Sciatic Nerve Block - Subgluteal Region
The sciatic nerve anterior (deep) to the gluteus maximus muscle is found just lateral to the origin of the biceps femoris muscle at the ischial tuberosity. Notice that the sciatic nerve is lateral to the greater trochanter. Sciatic nerve blockade in the subgluteal region is convenient and easily accessible. The nerve lies within a palpable groove in this location and is more superficial than in the gluteal region. The term subgluteal is often used interchangeably with infragluteal and both refer to the distal part of the gluteal region where the gluteus maximus muscle is thin.
 |
SN = sciatic nerve |
- 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.
Transverse View in the Subgluteal Region
 |
Arrowhead = sciatic nerve
IT = ischial tuberosity
GMM = gluteus maximus muscle
GT = greater trochanter
QFM = quadratus femoris muscle |
 |
- 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.
- 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 NEEDLE INSERTION 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 |
- 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.
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.
Needle Insertion and Local Anesthetic Injection
1. Out of Plane Approach
 |
|
2. Local Anesthetic Injection
 |
Arrowhead = sciatic nerve
GMM = gluteus maximus muscle |
 |
Arrowhead = sciatic nerve
GMM = gluteus maximus muscle
LA = local anesthetic |