Operations
on the lower extremities are commonly performed with either subarachnoid or
epidural block. Although conduction anesthesia has a high rate of success
and is relatively easy to perform, subarachnoid or epidural procedures may not
be indicated for certain groups of patients, including the elderly,
debilitated, arthritic, obese or critically ill. With such patients,
lower-extremity regional anesthesia can be accomplished by blocking the
lumbosacral plexus or its branches.
Unlike those
of the upper arm, the nerves of the lower extremity travel widely separate
courses from their origins, making it impossible to block them all with a
single approach, while attempts to block several nerves with multiple
injections give poor results because some are situated more deeply than
others. Meticulous attention to detail and impeccable technique are
necessary to overcome the pitfalls of multiple peripheral nerve blocking.
The history
of lower-extremity blocks is not as illustrious as that of upper-extremity
blocks. Labat, Bonica, Moore, Beck, Ichiyanagi and Raj have contributed
to various techniques of sciatic and femoral nerve blocks, while Winnie and
Chayen have described lumbosacral plexus blocks.
The lumbar
plexus is formed by the union of the ventral primary divisions of the upper
four lumbar nerves, with an occasional contribution by the 12th thoracic
nerve. The ventral primary division of the first lumbar nerve splits into
an upper and a lower branch. The upper branches from the 12th thoracic
nerve to form the common trunk of the iliohypogastric and the ilioinguinal
nerves, while the lower branch joins the branch of the second lumbar nerve to
form the genitofemoral nerve. The ventral primary division of the second
lumbar nerve also splits into an upper and lower branch. The upper branch
contributes to the genitofemoral nerve, while the lower branch gives rise to
the femoral, lateral femoral cutaneous, and obturator nerve.
The lower
branch of the second, the upper branch of the fourth, and the entire third
lumbar nerves split into ventral and dorsal subdivisions. The ventral
subdivisions of the second, third, and fourth lumbar nerves join to form the
obturator nerve; the larger branches of the dorsal divisions of the second,
third, and fourth lumbar nerves join to form the femoral nerve, while the
smaller branches of the dorsal divisions of the second and third lumbar nerves
form the lateral femoral cutaneous nerve. The smaller branch of the
ventral primary division of the fourth lumbar nerve joins the fifth to form the
lumbosacral trunk, which contributes to the sacral plexus.
The ventral
primary divisions of the upper four sacral nerves pass anteriorly into the
pelvis through the ventral sacral foramina; that of the fifth emerges from the
sacral canal through the sacral hiatus. The ventral primary divisions of
the upper three primary sacral nerve roots unites with the lumbosacral trunk to
form the sacral plexus, while the ventral primary divisions of the fourth and
fifth sacral nerves unite with the coccygeal nerve to form the pudendal
plexus. Parasympathetic white communicating branches pass from the
second, third and fourth sacral nerves and unite to form the nervi
erigentes. All the lumbosacral nerves receive sympathetic gray branches
which follow the nerves distally to supply the respective segments.
Approaches to the Lumbosacral Plexus
The
lumbosacral plexus can be blocked through a lumbar block or a psoas compartment
block. The main advantage of both is that they can be performed with a
single-needle technique. However, the reproducibility of the block is
uncertain. The lumbar plexus is reliably blocked, but the sacral plexus
is poorly blocked even with volumes greater than 40 ml.
Winnie1
described a combined lumbosacral block in the early 1980s.
The lumbar
plexus is formed by the anterior primary rami of the second, third and fourth
lumbar nerves as they emerge from the intervertebral foramina ventral to the
quadratus lumborum and dorsal to the psoas major muscles. As the three
major constituent branches of the lumbar plexus (femoral, obturator, and
laterofemoral cutaneous nerves) descend to the leg, they are thus sandwiched
between these muscles and invested by their fasciae. The lumbar plexus is
enveloped at its roots by the fasciae.
The sacral
plexus is formed by the lumbosacral trunk from the fourth and fifth lumbar
nerves and by the first, second and third sacral nerves.
If such
techniques as conduction anesthesia or inguinal paravascular or sciatic nerve
block are contraindicated, such as in the case of infection or CNS disease, the
combined lumbosacral plexus block may be attempted by a lumbar paravertebral
approach.
Position:
The patient is placed in the lateral position, lying on the side opposite the
one to be blocked.
Landmarks:
A line is drawn connecting the superior borders of both iliac crests.
This indicates the L3-4 or L4-5 interspace. A second line is drawn
through the posterior superior iliac spine, parallel to the spinous
process. The point where the intercristal line crosses the paraspinous
line is the needle entry site.
Procedure:
A 22 G, 9-cm needle is inserted perpendicular to the skin but in a slightly
medial direction. When the transverse process is encountered, the needle
is redirected slightly more caudad and advanced until paraesthesia is
obtained. This usually occurs at a depth of 5-6 cm beneath anesthetic
solution is injected.
A block of
the lower extremity is achieved when there is anesthesia in the distribution of
the femoral, obturator and lateral femoral cutaneous nerves.
A large
volume is needed to cover both the lumbar and sacral plexuses. The
sciatic nerve is usually inadequately blocked. A peripheral nerve
stimulator is helpful in the objective evaluation of nerve stimulation and
hence in the placement of the needle tip on the nerve.
A block of
the lumbar and sacral plexus can result in such complications as subarachnoid
or epidural block, intravascular injection causing CNS and CVS toxicity, lumbar
sympathetic block and neuropathy.
References
1. Winnie AP: Plexus
anesthesia: upper and lower extremity surgery. Presentation at
Regional Anesthesia Update. Boston, MA, October, 1983.
2. Chayen D, Nathan M, Chayen M:
The psoas compartment block. Anesthesiology 1976; 45:95.