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 nerves.
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.
Chayen et al2
described the psoas compartment block in 1976.
The psoas
compartment block provides adequate analgesia for surgery of the hip and the
anterolateral thigh. When combined with a sciatic nerve block, analgesia
of the whole leg is achieved. The combined approach is a useful
alternative to conduction blocks.
Position:
The patient
is placed on his side with the hips flexed and the side to be operated on
facing upward.
Landmarks:
The fourth
lumbar spine is identified by the intercristal line. The site of
injection is marked 2 cm cephalad and 5 cm lateral to this point.
Procedure:
After a
sterile prep and infiltration, a 20 G, 15-cm needle is inserted perpendicular
to the skin and advanced until it contacts the transverse process of L5.
The needle is then redirected slightly cephalad and advanced 1-2 cm
deeper. A glass syringe filled with 20 mg of air is attached to the
needle. With light tapping on the plunger, the needle is advanced until
resistance to the tapping disappears. The needle passes through the
quadratus lumborum muscle into the psoas compartment at this point, usually at
a depth of 12 cm. After aspiration to test for blood, CSF or air, 20 ml
of air is injected to dilate the psoas compartment. This is followed by
30 ml of anesthetic solution. For a combined psoas-sciatic block, a
volume of 25 ml is used for the compartment block and 15 ml for sciatic nerve
block.
Alternative
Technique:
The psoas
compartment can also be reached by the lateral technique or Reid described for
lumbar sympathetic block. When the needle touches the side of the
vertebral body, the operator retracts the needle 2-4 mm instead of pushing it
forward to slip off the body. Testing for blood, air, or CSF is carried
out and, if negative, the same procedure as in the original description of the
technique by Chayen et al2 is followed from this point.
Anesthesia
of the distribution of the lumbar plexus is a sign of successful block.
More
accurate placement of the needle can be achieved with a nerve stimulator.
When the needle tip is on the femoral nerve, there will be contractions of the
quadriceps femoris muscle with every stimulation.
Complications
possible with the psoas compartment block are similar to those for the lumbar
somatic nerve root or combined lumbosacral plexus block.
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.