MRI verification of ultrasound-guided infiltrations of local anesthetics into the piriformis muscle.

OBJECTIVE: Patients presenting with buttock pain syndromes are common. Up to 8% of these conditions may be attributed to piriformis syndrome. Included in several therapeutic and diagnostic approaches, injections directly into the piriformis muscle may be performed. Because the muscle lies very close to neurovascular structures, electromyographic, fluoroscopic, computed tomographic, and magnetic resonance imaging (MRI) guidance have been employed. In few studies, an ultrasound-guided technique was used to inject a local anesthetic into the piriformis muscle without impairing adjacent neuronal structures.

DESIGN: Feasibility study in healthy human subjects. Confirmation of ultrasound-guided injections by MRI.

SUBJECTS: In 10 male human subjects, ultrasound-guided injections of 3 mL of a local anesthetic into the piriformis muscle were performed.

METHODS: Directly after the injection, the subjects were placed in an MRI scanner, and the placement of the liquid depot was confirmed by MRI imaging. Somatosensory deficits were evaluated after the injection.

RESULTS: The MRI showed that 9 of 10 of the injections were correctly placed within the piriformis muscle. The distance of the depot to the sciatic nerve decreased over time due to dispersion, but the nerve itself was not reached in the MRI. Only one subject experienced slight, short-term sensorimotor deficits.

CONCLUSIONS: MRI confirmed the correct placement of the local anesthetic within the muscle. The dispersion of the fluid 30 minutes after the injection could be visualized. Moreover, only one subject experienced slight motor deficits without anatomical correlate. This ultrasound-guided method will be further employed in ongoing clinical studies.