How is a posterior sciatic nerve block administered in pain management?

Updated: Jun 19, 2018
  • Author: Anthony H Wheeler, MD; Chief Editor: Meda Raghavendra (Raghu), MD  more...
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Answer

Answer

A posterior sciatic nerve block into the subgluteal region is usually performed with the patient in a lateral decubitus position with the top leg flexed. Ultrasonography-guided needle placement enhances safety and provides more accurate needle position. In these cases, the ultrasound transducer is placed in the subgluteal region midway between the greater trochanter and ischial tuberosity. After the sciatic nerve is located, the skin is infiltrated with local anesthetic, a 22-gauge needle that is 10-12 mm long or a 25-gauge, 3.5-inch needle is directed in a perpendicular plane.

Needle movement can be ultrasound guided or may be gently and slowly advanced until it elicits paresthesia. If bone is encountered prior to paresthesia, the needle is redirected along a line joining the sacral hiatus and the greater trochanter. During redirection, the needle is steered deeper, not to exceed 2 cm.

Once paresthesia is elicited in the distribution of the sciatic nerve, the needle is withdrawn 1 mm, and the patient is observed to rule out any persistent paresthesiae. Further guidance and confirmation of tip placement can be obtained using electrical nerve stimulation. If a nerve stimulator is used, dorsiflexion and plantar flexion of the foot are noted. If paresthesiae resolve and careful aspiration is unrevealing, then 20-25 mL of 1% preservative-free lidocaine can be slowly injected.

If the pain has an inflammatory component, then the local anesthetic can be combined with 80 mg of methylprednisolone that is incrementally injected. Subsequent daily nerve blocks can be carried out in a similar manner substituting 40 mg of methylprednisolone before the initial 80 mg dose. Pressure should be applied to the injection site to decrease the incidence of postblock ecchymoses and hematoma formation.


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