contrast, the in-line technique, as
used in adults, is also the method of choice in larger children and
adolescents.26
++
++
++
Potential indications for posterior blockade of the lumbar plexus in
children include surgical treatment of the hip, femur, and knee joint. The
use of ultrasonography in psoas compartment blocks is justified by the risk
of epidural/intrathecal injection. Damage to the kidney is another potential
risk (the kidneys extend down to the L4 to L5 level in small
children).27
++
The technique of inguinal perivascular blockade of the femoral,
obturator, and lateral cutaneous femoral nerves was first described by
Winnie and colleagues in 1973.28 Much debate ensued about
whether this technique was really capable of blocking all three nerves and
on the spread direction of the local anesthetic. Originally, it was assumed
that the local anesthetic spread proximally from the inguinal puncture site
to the lumbar plexus along a muscle–fascia sheath. This assumption was
clearly refuted by our study group in an MRI investigation that demonstrated
that the local anesthetic exclusively spread in a lateral and medial
direction with this technique.29 The quality of sensory
blockade depends more on the needle insertion technique than on the volume
of local anesthetic.4,30 This finding in particular has
important implication in children, in whom the use of large doses of local
anesthetic may be prohibitive in this technique.
++
Unlike other lower limb peripheral blocks, however, the 3-in-1 technique
is relatively well documented with conventional methods of nerve
identification in children. Most commonly, this technique is used for muscle
biopsy in children (eg, in making the diagnosis of malignant hyperthermia or
neuromuscular diseases).31–33 Various investigators used
the technique for both single-shot and continuous applications of
perioperative pain therapy in femoral fractures.34–38
They obtained respectable success rates (up to 96%) with this
approach.37 However, an ultrasound-guided 3-in-1 block
offers a number of advantages:
++
The volumes of local anesthetic can be further reduced, and combined
nerve blocks can be performed more safely. For example, the sensory nerve
supply to the femoral shaft is affected by both the femoral and sciatic
nerves; hence the sciatic nerve should be additionally blocked for effective
pain therapy in femoral shaft fractures.
Because the femoral nerve is located very close to the femoral artery,
there is also a risk of arterial puncture.
Using ultrasound guidance, it should be possible to direct the spread of
the local anesthetic in a lateral and (to a smaller extent) medial
direction, such that a true 3-in-1 block is achieved. Also, it should be
possible to selectively block the femoral nerve only by reducing the amount
of local anesthetic (volume-dependent differential
blockade).
A high-frequency ultrasound probe is advanced to a point in the immediate
distal vicinity of the inguinal ligament to visualize the femoral artery
and, more laterally, the femoral nerve. The weight of the transducer alone is
usually sufficient to compress the femoral vein, which is located medial to
the artery. The space between the artery and the nerve harbors the
iliopectineal fascia, representing the deep folium of the inguinal ligament.
The femoral nerve is usually very close to the skin and it can be
visualized in its entirety only from a position immediately distal to the
inguinal ligament (Figure 52–25) because it soon divides into its
distal branches.
++
++
++
As in most peripheral blocks, the needle is inserted perpendicular to
the transducer (Figure 52–26). The needle can be placed either
lateral or medial to the femoral nerve. Note that the nerve is often
located very close to the artery, so the needle must be advanced with care
if a medial position (toward the artery) is selected. It is, therefore,
usually better to select a lateral position. The amount of local anesthetic
to be injected depends on how many nerves are targeted. For selective
blockade of the femoral nerve, it is sufficient if the injectate covers just
the surface of the nerve. However, if a 3-in-1 block is required, a larger
volume must be injected, so that the local anesthetic visibly spreads in a
lateral and medial direction. Note that blockade of the obturator nerve is
usually confined to its anterior ramus. Therefore, the 3-in-1 block is
really a “2.5-in-1” block (refer to Chapter 36). Direct visualization of the lateral cutaneous
femoral nerve requires the use of a high-resolution transducer and is only
possible in larger children. A limitation to this technique is that the
ultrasound-assisted technique for 3-in-1 blocks still relies on the lateral
and medial distribution pattern of the local anesthetic to reach the
obturator nerve because this nerve normally cannot be visualized by
ultrasonography due to its position between the adductor muscles and the
small diameter of its two branches.
++
++
The 3-in-1 technique is particularly well suited for continuous nerve
blockade. This approach was described for conventional methods of needle
guidance as well.34,35 The same puncture technique is used
as with the single-shot approach. Following injection of the local
anesthetic, the catheter is placed underneath the iliopectineal fascia under
ultrasound guidance. It is also possible to selectively block the femoral
nerve in this way. The catheter is not directly visible in the majority of
cases; however, a small amount of local anesthetic injected on a preliminary
basis will be enough to foresee the spread direction and whether successful
blockade can be expected, once the full volume is used. It is essential to
decide beforehand whether a selective femoral nerve block or a complete
3-in-1 block is required. For selective femoral nerve blockade, the local
anesthetic can be applied continuously through the catheter (eg, 0.1–0.2
mL/kg of local anesthetic in a low concentration). For 3-in-1 blockade, an
initial bolus dose is recommended, although low concentrations of a
long-acting substance, such as levobupivacaine (0.125%) or ropivacaine
(0.2% twice daily), usually is sufficient. Ultrasound monitoring can be
performed while the bolus is being applied through the catheter to identify
its position.
+++
Selective Saphenous Nerve Block
++
The saphenous nerve is best located with a high-resolution ultrasound
probe at the distal-medial thigh level in the transition zone between the
sartorius and gracilis muscles and their attachment of their respective
tendons (Figure 52–27). The puncture should be transverse to the
proximal aspect toward the transducer (Figure 52–28) or performed
using the in-line technique. Only small amounts of local anesthetic are
required for blockade. In addition, smaller volumes are advisable because a
larger volume of local anesthetic may cause excessive pressure in this
tissue compartment and, consequently, increase the risk of nerve injury.
++
++
++
Sciatic nerve blocks should form an integral part of every well-trained
anesthesiologist's repertoire. It is desirable that several approaches to
the sciatic nerve are mastered because the need for various approaches is
dictated by the nature of the surgical procedure, as well as by the
positioning of the child and the affected limb. In clinical practice, good
results were obtained with the subgluteal, the midfemoral, and the popliteal
access route. A clear-cut distinction between these three approaches cannot
always be made. Perhaps the greatest advantage of ultrasound guidance for
sciatic blockade is that it enables the anesthesiologist to block nerves at
any location without the need to use surface landmarks as reference.
Therefore, selecting a site of optimal sonographic visibility is the only
consideration in addition to selecting the level of blockade depending on
the type of surgery and patient positioning. The sciatic nerve is not
uniformly accessible to ultrasound imaging over its entire course.
++
The success rates achieved with conventional guidance techniques (mainly
identification of the nerve with a nerve stimulator) for sciatic nerve
blockade in children are excellent. For instance, Konrad and
Jöhr39 and Tobias and Mencio40
reported successful blocks in over 90% of cases. However, the large
volumes of local anesthetic (0.75–1.0 mL/kg) used in these studies may not
allow for the use the additional combined blocks (eg, femoral or 3-in-1
blocks). Furthermore, nerve stimulation or other indirect techniques of
nerve identification carry a certain risk of damaging the targeted nerve
during puncture due to the shear size of the sciatic nerve. Ultrasound
guidance may be superior in this regard because, theoretically, it should
allow the needle to be safely inserted, avoiding direct nerve contact.
++
When it comes to ultrasound-guided sciatic nerve block, the choice of
approach is different from those used with the nerve stimulator-guided techniques.
For instance, the approach described by Labat, which enjoys widespread
popularity in adults, should not be used in children because ultrasound
visualization may be limited in this area. This is because the nerve is
located underneath several muscle layers, such that the high-frequency
ultrasound probes cannot be used. A similar problem is encountered with the
anterior approach, in which the sciatic nerve is overlapped by the
trochanter minor. In addition, the anterior approach is very uncomfortable
due to the deep location of the nerve. For these reasons, we do not use the
anterior approach, even though Aizenberg and investigators reported on its
use in children.41 Dalens and coworkers demonstrated, in a
comparative study, that the posterior and lateral access routes were both
more reliable and more practical than the anterior approach in
children.42 Their findings are generally in agreement with
the following discussion of meaningful approaches to ultrasound-guided
sciatic nerve blocks.
++
The subgluteal approach is the most proximal approach we use for
sciatic nerve blockade under ultrasound guidance. Bösenberg employed a
nerve stimulator for this access route,38 thereby
achieving good success rates despite injecting relatively small amounts of
local anesthetic (0.5 mg/kg for unilateral and 0.3 mg/kg for bilateral
blocks). Gray and coworkers mention ultrasonography as an alternative option
to guide sciatic blocks through the subgluteal approach.43
++
The child may remain in a supine position for this technique, but a
prone or side position is also possible. We always try to meet individual
requirements, thus selecting the most comfortable position for the child.
++
The sciatic nerve is located close to the surface in the immediate
subgluteal area, requiring the use of a high-frequency linear ultrasound
probe. Figure 52–29 illustrates the position of the sciatic nerve
between the gluteus maximus and quadratus femoris muscles (lateral) on the
one hand and the biceps femoris muscles (medial) on the other. The posterior
cutaneous femoral nerve usually can be visualized medial and slightly more
superficial to the sciatic nerve. In association with a thigh tourniquet,
this nerve should also be blocked. Therefore, indications for this approach
include not only situations in which good ultrasound visibility is required
but also the use of a tourniquet.
++
++
Figure 52–30 illustrates the needle position relative to the
ultrasound transducer, the child being in a supine position with the hip and
knee flexed. After the needle has been placed medial to the sciatic nerve,
the local anesthetic usually spreads to the posterior cutaneous femoral
nerve. If the initial injection fails to reach the nerve, the needle is
repositioned more medial to the posterior cutaneous femoral nerve to
optimize the distribution of local anesthetic. As a rule, however, a single
shot will suffice.
++
++
The midfemoral approach to the sciatic nerve is usually selected only
if a subgluteal or popliteal approach is not possible, usually because some
segments of the sciatic nerve are not accessible to ultrasonography. A
clear-cut distinction between the midfemoral and popliteal approach cannot
always be made.
++
The same needle position is selected as with the subgluteal approach
(Figure 52–31). The in-line technique is a viable option for this
type of puncture. With increasing displacement of the transducer in a distal
(popliteal) direction, practical considerations will dictate that the needle
insertion be parallel to the long axis of the transducer.
++
++
It is essential to track the route of the sciatic nerve in a distal
direction, thereby visualizing its separation into the tibial and peroneal
nerves. Schwemmer and coworkers reported that the level at which this
separation takes place varies widely,44 which is in
accordance with our own observations. This variability is not known to
correlate with body weight or body height. Therefore, if complete blockade
of the sciatic nerve is required, the needle insertion site should be
proximal to this point of separation. Again, this requirement can only be
accurately met through direct sonographic visualization.
++
Figure 52–32 illustrates anatomic conditions in the midfemoral
access area proximal to the furcation site of the sciatic nerve as
visualized by ultrasound. The puncture itself is carried out in the same way
as the subgluteal puncture. It should be placed between the biceps femoris
and semimembranous muscles. Puncturing the muscles should be avoided to
reduce the risk of hematoma during blockade. Depending on how deep the nerve
is located, the use of a longer needle (70 mm) may be indicated. In the
overwhelming majority of cases, the needle has to be repositioned several
times to optimize the distribution of the local anesthetic.
++
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Popliteal Sciatic Block
++
The popliteal approach to the sciatic nerve, also referred to as a
fossa popliteal block, is very useful in pediatric
anesthesia. One of its advantages is that blockade from a dorsal route is
relatively painless in this rhombus-shaped knee area (whose upper sides are
formed laterally by the femoral biceps muscle and medially by the
semimembranous and semitendinous muscles), as no muscle bellies are
perforated during puncture. One shortcoming of conventional guidance by
nerve stimulation is, however, that no reliable information is obtained about at which
level the sciatic nerve bifurcates into the tibial and peroneal nerves. The
only practical way to identify this level, which varies widely between
children, is by ultrasonography.44
++
The conventional technique is described as proximal to the fold of the
popliteal fossa, with the puncture in a somewhat lateral position to the
midline at a 45-degree angle relative to the skin, using 0.75–1.0 mL/kg of
local anesthetic.45 According to Konrad and Jöhr, body
weight shows the best correlation with tibial nerve depth in
children.39
++
A high-frequency probe should be used for popliteal access to the sciatic
nerve in order to accurately identify the point at which the nerve furcates
into its two branches, Figure 52–33 illustrates the sciatic nerve
proximal to this furcation site. More distally, the peroneal nerve divides
at a very superficial level, and the tibial nerve courses distally and
deeper. The lateral approach to popliteal block is particularly suitable
(Figure 52–34) because the child may remain in a supine position
during the blockade, similarly to the technique in adult
patients.46,47 The exact puncture site is selected based
on the depth of the nerve visualized in the ultrasound image. The needle tip
is first positioned above and then below the sciatic nerve, which will
ensure an optimal distribution pattern of the local anesthetic. Naturally,
the advantage of not transversing the muscles is lost with this technique.
Therefore, a distinction must be made between the lateral (in-line) and
dorsal (cross-sectional) techniques.
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++