Renal Artery Stenosis (RAS) is the narrowing of one or both renal arteries, the blood vessels that deliver blood directly to the kidneys. The kidneys require a constant, high-volume blood supply to filter waste products and maintain proper fluid balance. When these arteries narrow, the restricted blood flow impairs kidney function and often leads to high blood pressure, making identifying the underlying cause important for treatment.
Atherosclerotic Plaque Buildup
The overwhelming majority of Renal Artery Stenosis cases (60% to 90%) are caused by atherosclerosis. This progressive disease involves the gradual accumulation of plaque on the inner lining of the renal artery wall. Plaque is composed primarily of fats, cholesterol, calcium, and cellular debris.
Plaque causes the artery wall to thicken and harden, restricting the lumen through which blood flows. This narrowing diminishes the amount of oxygen-rich blood reaching the kidney tissue. Since atherosclerosis is a systemic disease, this form of RAS is common in older individuals, particularly men over 45, who often have plaque buildup in other arteries. The plaque typically forms at the opening of the renal artery where it branches off the aorta, or within the first two centimeters of the vessel.
As the artery narrows, the reduced blood flow triggers a compensatory mechanism within the kidney. The organ mistakenly senses low overall blood pressure and releases a hormone called renin, which initiates a cascade to raise blood pressure throughout the body. This hormonal response often results in a form of high blood pressure that can be difficult to manage with standard medications. The chronic lack of sufficient blood flow can ultimately lead to scarring and a gradual loss of kidney function.
Fibromuscular Dysplasia
Fibromuscular Dysplasia (FMD) is the second most frequent cause of RAS, responsible for up to 30% of cases. FMD is a non-inflammatory and non-atherosclerotic condition, meaning the narrowing is not caused by cholesterol plaque or a systemic autoimmune reaction. Instead, the condition involves the abnormal growth of cells within the walls of medium-sized arteries, including the renal arteries.
This irregular cellular growth causes segments of the artery wall to thicken and become disorganized, which alternates with areas where the wall is weakened and bulges outward. On imaging, this pattern of alternating narrowings and enlargements creates a distinct “string of beads” appearance. While the exact underlying cause of FMD remains unknown, it may involve genetic, hormonal, or mechanical factors.
FMD most commonly affects younger individuals, particularly women diagnosed before age 50. The narrowing caused by FMD reduces the blood pressure sensed by the kidney, leading to the same renin-mediated hormonal response that causes high blood pressure. Unlike atherosclerotic RAS, FMD lesions usually occur further down the renal artery or within its branches.
Rarer Vascular Conditions
While atherosclerosis and FMD account for the vast majority of cases, a small percentage of RAS is caused by other, less common conditions that directly affect the renal artery structure.
Inflammatory diseases known as vasculitis can cause artery walls to swell and become damaged, leading to narrowing. Conditions such as Takayasu’s arteritis, a type of large-vessel vasculitis, can target the aorta and the origins of its branches, including the renal arteries. Another mechanism is arterial dissection, which involves a tear within the layers of the renal artery wall, creating a flap that obstructs the vessel’s center channel.
Narrowing can also occur due to extrinsic compression, where a mass or tissue outside the artery presses upon it. This may be caused by a tumor, an abnormal band of tissue, or compression by a structure like the diaphragmatic crura. In all these instances, the renal artery diameter is reduced, impeding blood flow to the kidney.
Systemic Conditions That Increase Risk
Certain health issues are not direct causes of physical narrowing but create an environment that accelerates the development of primary causes, especially atherosclerosis. These systemic conditions predispose an individual to plaque buildup within the arterial walls.
Chronic hypertension damages the inner lining of arteries, making them susceptible to plaque deposition. High cholesterol (hyperlipidemia) provides the fatty material that forms the core of atherosclerotic plaque. Diabetes mellitus contributes to widespread vascular damage and inflammation, accelerating the hardening and narrowing of arteries throughout the body, including the renal arteries.
The use of tobacco products, such as smoking, is a potent factor that injures the lining of blood vessels, increasing the risk of plaque formation and progression. Advanced age is also an undeniable risk factor, as the cumulative exposure to these other conditions and the natural processes of wear and tear contribute to arterial changes over decades. These conditions set the stage for atherosclerotic disease to take hold and progress rapidly.