Sciatic Nerve Block

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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.

image

Figure 26-1 Sciatic nerve anatomy.

(From Brown DL: Atlas of Regional Anesthesia, 3rd ed. Philadelphia, Saunders, 2005.)

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.

image

Figure 26-2 Sciatic nerve block: posterior approach.

(From Brown DL: Atlas of Regional Anesthesia, 3rd ed. Philadelphia, Saunders, 2005.)

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.

image

Figure 26-3 Sciatic nerve block: landmarks for anterior approach.

(From Brown DL: Atlas of Regional Anesthesia, 3rd ed. Philadelphia, Saunders, 2005.)

Lumbar Plexus Block—Parasacral Approach

The parasacral approach to a lumbar plexus block was described by Mansour in 1993.7 This block can be used for anesthesia and analgesia in patients having lower extremity hip, tibia and fibula, knee, ankle, and foot surgery and amputation at the level of the knee.8

The patient is positioned in the lateral decubitus position. Surface anatomic landmarks for parasacral lumbar plexus block are the posterior-superior iliac spine and the ischial tuberosity.

A line is drawn connecting the posterior-superior iliac spine and the ischial tuberosity. On this line, the needle is inserted 6 cm from the posterior superior iliac spine and advanced in the sagittal plane. A nerve stimulator with a standard setting of 2 Hz and 100 μsec can be used and will cause a contraction of the hamstring and posterior leg muscles. Make small adjustments of the needle position 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 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 desired stimulation, the needle should be redirected caudally. 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.

Sciatic Nerve Block at the Popliteal Fossa

The sciatic nerve consists of two separate nerve trunks—the tibial and common peroneal nerves (Fig. 26-4). These nerves share a common epineural sheath and are separated throughout their course by a thin fascial plane. In the popliteal fossa, the tibial and common peroneal nerves diverge. The level of this separation is highly variable and occurs from 5 to 12 cm proximal to the popliteal crease.9 The common peroneal nerve continues behind the fibular head and divides into the superficial and deep peroneal nerves. The superficial peroneal nerve innervates the lateral compartment muscles of the leg (peroneal longus and brevis). The deep peroneal nerve innervates the anterior compartment muscles of the leg. The tibial nerve travels down in the posterior aspect of the leg with the posterior tibial artery. During its course, it gives rise to the medial sural cutaneous and medial calcaneal branches. The tibial nerve terminates as the medial and lateral plantar nerves.

image

Figure 26-4 Sciatic nerve anatomy in the popliteal fossa.

(From Brown DL: Atlas of Regional Anesthesia, 3rd ed. Philadelphia, Saunders, 2005.)

To perform a sciatic nerve block at the popliteal fossa, the patient is positioned in the prone position. For patient comfort and improved visualization of the ankle, the ankle may be slightly elevated by placing a blanket under it.

Surface anatomic landmarks for sciatic nerve block at the popliteal fossa include tendons of semitendinosus and semimembranosus muscles medially; the tendon of the biceps femoris muscle laterally; and the popliteal crease inferiorly.

These landmarks are clearly identified and marked by asking the patient to flex the leg at the knee. The needle can be inserted at the midpoint between the tendons about 7 to 10 cm proximal to the popliteal crease. A nerve stimulator with a standard setting of 2 Hz and 100 μsec can be used. Stimulation of the common peroneal nerve will result in dorsiflexion and eversion at the ankle, whereas stimulation of the tibial nerve results in plantar flexion and inversion at the ankle. Either of these responses is acceptable. 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 subsides. This should usually be above 0.2 mA. After initial and intermittent negative aspiration, 25 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 corticosteroids with a lower volume of local anesthetic can be added when used for treatment of chronic pain syndromes.

Sciatic Nerve Block under Ultrasound Guidance

In a study performed by Perlas and colleagues, ultrasound guidance improved the success of sciatic nerve block at the popliteal fossa.10 This study showed the success rate for ultrasound-guided block was 89.2% versus a nerve stimulator-guided group with the success rate of 60.6%. Ideally, a lower frequency ultrasound probe between 2 and 5 MHz may be used for scanning and identifying deep structures such as the sciatic nerve.

Ultrasound-Guided Popliteal Sciatic Nerve Block

Surface anatomic landmarks for sciatic nerve block at the popliteal fossa follow:

The patient is positioned prone and the surface landmarks as described earlier are identified and clearly marked. The ultrasound probe is positioned 5 to 10 cm proximal to the popliteal crease to obtain a short axis or transverse view of the sciatic nerve.11 The sciatic nerve at this level will appear as a single circular or oval structure (Fig. 26-5). The popliteal artery appears as a hypoechoic pulsatile structure medial to the sciatic nerve. Attempt to visualize the sciatic nerve in the upper portion of the popliteal fossa prior to its division into the tibial and common peroneal nerves. The ultrasound probe should be moved cephalad until this point is located. Every effort should be made to visualize the sciatic nerve in the center of the sonogram.

In the more commonly performed posterior approach, the needle is placed perpendicular to the center of the ultrasound probe. The needle is visualized as a hyperechoic circular structure and the full length of the needle is not visualized. If using this method, it may be helpful to combine the use of a standard nerve stimulator with a setting of 2 Hz and 100 μsec. Alternatively, the needle can be inserted laterally, parallel to the ultrasound probe, but anterior to the tendon of the biceps femoris. With this approach, the entire length of the needle can be visualized under ultrasound. When the needle is in appropriate proximity of the sciatic nerve, plantar or dorsiflexion of the foot will be noted with less than 0.5 mA when a nerve stimulator is used. Either of these contractions is acceptable. Continue to reduce the stimulation current until disappearance of contraction 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. Spread of the local anesthetic around the sciatic nerve can be visualized on the ultrasound. Triamcinolone (40 to 80 mg) or other corticosteroids with a lower volume of local anesthetic can be added when used for treatment of chronic pain syndromes.

Ultrasound-Guided Mid-thigh Sciatic Nerve Block12

Surface anatomic landmarks for posterior approach are posterior superior iliac spine and greater trochanter of femur. The patient is positioned in the prone position. This block may also be performed with the patient in a lateral position with the operative side up.

A line is drawn that connects the posterior-superior iliac spine to the ipsilateral greater trochanter of the femur. A low-frequency ultrasound probe preferably 2 to 5 MHz may be used to scan from the midpoint of the line described earlier down to the popliteal crease with the goal to identify the sciatic nerve. The sciatic nerve should be visualized in the short axis. After anesthetizing the skin, the needle can be inserted just lateral to and parallel to the ultrasound probe. The needle can be advanced medially under direct vision on the sonogram. If a nerve stimulator is also used, contraction of the calf muscles along with plantar or dorsiflexion of the foot can be noted with less than 0.5 mA. The stimulation current can be reduced until the contraction subsides. This should be >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. Spread of the local anesthetic around the sciatic nerve can be visualized on the sonogram. 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.

Complications of Sciatic Nerve Block

Various studies analyzed complications following peripheral nerve blocks.13,14 Recent studies have also examined complications after continuous popliteal sciatic nerve blocks for postoperative analgesia.15,16

Some of the following common complications have been observed during or after the performance of sciatic nerve blocks:

Infection: The incidence of infection has ranged from 0.1% to 3.2% as reported by various studies.13,15,17 Longer duration of catheter insertion has been assumed to increase the incidence of infection; however, no definitive data are available to corroborate this assumption. Meticulous sterile prep, technique, and dressing have been shown to reduce the incidence of infection for central venous line catheters.18 We believe similar care would help avoid infections following the performance of peripheral nerve blocks.
Nerve injury: The incidence of nerve injury has been reported between 0.21% and 0.5% with the sciatic nerve blocks.15 Mechanisms for nerve injury include the use of pneumatic tourniquet on the upper third of the leg and direct nerve injury from needle and injection trauma. Techniques to avoid potential nerve injury would include slow needle advancement, avoidance of forceful injections, and avoiding injecting when the patient is insensate.

Other complications of the sciatic nerve block include a transient unintended pudendal nerve block, perforation of pelvic organs, and pressure necrosis of the heel.19,20

References

1. Williams A. Pelvic girdle gluteal region and hip joint area. In: Standring S., editor. Gray’s Anatomy: The Anatomical Basis of Clinical Practice 39th ed. Philadelphia: Elsevier, 2004.

2. Mansour N.Y. Reevaluating the sciatic nerve block: Another landmark for consideration. Reg Anesth. 1993;18:322-333.

3. di Benedetto P., Bertini L., Casati A., et al. A new posterior approach to the sciatic nerve block: A prospective, randomized comparison with the classic posterior approach. Anesth Analg. 2001;93:1040-1044.

4. Singelyn F.J., Aye F., Gouverneur J.M. Continuous popliteal sciatic nerve block: An original technique to provide postoperative analgesia after foot surgery. Anesth Analg. 1997;84:383-386.

5. Ilfeld B.M., Morey T.E., Wang R.D., Enneking F.K. Continuous popliteal sciatic nerve block for postoperative pain control at home: A randomized, double-blinded, placebo-controlled study. Anesthesiology. 2002;97:959-965.

6. White P.F., Issioui T., Skrivanek G.D., et al. The use of a continuous popliteal sciatic nerve block after surgery involving the foot and ankle: Does it improve the quality of recovery? Anesth Analg. 2003;97:1303-1309.

7. Mansour N.Y. Reevaluating the sciatic nerve block: Another landmark for consideration. Reg Anesth. 1993;18:322-323.

8. Morris G.F., Lang S.A., Dust W.N., Van der Wal M. The parasacral sciatic nerve block. Reg Anesth. 1997;22:223-228.

9. Vloka J.D., Hadzić A., April E., Thys D.M. The division of the sciatic nerve in the popliteal fossa: Anatomical implications for popliteal nerve blockade. Anesth Analg. 2001;92(1):215-217.

10. Perlas A., Brull R., Chan V.W., et al. Ultrasound guidance improves the success of sciatic nerve block at the popliteal fossa. Reg Anesth Pain Med. 2008;33(3):259-265.

11. Sinha A., Chan V.W. Ultrasound imaging for popliteal sciatic nerve block. Reg Anesth Pain Med. 2004;29(2):130-134.

12. Barrington M.J., Lai S.L., Briggs C.A., et al. Ultrasound-guided midthigh sciatic nerve block—a clinical and anatomical study. Reg Anesth Pain Med. 2008;33(4):369-376.

13. Capdevila X., Pirat P., Bringuier S., et al. Continuous peripheral nerve blocks in hospital wards after orthopedic surgery: A multicenter prospective analysis of the quality of postoperative analgesia and complications in 1,416 patients. Anesthesiology. 2005;103:1035-1045.

14. Auroy Y., Benhamou D., Bargues L., et al. Major complications of regional anesthesia in France: The SOS regional anesthesia hotline service. Anesthesiology. 2002;97:1274-1280.

15. Compère V., Rey N., Baert O., et al. Major complications after 400 continuous popliteal sciatic nerve blocks for post-operative analgesia. Acta Anaesthesiol Scand. 2009;53:339-345.

16. Borgeat A., Blumenthal S., Lambert M., et al. The feasibility and complications of the continuous popliteal nerve block: A 1001-case survey. Anesth Analg. 2006;103:229-233.

17. Neuburger M., Buttner J., Blumenthal S., et al. Inflammation and infection complications of 2285 perineural catheters: A prospective study. Acta Anaesthesiol Scand. 2007;51:108-114.

18. O’Grady N.P., Alexander M., Dellinger E.P., et al. Guidelines for the prevention of intravascular catheter-related infections. Infect Control Hosp Epidemiol. 2002;23:759-769.

19. Vloka J.D., Hadzić A. Block of the Sciatic Nerve in the Popliteal Fossa. In: Hadzić A., editor. The New York School of Regional Anesthesia Textbook of Regional Anesthesia and Acute Pain Management. New York: McGraw Hill; 2007:532-553.

20. Gaertner G., Fouche E., Chouquet, et al. Sciatic nerve block. In: Hadzić A., editor. The New York School of Regional Anesthesia Textbook of Regional Anesthesia and Acute Pain Management. New York: McGraw Hill; 2007:517-531.