Aside from filtering blood and making urine, the kidneys have other functions. One such function is monitoring blood pressure (performed by special cells called macula densa). Using a chemical messenger called angiotensin, these cells can help adjust blood flow throughout the body and maintain normal blood pressure.
The angiotensin acts by increasing muscle tone in small artery walls to help boost blood pressure. Angiotensin also stimulates the release of aldosterone, which helps the body retain sodium and water, increasing the amount of fluid within the blood vessels.
Most people with high blood pressure have essential hypertension (hyper = more + tension = pressure), meaning that the cause of the high blood pressure is unknown. However, one known cause of hypertension is renal artery stenosis (renal = kidney + stenosis = narrowing).
Each kidney gets its blood flow via a renal artery that arises from the aorta, the major blood vessel from the heart. If one of the renal arteries narrows, it may cause decreased blood flow to the kidney and to the macula densa (the specialized, blood-pressure sensing cells in the kidney). These cells falsely presume that this low blood flow is being experienced by the rest of the body and that overall blood pressure is too low. They respond by increasing secretion of angiotensin, triggering the body's response to increase blood pressure.
High blood pressure is known as the "silent killer." It has no specific symptoms, but over time stresses the major organs in the body and is a risk factor for heart attack and stroke.
Blood pressure elevation with renal artery stenosis is no different; however, the decreased blood flow the kidney(s) over time may cause damage to the kidney(s). Decreased renal function (azotemia), may present with symptoms of fatigue, malaise, and/or slight confusion due to a gradual buildup of waste products in the body.
The healthcare provider may be concerned about renal artery stenosis if high blood pressure has its initial presentation in a person older than age 50 or in a person under the age of 30.
The physical examination may give a clue if a bruit (a rustling sound produced by turbulent blood flow) is heard when the healthcare provider listens to the abdomen. If an artery is narrowed, it may cause turbulence as blood flows through the narrowing, causing a noise, like the rapids in a river. This noise is called a bruit.
Renal artery stenosis may also be considered a cause of elevated blood pressure if multiple anti-hypertension medications have failed to control high blood pressure.
The most common cause of renal artery stenosis is atherosclerosis, the same condition that causes narrowing of the arteries in coronary heart disease and stroke, and the risk factors are the same as well. Cholesterol plaques build up along the walls of the renal artery and gradually cause narrowing.
Risk factors for renal artery stenosis include:
Other causes of stenosis (narrowing) of the renal artery include:
- fibromuscular dysplasia, an abnormal thickening of the muscles of the artery wall, most often seen in young women;
- arteritis or inflammation of the artery;
- aneurysm of the artery;
- compression of the artery by an outside mass; for example, a tumor.
The decision whether to treat renal artery stenosis with medicine or with a surgical procedure depends on the situation.
In patients with renal artery stenosis and only one kidney, if kidney failure from renal artery stenosis is a possibility, opening the artery with a surgical procedure may be the preferred option. Similarly, if there is significant stenosis in both renal arteries and kidney function is compromised, surgical repair may be offered as the primary treatment.
If the renal artery stenosis causes less than 50% narrowing of the artery and if kidney function is maintained, medications that block the actions of angiotensin [for example, captopril (Capoten), enalapril (Vasotec), losartan (Cozaar)] may be used in association with routine monitoring of the renal artery status with ultrasound.
The most frequently used invasive procedure to open a narrowed renal artery uses the same type of procedure that is used in heart disease. Balloon angioplasty allows a radiologist to insert a catheter, or narrow tube, into the femoral artery in the groin and advance it to the area of narrowing in the renal artery. By quickly inflating the balloon, the cholesterol plaque causing the narrowing is compressed into the artery wall. A stent or wire mesh is then placed across the area of previous narrowing to keep it open.
If angioplasty fails or is not technically feasible, formal bypass surgery may be considered. In this situation, just like in a heart bypass operation, a surgeon will take a piece of normal vein or a synthetic tube and connect the aorta and kidney, bypassing the blocked area.
Unfortunately, if the renal artery stenosis is long standing and kidney function has been compromised for a prolonged period of time, return blood flow to the kidney may not return kidney function.