Sciatic Nerve Block

26 Sciatic Nerve Block



Sciatic nerve blockade is useful for surgical anesthesia, postoperative pain for lower extremity surgery, and for management of pain in certain lower extremity chronic pain syndromes such as sciatic neuropathy. This chapter describes the anatomy, various block approaches, and potential complications of this procedure.



Anatomy


The sciatic nerve is the largest nerve in the body and is found in the pelvis from the ventral rami of the fourth lumbar to the third sacral spinal nerves.1 It transverses through the sciatic foramen and below the piriformis to enter the lower extremity and then descends between the greater trochanter and the ischial tuberosity (Fig. 26-1). In some cases the common peroneal component may pass through the piriformis, whereas the tibial component passes below the muscle.



The sciatic nerve runs down the posterior aspect of the thigh and divides into the common peroneal nerve and tibial nerve. During this course, it is accompanied by the posterior femoral cutaneous nerve and the inferior gluteal artery. Here it lies deep to the gluteus maximus—running in the posterior ischial surface. Further down it turns posterior to the obturator internus, the gemelli, and the quadratus femoris. Distally it lies behind the adductor magnus and the long head of biceps femoris crosses it posteriorly. It corresponds to a line drawn from just medial to the midpoint between the ischial tuberosity and the greater trochanter to the apex of the popliteal fossa. There is considerable variation in the level of the division of the sciatic nerve into the common peroneal nerve and the tibial nerve because they are usually connected by a thin fascial plane. The sciatic nerve innervates the hip joint through the posterior capsule, the knee joint, and all the hamstring muscles. This nerve also supplies the muscles of the posterior compartment of the leg.





Sciatic Nerve Block—Posterior Approach


Patient is positioned in the lateral decubitus position. Surface anatomical landmarks for posterior approach are:




The needle insertion site is 4 cm below the midpoint of the line joining the posterior superior iliac spine (PSIS) to the ipsilateral greater trochanter (Fig. 26-2). The needle is inserted perpendicular to the skin. A nerve stimulator with a standard setting of 2 Hz and 100 μsec can be used and will initially cause a twitching of the gluteal muscles. On further advancement of the needle and with stimulation of the sciatic nerve, contraction of the hamstrings and calf muscles will be noted. This is observed as dorsiflexion of the ankle and foot. Small adjustments of the needle may become necessary to achieve stimulation at less than 0.5 mA. The stimulation current may be reduced until disappearance of stimulation is noted. This should usually be above 0.2 mA. After initial and intermittent negative aspiration 15 to 30 mL of local anesthetic is injected in increments of 4 to 5 mL. Injection of local anesthetic should be without any resistance and without pain or paresthesia. Triamcinolone (40 to 80 mg) or other corticosteroid with a lower volume of local anesthetic can be added when used for treatment of chronic pain syndromes.



In situations where prolonged postoperative analgesia is required, a catheter is threaded 5 to 10 cm beyond the needle tip and left in situ. Prior to injecting local anesthetic through the catheter, it should always be aspirated to confirm that the catheter tip is not intravascular.



Sciatic Nerve Block—Anterior Approach


The patient is positioned in the supine position. The leg is maintained fully extended. If the patient has an overhanging abdominal pannus, it may need to be either held up or taped to fully expose the inguinal area and femoral crease of the patient.


Surface anatomic landmarks for anterior approach are the femoral crease and the femoral artery pulse.


The femoral artery is palpated within the femoral crease. From this point, a line is drawn perpendicular to the femoral crease to identify a point 5 cm distal to the femoral crease (Fig. 26-3). At this point, a needle is inserted perpendicular to the skin plane. A nerve stimulator with a standard setting of 2 Hz and 100 μsec can be used and will initially cause contraction of the quadriceps muscles. On further advancement of the needle and stimulation of the sciatic nerve, contraction of the hamstrings and calf muscles will be noted. Make small adjustments of the needle if necessary to achieve stimulation at less than 0.5 mA. Continue to reduce the stimulation current until the contraction disappears. This should usually be above 0.2 mA. After initial and intermittent negative aspiration of blood 15 to 30 mL of local anesthetic can be injected in increments of 4 to 5 mL. Injection of local anesthetic should be without any resistance and without pain or paresthesia. If bone was contacted prior to obtaining the desired stimulation, the needle can be reinserted medially. Triamcinolone (40 to 80 mg) or other corticosteroid with a lower volume of local anesthetic can be added when used for treatment of chronic pain syndromes.



Jun 19, 2016 | Posted by in PAIN MEDICINE | Comments Off on Sciatic Nerve Block

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