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Sciatic Nerve Block - Gluteal Region

THE SCIATIC NERVE

 

ANATOMY

 

The sciatic nerve is the largest nerve in the human body, originating from the lumbosacral plexus (L4-5 and S1-3) and providing sensory and motor innervation to the lower extremity. The sciatic nerve exits the pelvis via the greater sciatic foramen below the piriformis muscle. In the gluteal region, the sciatic nerve courses between muscle layers. The nerve is deep (anterior) to the gluteus maximus muscle and is superficial (posterior) to the inner muscle layers (superior and inferior gemellus muscles, obturator internus muscle, quadratus femoris muscle).

 

It courses down the midline of the posterior thigh and branches into the tibial and common peroneal nerves usually in the popliteal fossa. Sciatic nerve block is most commonly performed for foot or ankle surgery

 

Posterior view of the gluteal region showing the relationship of the sciatic nerve to surrounding muscle layers:

 

1 = sciatic nerve

 

2 = piriformis muscle

 

3 = gluteus maximus muscle

 

4 = inner muscle layer (superior and inferior
gemellus muscles, obturator internus muscle, quadratus femoris muscle)

 

Transverse View of the Sciatic Nerve at the Ischial Spine Region

 

 

Posterior View of The Gluteal Region

 

The relationship of the sciatic nerve to surrounding bony structures. The red dotted box marks the target of the sciatic nerve for the Labat approach at the ischial spine level.

 

At the level of the ischial spine, the sciatic nerve lies on top of the ischial bone and lateral and posterior to the ischial spine. Important vascular landmarks medial to the sciatic nerve and immediately next to the ischial spine are the pudendal vessels (artery and vein).

 

SCANNING TECHNIQUE

  • Position the patient semi-prone (Sims’ position) with the hip and knee flexed and the operative side uppermost.
  • After skin and transducer preparation (see transducer preparation section), place a curved low frequency 2-5 MHz transducer firmly on the buttock region to capture the best possible transverse view of the sciatic nerve.
  • Optimize machine imaging capability. Select appropriate depth of field (usually > 4 cm from the skin surface), focus range and gain.
  • Observe on the screen the transverse view of the sciatic nerve (short axis). The sciatic nerve in the gluteal region appears predominantly hyperechoic (bright) and is often wide and flat in the transverse view on ultrasound.
Transducer over right gluteal region

 

GT = greater trochanter

 

PSIS = posterior inferior iliac spine

 

SH = sacral hiatus

 

ANATOMICAL CORRELATION

 

Lateral

 

Arrow = sciatic nerve

 

GMM = gluteus maximus muscle

 

IB = ischial bone

PN & PV = pudendal nerve & vessels

 

SN = sciatic nerve

 

NERVE LOCALIZATION

  • Perform a systematic anatomical survey from cephalad to caudad and from superficial to deep.
  • First identify the bony structures that are adjacent to the sciatic nerve. Locate the ischial bone (a hyperechoic line with the bony shadow underneath). Locate the widest portion of the ischial bone and the ischial spine medially.
  • The bony segment at the level of the ischial spine (A) is the widest compared to the bony segments cephalad (B) and caudad (C).
  • Locate the bulky gluteus maximus muscle (GMM) that is superficial and posterior to the sciatic nerve. The sciatic nerve is expected to lie between the GMM and ischial bone.
  • May use color Doppler to locate the pudendal vessels (artery and vein) that are adjacent to the ischial spine, all medial to the sciatic nerve.
  • Another vascular structure that may be noted immediately adjacent to the sciatic nerve is the inferior gluteal artery.
  • Angle the transducer slightly cephalad or caudad to capture the best possible transverse view of the sciatic nerve.
  • The bony segment at the level of the ischial spine (A) is the widest compared to the bony segments cephalad (B) and caudad (C).

    •  

    Arrowhead = sciatic nerve

     

    GMM = gluteus maximus muscle

     

    IB = ischial bone

  • Locating the sciatic nerve in this region can be challenging because of the required depth of penetration especially when there is an abundant amount of overlying adipose tissue.
  • The sciatic nerve can be wide but thin. It can be difficult to separate the thin sciatic nerve from the overlying gluteus maximus muscle layer in the transverse view.
  • Follow the nerve by scanning proximally (cephalad) and distally (caudad) to follow the course of the nerve. It may be necessary to first identify the nerve in the subgluteal region and then trace the nerve proximally should visualization be difficult.

NEEDLE INSERTION APPROACH

 

Ultrasound guided sciatic nerve block in the gluteal region is considered an ADVANCED skill level block.

The sciatic nerve may be difficult to visualize in this region because of the required depth of beam penetration and lower image resolution from the use of a lower frequency transducer. The overlying adipose tissue in the buttock may be sizable, the sciatic nerve may be quite flat in the transverse view and the required angle of needle penetration may be quite steep.

Both In Plane (IP) and Out of Plane (OOP) approaches are available.

 


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