Image-guided high-intensity focused ultrasound for conduction block of peripheral nerves

The objective of our work has been to investigate the use of ultrasound image-guided high-intensity focused ultrasound (HIFU) to non-invasively produce conduction block in rabbit sciatic nerves in vivo, a technique that could become a treatment of spasticity and pain. The work reported here involved the investigation of the duration of such conduction blocks after HIFU treatment and whether they resulted in axon degeneration. The right sciatic nerves of 12 rabbits were treated, under guidance of ultrasound imaging, with repeated 5-s applications of 3.2 MHz HIFU with in situ intensity of 1930 W/cm(2) (spatial-average, temporal-average) until conduction block was achieved. Survival endpoints were 0, 7, or 14 days after HIFU treatment, at which point the nerve conduction was assessed. Qualitative and quantitative histological analysis of nerve sections proximal and distal to the HIFU site was performed. Conduction block of all 12 nerves was achieved with average HIFU treatment time of 10.5+/-4.9 s (mean+/-SD). The volume of necrosis of adjacent muscle was measured to be 1.59+/-1.1 cm(3) (mean+/-SD). For all nerves, conduction block remained at the survival endpoint and the block resulted in degeneration of axons distal to the HIFU site, as confirmed by electrophysiological and histological methods. Potential clinical applications include treatment of spasticity in patients with spinal cord injury or pain in cancer patients.