Continuous Sciatic Nerve Block: Posterior Approach

The continuous sciatic nerve block is an advanced regional anesthesia technique and experience with the single-shot technique is recommended to ensure its efficacy and safety.

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Overview

Indications: Surgery on the knee, tibia, ankle, and foot Landmarks: Greater trochanter, superior posterior iliac spine, midline between the two Nerve stimulation: Twitch of the hamstrings, calf, foot, or toes at 0.2-0.5 mA current Local anesthetic: 20 mL Complexity level: Advanced

General considerations

The continuous sciatic nerve block is an advanced regional anesthesia technique and experience with the single-shot technique is recommended to ensure its efficacy and safety. The technique is quite similar to the single-shot injection, however, slight angulation of the needle caudally is necessary to facilitate threading of the catheter. Securing and maintenance of the catheter are easy and convenient. This technique can be used for surgery and postoperative pain management in patients undergoing a wide variety of lower leg, foot, and ankle surgeries. Perhaps the single most important indication for use of this block in our practice is for amputation of the lower extremity.

Regional anesthesia anatomy

The sciatic nerve is formed from the L4 through S3 roots. These roots of the sacral plexus form on the anterior surface of the lateral sacrum and are assembled into the sciatic nerve on the anterior surface of the priformis muscle. The sciatic nerve is the largest nerve in the body and measures nearly 2 cm in breadth at its commencement. It exits thepelvis through the greater sciatic foramen, below the piriformis; descends between the greater trochanter of the femur and the tuberosity of the ischium. The nerve then runs along the back of the thigh to about its lower one-third, where it diverges into two large branches, the tibial and common peroneal nerves. A course of the nerve can be estimated by drawing a line on the back of the thigh beginning from the apex of the popliteal fossa to the midpoint of the line joining the ischial tuberosity to the apex of the greater trochanter. The sciatic nerve also gives off numerous articular (hip, knee) and muscular branches.

In the upper part of its course, the sciatic nerve is situated deep in the major gluteal muscle, and rests first on the posterior surface of the ischium. The nerve crosses the external rotators, obturator internus gemelli muscle, and then passes on to the quadratus femoris, by which it is separated from the obturator externus and the hip joint. On its medial side, the sciatic nerve is accompanied by the posterior cutaneous nerve of the thigh and the inferior gluteal artery. More distally, it lies on the adductor magnus. It is crossed obliquely by the long head of the biceps femoris. The articular branches of the sciatic nerve arise from the upper part of the nerve, and supply the hip joint by perforating the posterior part of its capsule; they are sometimes derived directly from the sacral plexus. The muscular branches of the sciatic nerve are distributed to the biceps femoris, semitrendinosus, semimembranosus, and to the ischial head of the adductor magnus; the branches of the latter two arise by a common trunk. The nerve to the short head of the biceps femoris comes from the common peroneal division, the other muscular branches from the tibial division of the sciatic nerve.

TIPS:

• There are variations in the course of the sciatic nerve through the gluteal region. In some 15% of people, the piriformis muscle divides the nerve. The common peroneal componen passes through the muscle or above it, and only the tibial component passes below the muscle.
• The components of the sciatic nerve diverge at a variable distance from the knee joint. By and large, most nerves diverge at or above 7 cm above the popliteal fossa crease.

Distribution of anesthesia

Sciatic nerve blockade results in anesthesia of the skin of the posterior aspect of the thigh, hamstrings and biceps muscles, part of hip and knee joint, and entire leg below the knee, with the exception of the skin of the medial aspect of the lower leg. Depending on the level of surgery, the addition of a saphenous or femoral nerve block may be required.

Patient positioning

Proper positioning at the outset and maintenance of the position during the continuous sciatic nerve block is crucially important to allow for precise catheter placement. A slight forward pelvic tilt prevents the "sag" of the soft tissues in the gluteal area and significantly facilitates block placement.

Equipment

A standard regional anesthesia tray is prepared with the following equipment: Sterile towels and 4"x4" gauze packs 20-mL syringes with local anesthetic Sterile gloves, marking pen, and surface electrode One 1½" 25-gauge needle for skin infiltration A 10-cm long, insulated stimulating needle (preferably Tuohy-style tip) Catheter Peripheral nerve stimulator

Landmarks

The landmarks for a continuous sciatic block are the same as those in the single-shot technique: Greater trochanter Posterior-superior iliac spine Needle insertion site 4-cm caudal to the midpoint of the line between landmarks 1 and 2

Technique

The continuous sciatic block technique is similar to the single-shot technique. With the patient in the lateral decubitus position and a slight forward pelvic tilt, the landmarks are identified and marked with the pen. After a thorough skin cleaning with an antiseptic solution, the skin at the needle insertion site is infiltrated with local anesthetic. The palpating hand is positioned around the site of needle insertion. It is used to fix the skin and shorten the skin-nerve distance. A 10-cm long continuous block needle is connected to the nerve stimulator and inserted at a perpendicular angle to the skin sphere. The opening of the needle should face distally (pointing toward the patient's foot) to facilitate catheter insertion. The initial intensity of the stimulating current should be 1.0-1.5 mA. TIP: It is useful to inject some local anesthetic intramuscularly to prevent pain on advancement of larger gauge and blunt-tipped needles typically used for this block.

As the needle is advanced, twitches of the gluteus muscle are obtained first. Deeper needle advancement results in stimulation of the sciatic nerve. The principles of nerve stimulation and needle redirection are identical to those in the single-shot technique. After obtaining the appropriate twitches, the needle is manipulated until the desired response is seen or felt using a current of 0.2-0.5 mA. At this point, a bolus of local anesthetic is injected (20 mL) after negative aspiration for blood. This is followed by insertion of the catheter some 5-10 cm beyond the needle tip. The needle is then withdrawn back to the skin, while the catheter is advanced simultaneously to prevent its inadvertent removal. Before administering local anesthetic, the catheter is checked for inadvertent intravascular placement by a negative test for blood.

TIP: When insertion of the catheter proves difficult, lowering the angle of the needle can be helpful. A number of techniques to secure the catheter to the skin have been proposed. A benzoin skin preparation, followed by application of a clear dressing and a cloth tape is a simple and efficacious method. The infusion port should be clearly marked as "continuous sciatic block".

Continuous Infusion Continuous infusion is always initiated after an initial bolus of dilute local anesthetic through the catheter. For this purpose, we routinely use 0.2% ropivacaine (15-20 mL). Diluted bupivacaine or l-bupivacaine are also suitable, but can result in more motor blockade. The infusion is maintained at 10 mL/hr or 5 mL/hr when a PCA dose is planned (5 mL).

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TIPS: Breakthrough pain in patients on continuous infusion is always managed by administering a bolus of local anesthetic. Simply increasing the rate of infusion is never adequate. With patients on the ward, a higher concentration of a shorter acting local anesthetic (e.g., 1% lidocaine) is useful to both quickly treat the pain and test the position of the catheter. When the bolus injection through the catheter fails to result in blockade after 30 minutes, the catheter should be considered dislodged and it should be removed. Every patient receiving a sciatic nerve block infusion should be prescribed an immediately available alternative pain management protocol because incomplete analgesia and catheter dislodgment can occur. For inpatients, this is probably best done using a back-up IV PCA.

The course of the catheter (black arrows) and the dispersion of 2 mL of contrast solution (white arrow) injected through the catheter.

TIPS:

• Stimulation at a current intensity of less than 0.5 mA may not be possible in some patients. This is occasionally (but not frequently) the case in elderly patients and patients with long-standing diabetes mellitus, peripheral neuropathy, sepsis, or severe peripheral vascular disease. In these cases, stimulating currents up to 1.0 mA should be accepted as long as the motor response is specifically and clearly seen or felt.
• Muscles of the thigh that are supplied by the sciatic nerve. Since the level of the blockade with this approach is above the departure of these branches, twitch of any of the hamstrings muscles can be accepted as a reliable sign of localization of the sciatic nerve.

Interpreting Responses to Nerve Stimulation

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Choice of local anesthetic

Sciatic blockade requires a relatively low volume of local anesthetic to achieve anesthesia of the entire trunk of the nerve. The choice of the type and concentration of local anesthetic should be based on whether the block is planned for surgical anesthesia or pain management. Since the duration of sciatic blockade lasts longer than any other peripheral nerve block, we tend to use shorter acting local anesthetic more commonly. However, when prolonged pain relief is sought, longer acting local anesthetic may be more appropriate.

TIP: We suggest avoiding the use of epinephrine for the anterior approach to sciatic nerve block because of the risk of nerve ischemia due to the combined effects of the vasoconstrictive action of epinephrine, nerve stretching, and application of a tourniquet.

Block Dynamics and Perioperative Management

An anterior approach to a sciatic block can be associated with significant patient discomfort because the needle must transverse multiple muscle planes on its way to the sciatic nerve. We routinely use 2-6 mg of midazolam IV and narcotic (500 to 1000 µg of alfentanil) to make the patient comfortable and cooperative during this procedure. The need for this premedication is further exemplified by the fact that most of our indications for this block are in patients with lower extremity trauma, making patient positioning and leg manipulation even more uncomfortable. A typical onset time for this block is 10-25 minutes, depending on the type, concentration, and volume of local anesthetic used. Usually, the first sign of blockade onset is reported by the patient in the form of the foot feeling "different" or an inability to wiggle the toes.

TIP: Inadequate skin anesthesia despite an apparent timely onset of the blockade can occur. With some blocks, it can take up to 30 minutes for full sensory-motor anesthesia to develop. Local infiltration at the site of the incision by the surgeon is often all that is needed to allow the surgery to proceed.

Complications and How to Avoid Them

Some general and specific instructions on possible complications and methods to use to avoid them.

Bibliography

• Bridenbaugh PO., Wedel DJ: The Lower Extremity. Somatic Blockade . In Cousins, M.J., and Bridenbaugh PO (eds): Neuronal Blockade in Clinical Anesthesia and Management of Pain, 3rd edition. Philadelphia, Lippincott - Raven Publishers, 1998, pp 375-94.
• Chelly JE, Casati A, Fanelli G: Continuous peripheral nerve block techniques. An illustrated guide. London, Mosby International Limited, 2001.
• di Benedetto P, Casati A, Bertini L, Fanelli G, Chelly JE: Postoperative analgesia with continuous sciatic nerve block after foot surgery: a prospective, randomized comparison between the popliteal and subgluteal approaches. Anesth Analg. 2002; 94:996-1000.
• di Benedetto P, Casati A, Bertini L: Continuous subgluteus sciatic nerve block after orthopedic foot and ankle surgery: comparison of two infusion techniques. Reg Anesth Pain Med 2002; 27:168-72.
• Klein SM, Greengrass RA, Grant SA, Higgins LD, Nielsen KC, Steele SM: Ambulatory surgery for multi-ligament knee reconstruction with continuous dual catheter peripheral nerve blockade. Can J Anaesth 2001; 48:375-8.
• Souron V, Eyrolle L, Rosencher N: The Mansour's sacral plexus block: an effective technique for continuous block. Reg Anesth Pain Med 2000; 25:208-9.
• Sutherland ID: Continuous sciatic nerve infusion: expanded case report describing a new approach. Reg Anesth Pain Med 1998; 23:496-501.

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