Chapter 146. Proximal Sciatic Blocks

Anatomy (Figure 146-2):

• The sacral plexus is formed by part of the anterior ramus of L4, which with L5 forms the lumbosacral trunk, and S1 to S3
• The roots converge toward the greater sciatic foramen in a triangular sheet to form the sciatic nerve anterior to the piriformis muscle
• The premature division of the nerve in the pelvis or the proximal part of the thigh is fairly common. In the case of a division in the pelvis, the two contingents of the sciatic nerve have different relationships with the piriformis muscle (in most cases, perforation of the piriformis muscle by the common peroneal nerve)
• The posterior cutaneous nerve of the thigh (PCNT) is separate from the sciatic nerve at this level. The more distal the sciatic block, the less constant the block of the PCNT:
  • Parasacral > classic > subgluteal » anterior

Indications:

Anesthesia or analgesia for surgery on the lower extremity, usually in conjunction with a lumbar plexus block (femoral nerve block or psoas compartment block).

Contraindications:

Depending on the approach used, sciatic blocks are more or less deep; for deep approaches (anterior, parasacral), coagulopathy or anticoagulation is a relative contraindication.

Techniques using NS:

• Classic (Labat) approach (Figure 146-3):
  • Patient prone or in lateral decubitus
  • Draw line A from posterior superior iliac spine (PSIS) to greater trochanter (GT)
  • Draw line B from sacral hiatus (SH) to GT
  • Measure the middle of line A and drop a perpendicular C
  • The intersection of lines B and C is the needle insertion point
  • After prepping and local anesthesia, a 100 mm needle is inserted perpendicular to the skin of the buttock, setting the PNS at 1.4 mA, 2 Hz, 0.1 millisecond
  • Once a hamstring, tibial, or peroneal response is obtained, adjust needle position while decreasing current. If a response is still present ...