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Monday, May 17, 2010
A severe renal artery stenosis is suggested by a much higher aortic blood pressure reading than in the renal artery (pulse rate of 94 compared with 35).
RAS is often clinically “silent.” When it does cause clinical manifestations, the most frequent one is hypertension; in fact, RAS is the most common secondary vascular cause of hypertension. It can lead to cardiac disturbances, such as flash pulmonary edema and unstable angina. Finally, RAS can cause renal insufficiency. A small but significant number of patients with RAS will even progress to occlusion and require dialysis. While much less common than essential hypertension, RAS should be considered in middle-aged or older patients whose blood pressure control becomes more difficult to manage, or when the patient presents with severe hypertension.
There is considerable debate among physicians about which patients with RAS should be treated with percutaneous transluminal renal angioplasty (PTRA) and stent placement, and at what threshold. Specialists at the Massachusetts General Hospital Vascular Center support the use of PTRA with stenting, but only when performed by highly experienced clinicians and only for patients who are likely to derive a clinical benefit. Opening the narrowed artery simply to restore the normal caliber is not recommended.
Etiology of RAS
Atherosclerosis is the predominant cause of RAS. Patients with RAS typically have a buildup of plaque in other arterial beds as well. Atherosclerotic RAS is becoming increasingly prevalent as the population ages.
While a narrowing can occur within the body of the kidney artery, the most frequent site is at its origin coming off the aorta. Stenosis at this ostial location is caused at least partially by aortic plaque that extends over and constricts the origin of the kidney artery. Patients may have unilateral or bilateral RAS.
Fibromuscular dysplasia (FMD) is a less common but very important cause of RAS. FMD is a genetic condition that affects more women than men and can cause RAS leading to high blood pressure. While it may be present in patients of all ages, FMD often occurs in young patients and should be strongly considered in those younger than 30 years of age who present with severe hypertension. There are five different subtypes of FMD, each with its own distinctive pattern of stenosis. The most common is known as medial fibroplasia, which causes weblike narrowings within the renal artery.
Other less prevalent conditions can also cause RAS.
Clinical Manifestations and Risk Factors
Patients whose hypertension is controlled and renal function remains normal may not need diagnostic evaluation for RAS. Clinicians should monitor these patients and pursue diagnostic testing if hypertension becomes difficult to manage and/or renal function deteriorates.
Multidisciplinary, Subspecialty Care
At the Mass General Vascular Center, a multidisciplinary team of physicians, including nephrologists, interventional radiologists, interventional cardiologists, vascular surgeons, and vascular medicine physicians who specialize in hypertension, works with the patient’s primary care physician to diagnose RAS and provide evidence-based medical and interventional care.
The renal artery duplex ultrasonography study is often the first test used to diagnose and grade the severity of RAS. These noninvasive tests are cost-effective, sensitive, and specific. Duplex ultrasonography can be used to determine the size of the kidney, preservation or loss of the renal cortex, and the degree of artery narrowing.
Other diagnostic tests include computed tomographic angiography (CTA) and magnetic resonance angiography (MRA). These tests provide actual images of the renal artery. However, they are more costly than duplex ultrasound and have liabilities. CTA exposes patients to contrast dye and radiation, and MRA is not accurate at assessing the degree of narrowing, although it can detect stenosis.
RAS is often diagnosed during invasive angiograms to evaluate the coronary or lower extremity arteries. In 2006, the American Heart Association issued a scientific advisory recommending that physicians evaluate the renal arteries in certain patients during coronary catheterization procedures.
Medical Therapy May Be Sufficient
Medical treatment for RAS is very similar to treating other forms of vascular disease. Physicians at the Vascular Center may prescribe one or more medications to manage hypertension, dyslipidemia, and diabetes. They may recommend that patients take aspirin and they may provide lifestyle counseling about losing excess weight, exercising regularly, and quitting smoking.
The Role of Interventional Therapy
After the renal artery stenosis has been corrected, the blood pressure in the aorta (pulse rate of 102) is nearly identical to that of the distal renal artery (pulse rate of 101).
Conflicting data regarding treatment outcomes has led to confusion and disparate views among physicians about the use of interventional procedures to treat patients with RAS. Some physicians advocate medical therapy alone and claim patients do not benefit from and may actually be harmed by procedure-related complications, such as atheroembolism and contrast nephropathy.
Mass General Vascular Center specialists, however, support the use of PTRA with stenting as an effective revascularization strategy that can significantly improve outcomes in specific patients with refractory hypertension, rapidly declining kidney function, or cardiac disturbance syndromes, such as flash pulmonary edema, congestive heart failure, or angina. In particular, Vascular Center physicians suggest early intervention in patients with a kidney transplant, with critical stenosis of both renal arteries, and with narrowing to a single functioning kidney (with the other kidney removed due to cancer, trauma, or shrunken from prior arterial blockage).
In these patients, renal artery stenting may help reduce blood pressure, prevent or delay the long-term effects of hypertension, and allow some patients to discontinue or reduce antihypertensive medications. Renal artery stenting may also help preserve renal function. Some patients are able to avoid dialysis and, although rarely, others can actually regain enough kidney function to come off dialysis once their renal arteries are opened.
Balloon angioplasty alone is generally not sufficient for obstructions in the origin of the renal artery, the most common site for atherosclerotic RAS. Plaque in this area is thick, resistant, and likely to reform. Stents act like a scaffold to maintain the patency; stents are employed in nearly every patient whose renal arteries are narrowed from atherosclerosis.
Patients with RAS from FMD, in contrast, usually respond well to balloon angioplasty alone and rarely require stenting. Following interventional treatment, patients with FMD often have normal or nearly normal blood pressure, thus reducing the amount of or avoiding the need for lifelong medication.
Renal angioplasty and stenting procedures are technically demanding and require significant expertise to achieve optimal results and avoid complications. For example, positioning the stent precisely so that it protrudes slightly into the aorta (as is desirable) can be challenging. Likewise, minimizing manipulation to avoid atheroembolism requires a high level of expertise. Physicians in the Vascular Center sometimes use embolic protection devices to capture and remove plaque debris.
Massachusetts General Hospital: At the Forefront of National RAS Efforts
In addition, physicians at the Mass General Vascular Center, as experts in noninvasive ultrasound imaging, have taken the national leadership role in developing what are now widely accepted criteria for defining significant RAS.
Mass General physicians have also been involved in the National Institutes of Health-sponsored Cardiovascular Outcomes in Renal Atherosclerotic Lesions (CORAL) trial, which seeks to determine which patients with RAS truly need to be stented in order to prevent death, kidney failure, or other adverse outcomes, and which patients can forego invasive treatment.
Finally, Mass General physicians are studying newer devices such as covered stents for patients with restenosis, working with the Food and Drug Administration to develop performance criteria for any new stents approved for renal angioplasty procedures, and developing a registry of renal artery stent patients.
Massachusetts General Hospital Vascular Center
Massachusetts General Hospital established the Vascular Center to provide comprehensive care for patients with vascular disease. As a dedicated vascular center with specialists in every area of vascular disease, the Vascular Center offers a uniquely focused, multidisciplinary approach to vascular medicine. Specialists in seven critical disciplines work together. These include cardiac surgery, cardiology/vascular medicine, nephrology, neurology, neurosurgery, vascular and endovascular surgery, and vascular radiology. Treatments and Services: Aortic Disease Program; Brain Aneurysm and Arteriovenous Malformations Program; Peripheral Artery Disease Program; Stroke and Carotid Artery Disease Program; Venous Disease Program; Visceral Vascular Disease Program.
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