Scanning Technique

• Position the patient semi-prone with the block limb uppermost. Mark the greater trochanter (GT) laterally and the ischial tuberosity (IT) medially. The midpoint marks the approximate sciatic nerve location.
• After skin and transducer preparation, place a curved 5 MHz transducer over the subgluteal region in a transverse plane to image the sciatic nerve.

• Optimize machine imaging capability. Select the appropriate depth of field (usually within 7 cm), focus range (usually within 5 cm) and gain.
• Obtain a transverse view of the sciatic nerve. The sciatic nerve is hyperechoic commonly found inside a space lined by a hyperechoic margin corresponding to the fascial sheaths of surrounding muscles.

Anatomical Correlation

Transverse View in the Subgluteal Region

Arrowhead = sciatic nerve IT = ischial tuberosity GMM = gluteus maximus muscle GT = greater trochanter QFM = quadratus femoris muscle

Nerve Localization

• Perform a systematic anatomical survey of structures from superficial (skin) to deep and medial to lateral.
• Identify the gluteus maximus muscle immediately underneath the layer of adipose tissue of varying thickness.
• Identify the bony structures, ischial tuberosity medially and greater trochanter laterally.
• The sciatic nerve is often hyperechoic and lip shaped, commonly found inside a space lined by a hyperechoic margin formed by surrounding muscles.
• It is important to note that the sciatic nerve can be thin and wide and is immediately deep to the gluteus maximus muscle at this location. It is not always visibly distinct on the transverse view (30% of the time based on personal experience).
• When visualization is difficult in the transverse view, it is helpful to turn the patient prone (from the original semi-prone position) and scan the sciatic nerve longitudinally along its long axis. This is useful if the nerve is wide but thin (i.e., a short anterior-posterior distance but a wide medial-lateral distance).
• Additionally, nerve stimulation guidance is very helpful to identify the sciatic nerve when it is not clearly visible.

Needle Insertion Approach

• Ultrasound guided sciatic nerve block in the subgluteal region is considered an INTERMEDIATE skill level block.
• The sciatic nerve may be difficult to visualize in this region because of the required depth of beam penetration and the use of a lower frequency transducer. The overlying layer of adipose tissue in the buttock may be sizable. The sciatic nerve may be quite flat in the transverse view. Visualization of the block needle can be challenging because of a steep angle of needle penetration.
• Both In Plane (IP) and Out of Plane (OOP) approaches are available. The OOP approach is commonly used for catheter placement.

In Plane Approach

• Depending on the depth, use a 5 or 8 cm 22 G insulated needle and advance the needle inline with the ultrasound transducer.
• When the needle makes contact with the nerve as indicated by nerve movement, stimulate the nerve electrically to confirm needle proximity and check the nature of nerve stimulation, tibial vs. peroneal component.

	Arrowhead = sciatic nerve AT = adipose tissue GMM = gluteus maximus muscle GT = greater trochanter
	Arrows = block needle Arrowhead = sciatic nerve GMM = gluteus maximus muscle GT = greater trochanter

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Out of Plane Approach

The OOP approach is appropriate for both the single shot and catheter techniques. Both hydro location and nerve stimulation will help determination of the needle tip location.

Local Anesthetic Injection

Arrowhead = sciatic nerve GMM = gluteus maximus muscle

Arrowhead = sciatic nerve GMM = gluteus maximus muscle LA = local anesthetic

Local Anesthetic Injection

• Once satisfied with nerve stimulation and motor response, inject 20-30 mL of local anesthetic under ultrasound observation. Aim to see circumferential spread of hypoechoic local anesthetic solution around the nerve.
• While it is desirable to see circumferential local anesthetic spread in the subgluteal region, it is not always possible because moving the needle deep to the nerve can be technically challenging.
• Two separate needle insertion sites may be necessary to place the needle on both sides of the nerve.
• Scan along the nerve proximally and distally to check the extent of longitudinal local anesthetic spread.

Clinical Pearls

Nerve Localization

1. Prone Position

Visualization of the sciatic nerve in the subgluteal region can be difficult (up to 30% of the time in the author’s opinion) due to poor tissue echogenicity. To localize the nerve, it may be worthwhile to scan and trace the course of the sciatic nerve from distal to proximal (i.e., from the popliteal fossa to the subgluteal region). Nerve tracing is often more accurate and easier when the patient is lying prone since the nerve is identified in its anatomical location as compared to the lateral decubitus position.

2. Fascial Plane

The surrounding quadratus femoris and gluteus maximus muscles and their fascial planes can aid localization of the sciatic nerve in the subgluteal region.

3. Long Axis View

It is difficult to visualize the nerve in the transverse view if the sciatic nerve is wide but thin. A long axis view will be appropriate and very helpful.

Catheter Insertion

• The subgluteal region is a convenient location for catheter placement and continuous sciatic nerve block. The nerve is more superficial at this location than the gluteal region and the catheter is easily anchored with less risk of dislodgment.
• The principles of catheter placement in this location are the same as those general principles described on Catheter Technique. The procedures for patient position, skin preparation and sterilization, and transducer selection (2-5 MHz) are identical.
• With the patient lying lateral decubitus, insert a 8 cm 17 G insulated needle using the out of plane approach.
• Needle to nerve contact is indicated by nerve movement (+/- nerve stimulation). Then inject 5-10 mL of local anesthetic or D5W (if nerve stimulation is desired) to distend the perineural space.
• A 20 G catheter is then inserted often without real time ultrasound guidance unless an assistant is available to hold the ultrasound transducer in place.
• Aim to advance the catheter 3-5 cm into the perineural space with or without nerve stimulation guidance.
• After the needle is withdrawn, real time assessment of local anesthetic spread during injection through the catheter helps to check if the catheter tip is located in the optimal position. Circumferential local anesthetic indicates optimal catheter location.

	A. Patient in semi-prone position. GT = greater trochanter IT = ischial tuberosity
	B. Out of plane needle approach.
	C. Catheter is advanced with the help of an assistant.
	D. Transverse scan of the nerve after hydro dissection. Arrowhead = nerve NT = needle tip GT = greater trochanter
	E. Transverse scan showing catheter and local anesthetic (LA). Arrowhead = nerve
	F. Longitudinal scan showing catheter and local anesthetic (LA). Arrowhead = nerve

Image Gallery

1. Local Anesthetic Spread Around the Sciatic Nerve

	A. Pre Injection The sciatic nerve is predominantly hyperechoic and elliptical in this transverse view. Arrowhead = sciatic nerve
	B. Post Injection The sciatic nerve is now surrounded by a hypoechoic collection of local anesthetic (LA). The nerve is now round in shape. Local anesthetic spread is not circum-ferential. Arrowhead = sciatic nerve

2. Apparent Nerve Enlargement After Local Anesthetic Injection

The nerve diameter is noted to be wider after injection suggesting some degree of unintentional intraneural injection. Arrowhead = sciatic nerve LA = local anesthetic

3. Poor Nerve Visualization

	Pre Injection The subgluteal sciatic nerve may not be clearly visualized (up to 30% in the author’s opinion). However, it is always possible to identify the muscular and bony landmarks. GMM = gluteus maximus muscle GT = greater trochanter IT = ischial tuberosity
	Post Injection The sciatic nerve (arrowhead) is now visualized after local anesthetic injection. GMM = gluteus maximus muscle GT = greater trochanter IT = ischial tuberosity

Notes: When nerve visualization is difficult, it is extremely helpful to combine nerve stimulation with ultrasound for nerve localization. The sciatic nerve is expected to lie deep to the gluteus maximus muscle (GMM) and between the greater trochanter (GT) and ischial tuberosity (IT).

4. A Thick Adipose Layer

Arrows = thickness of the adipose tissue Arrowhead = sciatic nerve

Video Gallery

Selected References

• Tammam TF. Ultrasound-guided infragluteal sciatic nerve block: a comparison between four different techniques. Acta Anaesthesiol Scand 2013;57:243-8.
• Gurnaney H, Ganesh A. Incidence and effects of unintentional intraneural injection during ultrasound-guided subgluteal sciatic nerve block. Reg Anesth Pain Med 2013;38:72.
• Hara K, Sakura S, Yokokawa N, Tadenuma S. Incidence and effects of unintentional intraneural injection during ultrasound-guided subgluteal sciatic nerve block. Reg Anesth Pain Med 2012;37:289-93.
• Karmakar M, Li X, Li J, Sala-Blanch X, Hadzi? A, Gin T. Three-dimensional/ four-dimensional volumetric ultrasound imaging of the sciatic nerve. Reg Anesth Pain Med 2012;37:60-6.
• Moayeri N, van Geffen GJ, Bruhn J, Chan VW, Groen GJ. Correlation among ultrasound, cross-sectional anatomy, and histology of the sciatic nerve: a review. Reg Anesth Pain Med 2010;35:442-9.
• Wadhwa A, Tlucek H, Sessler D. A Simple Approach to the Sciatic Nerve That Does Not Require Geometric Calculations or Multiple Landmarks. Anesth Analg 2010;110:958-63.
• Danelli G, Ghisi D, Fanelli A, Ortu A, Moschini E, Berti M, Ziegler S, Fanelli G. The effects of ultrasound guidance and neurostimulation on the minimum effective anesthetic volume of mepivacaine 1.5% required to block the sciatic nerve using the subgluteal approach. Anesth Analg 2009;109:1674-8.
• Saranteas T, Chantzi C, Paraskeuopoulos T, Alevizou A, Zogojiannis J, Dimitriou V, Kostopanagiotou G: Imaging in Anesthesia: The Role of 4 MHz to 7 MHz Sector Array Ultrasound Probe in the Identification of the Sciatic Nerve at Different Anatomic Locations. Reg Anesth Pain Med 2007; 32: 537-8
• Chantzi C, Alevizou A, Saranteas T, Zogogiannis J, Iatrou C, Dimitriou V: Usefulness of the two to 5 MHz ultrasound probe in examination and block of the sciatic nerve in orthopedic trauma patients: a preliminary study. J Clin Anesth 2007; 19: 486-8.
• Chan VW, Nova H, Abbas S, McCartney CJ, Perlas A, Quan XD: Ultrasound Examination and Localization of the Sciatic Nerve: A Volunteer Study. Anesthesiology 2006; 104: 309-14.
• Graif M, Seton A, Nerubai J, Horoszowski H, Itzchak Y: Sciatic nerve: sonographic evaluation and anatomic-pathologic considerations. Radiology 1991; 181: 405-8.
• Hullander M, Spillane W, Leivers D, Balsara Z: The use of Doppler ultrasound to assist with sciatic nerve blocks. Reg Anesth. 1991; 16: 282-4.