Renal sympathetic denervation

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: João André Alves Silva, M.D. [2]

Synonyms and keywords: RDN; Renal denervation

Overview

Renal denervation (RDN) is a minimally invasive, endovascular catheter-based procedure. It most commonly uses a radiofrequency energy source and aims to treat hypertension in selected patients whose blood pressure can't be controlled with a combination of lifestyle changes and antihypertensive therapy. A patient is said to have resistant hypertension when despite these changes and taking 3 or more antihypertensive drugs, including a diuretic drug, at their maximal tolerated doses, without secondary hypertension, has a baseline systolic blood pressure of more that 160 mm Hg (or more that 150 mm Hg, in case of concomitant type 2 diabetes mellitus). This treatment is based on the fact that these patients have an increase in sympathetic outflow. By applying radiofrequency energy pulses to the renal arteries, the nerves in the vascular wall (adventitia layer) can be ablated. This causes a reduction of renal sympathetic afferent and efferent activity with the intent to decrease blood pressure. This specific interruption of the renal sympathetic nerves appears to offer a durable blood pressure decrease, without severe consequences. This might be due, not only to the overall decrease in sympathetic stimulus, along with the concomitant decrease in renin and vasoconstriction, but also to the reduction of release of norepinephrine by nerve endings, at the renal epithelial cells, thereby decreasing sodium and and water reabsorption. Early data from international clinical trials shows promising results, demonstrating an average blood pressure reduction of approximately 30mm Hg at three year follow up in patients with treatment-resistant hypertension. Since 2007, over 4000 patients have undergone catheter-based renal denervation with the Medtronic Symplicity™ Renal Denervation System. Despite its importance in drug-resistant hypertensive patients, by reducing renal sympathetic nerve flow, this procedure will also reduce overall sympathetic activity, thereby affecting multiple organs, which is why renal denervation might also be beneficial in other disease states, such as congestive heart failure, chronic kidney disease, metabolic syndrome and others. Until now, few complications have been reported, being inconsequential. It's long-term efficacy is still yet to be determined, however, studies show a permanence of effect at least of 2 to 3 years.

History

Prior to pharmacological management of hypertension, surgical sympathectomy was a recognized treatment for hypertension. Before modern pharmacotherapy, the mortality rate within 5 years of malignant hypertension was close to 100%. Surgical treatment of hypertension was suggested by several independent researchers in 1923, yet the first patient with malignant hypertension was only treated with surgical sympathectomy, by Adson in 1925. Isolated bilateral kidney denervation was only performed in 1934 by Page and Heuer, however, because the results were considered unsatisfactory, surgical renal denervation was replaced by a more aggressive technique, the surgical removal of splanchnic nerves or splanchnicectomy, which showed effective results. Subsequently, thoracolumbar splanchnicectomy became the procedure of choice for malignant hypertension, which did not respond to diet and the limited pharmacological therapy of the time, for the following 2 decades. Between 1938 and 1947, other studies were made by Smithwick and Thompson, who published results from studying 3500 patients with malignant hypertension. Of those, 2400 were treated with thoracolumbar splanchnicectomy, while the others took the pharmacological therapy available at the time. The group who underwent the surgical procedure had an inferior mortality rate and substantial blood pressure reduction, when compared to the pharmacologically treated group. This technique was often successful in reducing blood pressure but due to its non-selective nature, the high operative mortality and side effects were considerable. These included orthostatic hypotension, palpitations, anhidrosis, intestinal disturbances, impotence, thoracic duct injuries and atelactasis. It was only until mid-1950's that the first oral antihypertensive medication became available. This allowed for a well-tolerated treatment regimen that patients could follow on the long term. For the last 50 years, medication has improved the control of hypertension in thousands of patients throughout the world, however, evidence from the National Health and Nutrition Examination Survey along with large randomized clinical trials shows that an estimated 20% to 30% of hypertension cases require up to 3 or more antihypertensive drugs to achieve blood pressure targets. Recent data from National Health and Nutrition Examination Survey shows that 12.8% of hypertensive patients fulfill the criteria for resistant hypertension, which represents about 120 million patients worldwide. Failure to reach normal blood pressure values puts these patients at an increased risk for development of major cardiovascular complications. Subsequently, the number of hypertensive patients being referred for splanchnicectomy has decreased considerably in recent years. During this time, a major effort in understanding the role of the SNS in hypertension has been made, particularly the role of renal sympathetic nerves in this process. Several models are pointing to a considerable role of the SNS overactivity in the development and maintenance of hypertension, to which renal sympathetic nerves are an important contributor. This overactivity is involved in several others diseases, described below, which justifies the need for more studies to evaluate how relevant this procedure might be in the treatment of other conditions.

Device

As of today, several percutaneous renal sympathetic nerve ablation systems are being studied and tested, 6 of them have already received CE marking to be used for renal nerve ablation:

• Medtronic's Simplicity™ System - produced by Medtronic (formerly Ardian), was the first device to be used in humans, receiving market approval in 2010. It uses a radio frequency catheter (6F) inserted percutaneously through a femoral sheath, under fluoroscopic control. Despite being easily used, it has a tendency to create lesions with a less predictable pattern. This device now has over 5 years of clinical experience and 3 years of follow up data. The device has received favourable reviews on WhichMedicalDevice, but concerns have been reported regarding availability and financial reimbursement for the procedure.
• St. Jude's EnligHTN system - also uses a radio frequency catheter inserted percutaneously through a femoral sheath, under fluoroscopic control, however, is equipped with 4 electrodes on a basket structure. This allows it to create lesions in a more circumferential pattern, being able to create thermal injury and fiber interruption in a more predictable way.
• Vessix's V2 system - also uses a radio frequency catheter inserted percutaneously through a femoral sheath, under fluoroscopic control, however, the electrodes are mounted in a balloon, allowing for a good distribution of the energy.
• Covidien's One Shot system - also uses a radio frequency catheter inserted percutaneously through a femoral sheath, under fluoroscopic control, however, the electrodes are mounted in a balloon, allowing for a good distribution of the energy.
• Iberis system - also uses a radio frequency catheter and a 4-French shaft, enabling radial access.
• Recor's Paradise system - uses an ultrasound technology catheter inserted percutaneously through a femoral sheath.

Any of these systems must be manipulated by skilled operators, in an appropriately equipped catheterization lab. Currently they are available in parts of Europe, Asia, Africa, Australia and the Americas. So far, no renal denervation device has had FDA approval.

Procedure

Considering the factors: drug-resistant hypertension, SNS involvement in hypertension, importance of renal nerves for the overall sympathetic activity of the body, along with the ease of approach of the renal nerves through catheter techniques, hypertension is a good candidate for a catheter-based interventional approach. Knowing that sympathetic nerve fibers are located in the adventitia of the renal arteries, they can be easily reached by a catheter through a transvascular approach and interrupted using thermal energy. However, considering that sympathetic nerves share their location with C-pain fibers, analgesia and sedation, but not anesthesia, are mandatory for this procedure. This technique must take place in a well equipped catheterization lab center, with skilled operators, experienced in handling possible surgical complications. Preprocedural examinations require the exclusion of secondary hypertension, as a "hidden cause" of resistance, as well as confirmation of uncontrolled blood pressure, while on medical treatment. This last criterion may involve the testimonial of a third party, confirming that the patient took the medication, since a common cause of "resistance" is noncompliance with the regimen.

Before the procedure can take place, renal artery suitability must be assessed, which is done with a duplex ultrasound or MRI, and adequate anticoagulation should be attained and confirmed by activated clotting time test (with a target of 200-250 seconds). Ideally the renal artery must be >20 mm in length and >4 mm in diameter. Renal function tests are also required, to confirm kidney's ability to sustain preprocedural contrast medium exposure. During the procedure, vital signs such as blood pressure, heart rate and oxygen saturation must be monitored and continuous transcutaneous oxygen should be provided. Although it has not been confirmed by clinical trials, it has been suggested the administration of periprocedural anti-platelet therapy, for up to 4 weeks after the procedure, to prevent thrombus formation.

Under fluoroscopic guidance, through a femoral sheath, the electrode is positioned in each of the renal arteries. In patients with single renal arteries, 4 ablation points are recommended. The catheter should be placed at the periphery of the second order renal artery branch point, with the help of a guide wire. The lesions made in the artery wall should have a circumferential pattern, to decrease the risk of artery stenosis, which can be achieved by rotating the catheter while pulling it back to the ostium of the artery, at the same time that the energy is being delivered. For a successive and safe ablation, the points of lesion should be distanced by >5 mm. In areas of atherosclerotic plaque, the ablation should be avoided. The energy used for the ablation also generates heat within the vessels, yet the system is cooled by the high rate of blood flow. The procedure lasts for about 30 to 60 minutes. Up until now, the devices used for this procedure have used either a single-tip electrode catheter, or a multielectrode system. The multielectrode systems have simplified this process by making it less painful and faster than the single-tip electrode version.

After the procedure, the patient should be monitored until the sedation wears off, and closely followed to access the safety and efficacy of the procedure. Some studies also recommend the evaluation of the renal arteries, using duplex ultrasound, in order to exclude renal artery stenosis. Despite having been reported in single cases, this complication might not be due to the technique itself, but to pre-existing atherosclerotic plaques.

Outcomes

Renal sympathetic denervation technique's efficacy was first evaluated by a proof-of-concept study in 45 patients with resistant hypertension. These patients were being treated with an average of 5 antihypertensive drugs, however, their blood pressure failed to be controlled, with baseline values of 177±20/110±15 mmHg. After undergoing the procedure, 1 month after survey, the patients' office blood pressure had significantly reduced by 27±16/17±11 mmHg at the first year.

The two largest studies to date have been the Symplicity HTN-1 and Symplicaty HTN-2 trials, conducted with the Medtronic's Symplicity RDN System:

• Symplicity HTN-1 - was design to be a first-in-human observational evaluation of the efficacy and safety of selective renal sympathetic denervation. It looked at outcomes of 45 patients, with a mean age of 58 ± 9 years and mean blood pressure of 177±20/101±15 mm Hg, taking on average 5 antihypertensive drugs, who underwent catheter-based renal denervation. The patients were then evaluated at 1, 3, 6, 9 and 12 months and the procedure was noted to be associated with a significant decrease in office blood pressure, with mean decreases of 14±4/10±3 (1st month), 21±7/10±4 (3rd month), 22±10/11±5 (6th month), 24±9/11±5 (9th month) and 27±16/17±11 (12th month) mm Hg. Three-year follow-up data have demonstrated an average blood pressure reduction of 33/19mm Hg. This trial confirmed the durability of the procedure, contradicting the hypothesis that sympathetic nerve regrowth would nullify the effect. It has also noted a reduction of 47% in renal norepinephrine spillover, accompanied by a decrease in overall body norepinephrine spillover, confirming a reduction in central sympathetic activity after the renal denervation.

• Symplicity HTN-2 - was a randomized, controlled trial, with a total of 106 patients from Australia and Europe, that compared 54 control patients with 52 patients who underwent catheter-based renal denervation. Six month follow-up data demonstrated a blood pressure reduction of -32/12mm Hg in the treated group compared with a change of 1/0 mm Hg in the control group.

• Symplicity HTN-3 trial is currently underway in the US. This is a 530-patient, multi-centre, prospective, single-blind, randomized, controlled study (Clinical Trial No. NCT01418261).

Longer follow-up, more patients, and improved study design are necessary to further validate catheter-based renal denervation.

Meta-analyses of renal denervation have yielded conflicting results. While office systolic blood pressure reductions typically average around 30 mmHg, reductions observed on ambulatory blood pressure monitoring are typically much smaller, around 10 mmHg. The reasons for this disparity are so far unclear. Proposed theories include renal denervation obliterating the white coat response, thereby disproportionately reducing clinic values, or the disparity rather being an anomaly due to deficiencies in renal denervation trial designs to date.

Risks

The Symplicity HTN-1 and HTN-2 trials have demonstrated a good safety profile for catheter based renal denervation. Patients may experience pain during application of radiofrequency pulses and intraprocedural bradycardia requiring atropine has also been reported. Other documented procedure related complications include femoral artery pseudoaneurysm and renal artery dissection.

Of particular concern is the theoretical risk of damage to renal arteries during delivery of radiofrequency energy. An animal study using swine showed no damage to the renal arteries at 6 month follow up. This finding is further supported in human studies in the HTN-1 and HTN-2 trial where follow up imaging has not demonstrated renal vascular damage.

Uses of Renal Denervation beyond Hypertension

Hypertension is associated with an overactive sympathetic drive and renal denervation is the ablation of the renal nerves stopping the cross-talk between the kidneys and brains, thus reducing the sympathetic drive. Similar to hypertension, congestive heart failure (CHF), left ventricular hypertrophy (LVH), atrial fibrillation (AF), obstructive sleep apnea (OSA), and insulin resistance/type 2 diabetes mellitus (DM) all have been associated with an overactive sympathetic drive. Current clinical trials are researching the effect of renal denervation in these clinical conditions as well.

• Milder hypertension

• Heart failure

• Kidney disease

• Cardiac arrhythmias

• Obstructive sleep apnea

• Polycystic ovary disease

• Metabolic disease/ Diabetes mellitus

• Hypertensive end-organ damage

• Improved exercise tolerance

• Improved peripheral circulation