Evaluation of therapeutic low-frequency transcranial ultrasound in an embolic MCA occlusion model in rats

Background: Therapeutic application of diagnostic ultrasound has been shown to improve recanalisation rates in patients with acute cerebral vessel occlusion. There is experimental evidence that low-frequency ultrasound may be superior to diagnostic ultrasound. This study was designed to evaluate therapeutic efficacy and safety of low-frequency ultrasound in an embolic MCA occlusion model in rats. A parameter setting was used that, in a previous study, had not shown any side effects and interactions with healthy rat brain tissue.

Methods: Male Wistar rats were submitted to MCA clot embolism. After induction of ischemia, animals were transcranially treated with 20 kHz cw ultrasound (0.2 W/cm), either alone or in combination with rtPA. Control groups received no treatment or rtPA alone. Ten surviving animals were included in each group. Outcome assessment consisted of determination of infarct volume and of a neurological evaluation, including parallel bar crossing and a neurological score with 10 different motor, coordinative and sensory items. Follow-up period was 6 days. Animals were perfusion-fixed for histology on day 7 (blinded examination).

Results: Six animals treated with ultrasound died during the follow-up, as compared to no animals in the control groups. This difference was statistically significant (p=0.02). The dying animals displayed a secondary worsening after an initial period of normal vigilance. In surviving animals, no benefit of treatment could be demonstrated. With regard to lesion size, there was a non-significant trend towards a larger lesion volume in animals treated with ultrasound (p=0.3).

Conclusions: In this study, 20 kHz ultrasound worsened clinical outcome and caused death in a significant number of animals. The mechanism of damage remains unclear, but is possibly due to induction of brain edema in ischemic brain tissue, as the same set of parameters had not shown any tissue damage in healthy rat brain. Since higher intensities induce brain edema in healthy brain tissue, 20 kHz ultrasound does not seem to be suitable for therapeutic cerebral applications. The data underline the necessity to obtain further animal data to establish safety limits of frequency and power output before applying this promising technique in humans.