What You Shoud Know About Sciatic Nerve Block

Tue, 20 Dec 2011 11:50:19 +0100

Sciatic nerve block is useful in the evaluation and management
of distal lower extremity pain thought to be subserved
by the sciatic nerve. The technique is also useful to
provide surgical anesthesia for the distal lower extremity
when combined with lateral femoral cutaneous, femoral,
and obturator nerve block or lumbar plexus block. It is
used for this indication primarily with patients who would
not tolerate the sympathetic changes induced by spinal or
epidural anesthesia who need distal extremity amputations
or de´bridement. Sciatic nerve block with local anesthetic
can be used diagnostically during differential neural
blockade on an anatomic basis in the evaluation of distal
lower extremity pain. If destruction of the sciatic nerve is
being considered, this technique is useful as a prognostic
indicator of the degree of motor and sensory impairment
that the patient may experience. Sciatic nerve block with
local anesthetic may be used to palliate acute pain emergencies,
including distal lower extremity fractures and for
postoperative pain relief, while waiting for pharmacologic
methods to become effective. Sciatic nerve block with
local anesthetic and steroid is occasionally used in the
treatment of persistent distal lower extremity pain when
the pain is thought to be secondary to inflammation or
when entrapment of the sciatic nerve at the level of the
lesser trochanter is suspected. Destruction of the sciatic
nerve is occasionally indicated for the palliation of persistent
distal lower extremity pain secondary to invasive
tumor that is mediated by the sciatic nerve and has not
responded to more conservative measures.
The sciatic nerve innervates the distal lower extremity
and foot with the exception of the medial aspect of the calf
and foot, which are subserved by the saphenous nerve.
The largest nerve in the body, the sciatic nerve is derived
from the L4, L5, and the S1-3 nerve roots. The roots fuse
together in front of the anterior surface of the lateral
sacrum on the anterior surface of the piriform muscle.
The nerve travels inferiorly and leaves the pelvis just
below the piriform muscle via the sciatic notch. The sciatic
nerve lies anterior to the gluteus maximus muscle
and, at this muscle’s lower border, lies halfway between
the greater trochanter and the ischial tuberosity. The sciatic
nerve courses downward past the lesser trochanter to
lie posterior and medial to the femur. In the mid-thigh,
the nerve gives off branches to the hamstring muscles and
the adductor magnus muscle. In most patients, the nerve
divides to form the tibial and common peroneal nerves in
the upper portion of the popliteal fossa, although these
nerves sometimes remain separate through their entire
course. The tibial nerve continues downward to provide
innervation to the distal lower extremity, whereas the
common peroneal nerve travels laterally to innervate a
portion of the knee joint and, via its lateral cutaneous
branch, provides sensory innervation to the back and
lateral side of the upper calf.

The Anterior Approach
Sciatic nerve block via the anterior approach is used for
patients who cannot assume the Sims’ or lithotomy position
because of lower extremity trauma. To perform sciatic
nerve block using the anterior approach, the patient is
placed in the supine position with the leg in neutral position.
The greater trochanter and the crease of the groin on
the involved side are identified by palpation. An imaginary
line is then drawn parallel to the crease of the groin that
runs from the greater trochanter to the center of the thigh.
This center point is then identified and prepared with
antiseptic solution. A 25-gauge, 31=2-inch needle is then
slowly advanced perpendicular to the skin until it
impinges on the femur. The needle is then ‘‘walked’’
slightly superiorly and medially until it walks off the top
of the lesser trochanter (Fig. 315-1). A paresthesia in the
distribution of the sciatic nerve will be elicited; if a nerve
stimulator is used, dorsiflexion and plantar flexion of the
foot will be noted. The patient should be warned to expect
paresthesia and should be told to say ‘‘There!’’ immediately
on perceiving the paresthesia. Paresthesia is usually
elicited at a depth 1 inch beyond initial bony contact.
Once paresthesia is elicited in the distribution of the sciatic
nerve, the needle is withdrawn 1 mm, and the patient
is observed to rule out any persistent paresthesia. If no
persistent paresthesia is present and after careful aspiration,
15 to 18 mL of 1.0% preservative-free lidocaine is
slowly injected. Care must be taken not to advance the
needle into the substance of the nerve during the injection
and inject solution intraneurally.
If the pain has an inflammatory component, the local
anesthetic is combined with 80 mg of methylprednisolone
and is injected in incremental doses. Subsequent
daily nerve blocks are carried out in a similar manner,
substituting 40 mg of methylprednisolone for the initial
80-mg dose. After injection of the solution, pressure is
applied to the injection site to decrease the incidence of
postblock ecchymosis and hematoma formation.

The Posterior Approach
To perform sciatic nerve block using the posterior
approach, the patient is placed in the Sims’ position
with the upper leg flexed. The greater trochanter and
the ischial tuberosity on the involved side are identified
by palpation. The sciatic nerve lies midway between these
two bony landmarks (Fig. 315-2). This midpoint is
then identified and prepared with antiseptic solution.
A 25-gauge, 31=2-inch needle is then slowly advanced perpendicular
to the skin until paresthesia is elicited; if a
nerve stimulator is used, dorsiflexion and plantar flexion
of the foot are noted. The patient should be warned to
expect paresthesia and should be told to say ‘‘There!’’
immediately on perceiving the paresthesia. Paresthesia is
usually elicited at a depth of 21=2 to 3 inches. If the needle is
felt to impinge on the bone of the sciatic notch, the needle
is withdrawn and redirected laterally and slightly superiorly
until paresthesia is elicited. Once paresthesia is
elicited in the distribution of the sciatic nerve, the needle
is withdrawn 1 mm, and the patient is observed to rule out
any persistent paresthesia. If no persistent paresthesia is
present and after careful aspiration, 15 to 18 mL of 1.0%
preservative-free lidocaine is slowly injected. Care must
be taken not to advance the needle into the substance of
the nerve during the injection and inject solution
intraneurally.
If the pain has an inflammatory component, the local
anesthetic is combined with 80 mg of methylprednisolone
and is injected in incremental doses. Subsequent daily
nerve blocks are carried out in a similar manner, substituting
40 mg of methylprednisolone for the initial 80-mg
dose. After injection of the solution, pressure is applied to
the injection site to decrease the incidence of postblock
ecchymosis and hematoma formation.
The main side effect of sciatic nerve block using the
anterior approach is postblock ecchymosis and hematoma.
As mentioned, pressure should be maintained on the
injection site post block to avoid ecchymosis and hematoma
formation. Because this technique elicits paresthesia,
needle-induced trauma to the sciatic nerve remains possible.
By advancing the needle slowly and withdrawing the
needle slightly away from the nerve, one can avoid needle induced
trauma to the sciatic nerve.

sciatic-nerve-pain---sciatic-nerve-block

What You Shoud Know About Sciatic Nerve Block