Hypertension Research | 2019
Observation of renal sympathetic nerves by intravascular ultrasound
Abstract
Renal denervation (RDN) has been found to be effective for reducing the blood pressure in recent clinical trials, including the SPYRAL HTN—ON MED trial [1], the SPYRAL HTN—MED trial [2], and the RADIANCE-HTN SOLO trial [3]. By overcoming the disappointing results of the SYMPLICITY HTN-3 trial [4], treatment of hypertension with RDN entered a new era with these three clinical trials [5]. In the two SPYRAL trials, a helical multielectrode catheter was used for RDN [1, 2]. The four electrodes on a Spyral catheter distribute radiofrequency energy to the four quadrants of the target renal artery, allowing ablation of sympathetic nerves through the vessel wall. However, the Paradise system used in the RADIANCE trial provides circumferential ultrasonic cauterization of the renal artery to a depth of 1–6 mm with a cooling balloon [3]. With both systems, there is no way to assess whether or not renal sympathetic nerves have been cauterized by the procedure, making it difficult to judge the success or failure of RDN. One reported method of selecting the target sites for ablation and the endpoint of RDN is electrical autonomic nervous stimulation conducted from inside the renal artery [6]. Although this method is presently considered to be the most useful method, it has not reached the stage of clinical application. Observation of nerves by ultrasound has already been done clinically during lumbar nerve block or diagnosis of carpal tunnel syndrome. Intravascular ultrasound (IVUS) is widely used for percutaneous coronary intervention, but neither the observation of nerves around the coronary arteries nor the observation of renal sympathetic nerves around the renal artery have been attempted. However, it might be possible to identify the renal sympathetic nerves as structures running parallel to the renal artery by careful observation with IVUS. Therefore, we reviewed IVUS images obtained during percutaneous transluminal renal angioplasty (PTRA) and searched for evidence of renal sympathetic nerves. During PTRA for treatment of renal artery stenosis, we used a VISICUBE and Navifocus (TERUMO Corporation) to observe the vessel diameter and the features of the culprit lesion. During PTRA, IVUS is usually done with the settings shown in Table 1 (Configuration A). By reviewing IVUS images from PTRA, we found multiple hypoechoic structures running along the renal artery at approximately 1–4 mm outside the vessel (Fig. 1). Observation of IVUS images revealed low echoic structures running along the renal artery approximately 68