Microwaving metal! Can you get away with it? Renal denervation after stenting

Despite over 20 years of research into the utility of percutaneous intervention in patients with renal artery stenosis (RAS), there is still a lack of definitive proof. A meta-analysis of six key randomized control trials comprising 1,208 patients comparing medical therapy and renal artery revascularization versus medical management alone failed to show any benefit of revascularization with respect to blood pressure reduction or renal function improvement [1]. These trials have had shortcomings, which may have resulted in their negative results [1]. The CORAL (Cardiovascular Outcomes in Renal Atherosclerotic Lesions) Trial, which was designed to overcome these limitations, is completed and we are awaiting their results. The disappointing results conflict with the theoretical background that improved renal perfusion would positively affect outcomes and renal function. This could be explained by neurohumoral activation in chronic RAS that perpetuate glomerular damage that cannot be reversed by restoring blood flow alone [1]. This notion is not new in cardiology with similar conclusions drawn from trials investigating the role of coronary revascularization as a treatment for heart failure. Sympathetic hyperactivity, a key adverse component of the neurohumoral activation in many conditions including hypertension, renal failure, and heart failure, can now be directly modulated through percutaneous renal artery denervation (RDN). RDN has had more of a convincing impact on blood pressure in the 5 years, this modality has been clinically available than renal revascularization has had in the previous 2 decades. This therapy needed just one multi-center randomized controlled trial of 106 patients with resistant hypertension to prove its value in significantly reducing blood pressure by on average 32/12 mm Hg [2]. However, this technique is contraindicated in patients with RAS or renal artery stents. In this issue of Catheterization and Cardiovascular Intervention, Ziegler et al., report a successful case of RDN using radiofrequency (RF) energy in a patient with refractory hypertension despite bilateral renal artery revascularization [3]. There is a concern that applying RF energy within a stent is akin to putting metal within a microwave oven (though microwaves have a larger magnitude of frequency). Doing this would cause heat deposition in the stent and coagulative necrosis circumferentially around it [4]. There is a role of RF being applied to metal stents in the treatment for periluminal tumors [4]. However, it is discouraged in renal arteries as the damage would likely extend transmurally through the artery and beyond rather than being limited to the peripheral sympathetic nerves. RDN targets the afferent and efferent renal sympathetic nerves which lie in the renal artery wall and perivascular soft tissue [5]. Human cadaveric studies have shown that the nerves are evenly distributed around the circumference of the artery, though they do arborize from fewer proximal common nerve trunks to distal multiple innervations [5]. Also the nerves tend to migrate toward the arterial lumen as they move distally [5]. The decision of where anatomically to apply RF in the renal artery is a play-off between a proximal strategy where there are fewer and large nerves to target but there is a risk of missing an important one versus a distal one, where the nerves are more superficial in the lumen and despite their being more target nerves, missing one is likely not to affect the results [5]. What is without question is the necessity of a circumferential ablative strategy and ideally with as many points as safely possible. There is no definite evidence that an ostial and