An anatomical study of the parasacral block using magnetic resonance imaging of healthy volunteers

Anesth Analg. 2009 May;108(5):1708-12. doi: 10.1213/ane.0b013e31819b065e.


Background: The parasacral approach to sciatic blockade is reported to be easy to learn and perform, with a high success rate and few complications.

Methods: Using magnetic resonance imaging, we evaluated the accuracy of a simulated needle (perpendicular to skin) in contacting the sacral plexus with this approach in 10 volunteers. Intrapelvic structures encountered during the simulated parasacral blocks were also recorded.

Results: The sacral plexus was contacted by the simulated needle in 4 of the 10 volunteers, and the sciatic nerve itself in one volunteer. The plexus was accurately located adjacent to a variety of visceral structures, including small bowel, blood vessels, and ovary. In the remaining five volunteers (in whom the plexus was not contacted on first needle pass), small bowel, rectum, blood vessels, seminal vesicles, and bony structures were encountered. Historically, when plexus is not encountered, readjustment of the needle insertion point more caudally has been recommended. We found that such an adjustment resulted in simulated perforation of intrapelvic organs or the perianal fossa.

Conclusions: These findings question the reliability of the anatomical landmarks of the parasacral block and raise the possibility of frequent visceral puncture using this technique.

MeSH terms

  • Adult
  • Computer Simulation
  • Female
  • Humans
  • Injections
  • Lumbosacral Plexus / anatomy & histology*
  • Magnetic Resonance Imaging, Interventional*
  • Male
  • Nerve Block / adverse effects
  • Nerve Block / methods*
  • Reference Values
  • Sciatic Nerve / anatomy & histology*
  • Wounds, Penetrating / etiology
  • Wounds, Penetrating / prevention & control