Sciatic nerve block following piriformis injection under ultrasound and nerve stimulator guidance-a case report

Purpose/Objective: Piriformis syndrome is thought to cause of 6-8% of sciatica. Injection of local anaesthetic and steroid into piriformis muscle is an established treatment option. A combined technique of ultrasound with motor stimulation has been advocated for accurate confirmation of piriformis pain and avoidance of sciatic nerve block. We report a case of sciatic nerve block despite taking the above precautions. Materials and Methods: The patient was positioned prone with intravenous access and standard monitoring in place. A pillow under the pelvis optimised piriformis visualization. A 2-5 Hz curvilinear probe placed horizontally over the posterior superior iliac spine was moved downwards and laterally to identify piriformis and the sciatic nerve. Under ultrasound guidance a 100mm insulated needle was passed into piriformis. A current of 2mA resulted in contraction of piriformis and pain. No muscle contractions corresponding to tibial or peroneal nerve stimulation were observed. 10ml of 0.25% levobupivacaine and 80mg of triamcinolone were injected. Results: 30 minutes after injection, a sciatic nerve block became obvious. This resolved completely after 8 hours. The patient reported a 70% reduction in her presenting symptoms. Conclusions: Nerve localisation studies have shown that an absence of muscle contraction at 1-2m. A stimulation indicates that the nerve is 0.5-1cm away from the needle tip. However, MRI studies report that a volume of 10mls is required for even spread within the muscle and in 10% of the population, the peroneal portion of the sciatic nerve traverses piriformis. In our case, injectate spread is likely to have caused the sciatic block.