Ultrasound-guided HIFU neurolysis of peripheral nerves to treat spasticity and pain

Spasticity, a major complication of disorders of the central nervous system (CNS) signified by uncontrollable muscle contractions, is difficult to treat effectively. We report on the use of ultrasound image-guided high-intensity focused ultrasound (HIFU) to target and suppress the function of the sciatic nerve of rabbits in vivo as a possible treatment of spasticity and pain. The image-guided HIFU device included a 3.2 MHz spherically curved transducer (focal dimensions of 5.1 mm×0.76 mm) integrated with an intraoperative imaging probe (CL10-5, Philips HDI-1000), such that the HIFU focus was within the image plane. The sciatic nerve was imaged in cross-section and identified between two muscle planes, and the HIFU treatment was directed to the nerve and monitored in real time. In situ focal acoustic intensity of 1480-1850 W/cm² was applied using a scanning method (scan rate of 0.5-0.6 mm/s). The force response of the plantarflexion muscles in the rabbit foot to electrical stimulation of the sciatic nerve was measured both before and after HIFU treatment using a force gauge perpendicularly coupled to the metatarsal joint of the rabbit foot. The force response was approximately 0.55 N before HIFU treatment, and complete suppression of this force was achieved after HIFU treatment, indicating complete conduction block. HIFU treatment time of 36±14 s (mean ± standard deviation) was effective in achieving complete conduction block in 100% of the 22 nerves treated (11 rabbits). Gross examination showed blanching of the nerves at the HIFU treatment site and lesion volumes of 2.8±1.4 cm² encompassing the nerves. Histologic results indicated axonal demyelination and necrosis of Schwann cells as probable mechanisms of nerve block. With accurate localization and targeting of peripheral nerves using ultrasound imaging, HIFU could become a promising tool for the suppression of spasticity and pain.