Ultrasound-Guided HIFU Neurolysis of Peripheral Nerves to Treat Spasticity and Pain Jessica L. Foley, James W. Little, Frank L. Starr III, Carie Frantz, Shahram Vaezy Departments of Bioengineering, Center for Industrial and Medical Ultrasound, and Rehabilitation Medicine University of Washington Seattle, WA Email: jlf2@u.washington.edu Abstract— 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 x 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/cm 2 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 cm 3 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. I. INTRODUCTION Spasticity is a complication of disorders of the central nervous system (CNS), such as multiple sclerosis, cerebral palsy, stroke and traumatic injury to the brain or spinal cord, that is displayed by uncontrollable muscle contractions. Spasticity results from the generation of hyperactive nerve reflexes in pathways below the site of spinal cord or brain injury. Mild or moderate spasticity can often be managed with physical methods or oral spasmolytic medications. However, these treatments are not adequate in 25 to 50% of patients where spasticity is severe. Such severe spasticity interferes with mobility and self-care and compromises quality of life by causing pain and interfering with sleep. Current treatments for severe spasticity have significant limitations including high cost, adverse side-effects or only short-term benefit. Clinicians need more effective, lower cost, lower risk treatments for severe spasticity. HIFU is a noninvasive therapeutic modality that can be applied percutaneously to achieve focal heating and necrosis of a small volume of internal tissue [1]. When HIFU is used with ultrasound image-guidance, the focal heating can be directed to a specific anatomic structure of interest and the treatment can be monitored in real time by visualizing the hyperechoic spot that develops with the application of HIFU [2]. We report on the development and use of a device that provides ultrasound image-guided HIFU to block conduction in nerves as a new treatment for spasticity and pain. In the last 50 years, several groups of researchers have shown interest in the development of therapeutic ultrasound devices to treat neurological disorders [3-5]. The interest has led to many studies investigating ultrasound’s effects on the brain, spinal cord, and peripheral nerves. These studies provide significant insight into the effects of ultrasound on neural structures. In studies using ex vivo peripheral nerves, suppression of evoked potentials was often observed after ultrasound application. This indicates the possible use of ultrasound to block nerve conduction or to destroy neural tissue for therapeutic purposes, such as neurolysis of peripheral nerves involved in hyperactive spinal reflexes and pain. Although high intensities of ultrasound have been shown to suppress evoked potentials in peripheral motor nerves, the subsequent effect of this suppression on the function of the nerves to stimulate muscle contraction has not been presented. We report the results of in vivo studies investigating the effects of HIFU on the motor function of peripheral nerves. The image-guided HIFU device that we have developed allows expansion of the current research by investigation of HIFU- induced effects on neural structures in vivo and during long- term studies. It also provides real-time image-guidance that is necessary to target and treat these structures accurately. II. MATERIALS AND METHODS The ultrasound imaging transducer and the HIFU transducer were attached via a custom-designed holder so that the geometrical orientations of the two beam patterns were coplanar, allowing the HIFU focus to be visualized in the image plane (Fig. 1). The compact linear array imaging transducer had a broadband frequency of 5 - 10 MHz. The 3.2 MHz single-element HIFU transducer had a focal length of 3.5 cm and an F-number of 1. Specially molded polyacrylamide gels were used with the device to provide efficient coupling of the HIFU energy into tissue, enabling transmission of the HIFU This work was supported by US Army MRAA (DAMD 17-02-2-0014) and the Seattle Foundation 1351 0-7803-8412-1/04/$20.00 (c)2004 IEEE. 0-7803-8412-1/04/$20.00 (c)2004 IEEE. 2004 IEEE International Ultrasonics, Ferroelectrics, and Frequency Control Joint 50th Anniversary Conference 2004 IEEE Ultrasonics Symposium