Overview of renal artery stenosis and kidney function

Renal Artery Stenosis (RAS) refers to the narrowing of one or both renal arteries, reducing the flow of oxygen-rich blood to the kidneys.^1,2 Adequate blood flow is essential for kidney function, in order to filter waste products and to remove excess fluids.² Reduced blood flow, as in RAS, may injure kidney tissue, resulting in hypertension (increased blood pressure) throughout your body.² In patients with mild hypertension, RAS has a prevalence of only less than 1%, but this can increase to 10-40% prevalence in patients with acute, severe, or refractory hypertension.¹

Causes of renal artery stenosis

The two major causes of RAS are atherosclerosis and fibromuscular dysplasia (FMD):

Atherosclerosis (60-90%)

Atherosclerosis is the build-up of fat, cholesterol, and other substances in and on your kidney artery walls. As these plaques get larger, and harden, it narrows the artery, reducing blood flow, resulting in kidney scarring and injury. Atherosclerosis is the most common cause of renal artery stenosis, and primarily affects men over the age of 40 years but can affect any patient. Risk factors for atherosclerosis include dyslipidemia (abnormal lipid levels in the blood), cigarette smoking, viral infections, and immune injuries.¹

Fibromuscular dysplasia (20-30%)

Fibromuscular dysplasia occurs when the muscle in the artery wall doesn’t grow as it should. This causes renal artery narrowing, resulting in a bead-like appearance in artery images, with narrow sections alternating with wider sections.² The cause of fibromuscular dysplasia remains unknown, but it often begins in childhood and mostly affects women under 50 years old.¹ The narrowing of the renal artery in patients with fibromuscular dysplasia causes high blood pressure at a young age.

3. Other causes (<10% cases)

RAS can result from other conditions such as inflammation of the blood vessels or a growth developing on your abdomen, resulting in pressure on the renal arteries.²

Symptoms of renal artery stenosis

How to recognise early signs

In the early stages of RAS, there are often no signs or symptoms, meaning that it is often diagnosed in early stages when testing for unrelated conditions.^3,4 Signs that would cause a healthcare provider to suspect RAS include:

• Sudden high blood pressure, or a worsening of high blood pressure without explanation 
• High blood pressure beginning at a young age (<30) or an older age (>50)

As renal artery stenosis progresses

As RAS progresses, your doctor may diagnose RAS based on the following symptoms:^4,5

• High blood pressure that’s hard to control
• Signs of issues with kidney function, such as elevated protein levels in the urine
• Treatment for high blood pressure resulting in worsening kidney function
• Swelling in body tissues
• Treatment-resistant heart failure

The doctor may also place a stethoscope over your kidneys, on the front or side of the abdomen to listen for a whooshing sound, which would be produced as blood flows through a narrowed vessel.⁴ High blood pressure with no family history of high blood pressure also leads to RAS being considered.

Symptoms of a significant decrease in kidney function can include: 

• A change in frequency of urination
• Oedema
• Headaches
• Weight loss
• Appetite loss⁴

Risk factors for renal artery stenosis

Risk factors to make narrowed arteries more likely in your kidneys and other body parts include:^4,6

• Aging
• High blood pressure
• High blood cholesterol levels
• Diabetes
• Being overweight, or obesity
• Smoking and other tobacco use
• A family history of early heart disease
• Lack of exercise and physical activity
• For men, being older than 45 or for women, being older than 55

Complications of RAS

RAS can cause high blood pressure. Prolonged periods of high blood pressure can cause damage to blood vessels and organs, which can lead to further complications including heart attack and stroke.⁵

Additionally, the lack of blood flow to the kidney, due to narrowed renal arteries can cause kidney failure which may require treatment with dialysis or a kidney transplant. Chronic kidney disease (CKD) is a potential complication, which is a condition involving reduced kidney function over some time.RAS can also result in swelling of your ankles and feet. 

It can also cause shortness of breath due to a sudden build-up of fluid in the lungs.⁵

Fibromuscular dysplasia

Blood vessels are made up of flexible, strong, epithelial cells, which are important to allow vessels to pump blood adequately around the body. Fibromuscular dysplasia is a rare blood vessel disorder that causes abnormal cell development in the artery wall, in which some of the strong, flexible cells of arteries are replaced with more fibrous, weaker, and less flexible cells.^6,7,3 This makes arteries more prone to damage, as they are stiffer and less flexible.

Fibromuscular dysplasia can result in narrowing (stenosis) or tears (dissection), in affected arteries. Narrowing and tears can reduce blood flow, resulting in the aforementioned symptoms.⁷

Many vascular diseases are caused by inflammation and plaque (atherosclerosis), where a build-up of fatty plaque inside arteries hardens and narrows them. However, FMD is different from these inflammatory diseases of the artery walls, as it doesn’t involve plaque build-up.³

Types of fibromuscular dysplasia

FMD can be classified into two types, based on artery imaging studies.^6,8

1-Multifocal FMD 

The most common type. Characterised by alternating areas of narrowing and bulging artery, resulting in a ‘string of bead-like’ appearance.^6,8

2-Focal FMD 

Focal FMD is rare among adult patients, and more commonly seen among children. It is characterised by a single narrowing within the diseased artery.^6,8

How does FMD lead to RAS?

Fibromuscular dysplasia is most commonly found in the renal arteries, which supply the kidneys with blood (as well as the carotid and vertebral arteries found in the neck). Therefore, FMD can lead to renal artery stenosis.

It is unknown what the risk factors that lead to RAS caused by FMD are, however, FMD is most common in women aged 25 to 50.⁶

Diagnosis of RAS and FMD

• Non-invasive tests (ultrasound, CT, angiography, MRI)
• Invasive tests (angiography)

Treatment Options for RAS

Treatment for RAS aims to prevent the worsening of the disease and relieve the blockage of the renal arteries.⁴ Some treatment options help disease progression only in renal artery stenosis caused by atherosclerosis or plaque build-up, whereas some also help with stenosis caused by fibromuscular dysplasia.

The following treatment options are commonly used:

Medications

• Blood pressure control to slow the progression of kidney disease
• Cholesterol management to prevent plaque from building up in the arteries
• Blood-thinning medications to allow blood to flow more easily through the arteries

Lifestyle changes

To promote healthy renal arteries and to prevent plaque build-up the following lifestyle changes can be made:

• Following a healthy diet and engaging in regular exercise
• Quitting smoking

Surgical Options

Surgery hasn’t been shown to be beneficial for RAS caused by atherosclerosis, but it is beneficial for patients with RAS caused by FMD. Types of surgery for RAS can include:

• Angioplasty and stenting: a catheter can be inserted into the renal artery, to strengthen the artery and help blood flow. In cases of RAS caused by atherosclerosis, a tiny balloon can also be inserted at the end of the catheter, to flatten the plaque against the artery wall, followed by a small mesh tube called a stent being positioned inside the artery to maintain the flattened plaque and keep the artery open
• Endarterectomy: a process where the plaque is cleaned out of the artery
• Bypass surgery: a vein or synthetic tube can connect the kidney to the aorta, as an alternative route to provide blood for the kidney

Monitoring and long-term management

Renal artery stenosis, whether caused by atherosclerosis or fibromuscular dysplasia, requires careful long-term management, to prevent further complications. This may involve frequent check-ups to monitor by checking blood pressure and responding by altering medications and lifestyle modifications.

Patients should be monitored every 3-6 months, depending on the disease severity, and treatment should follow a multidisciplinary approach, combining care from a variety of healthcare professionals, including nephrologists, radiologists and primary care physicians.

With careful management, regular follow-up appointments and communication between patients and healthcare teams, patients can maintain stable kidney health, resulting in an ability to have a good quality of life. 

Summary

Renal Artery Stenosis (RAS) occurs when one or both arteries that supply blood to the kidneys become narrowed, reducing blood flow and leading to high blood pressure and potential kidney damage. The two main causes are atherosclerosis, which results from plaque build-up and mainly affects older adults, and fibromuscular dysplasia (FMD), a condition that affects artery walls and is more common in younger women. Early stages of RAS often show no symptoms, but as it progresses, it can cause uncontrolled high blood pressure, swelling, and reduced kidney function.

Diagnosis usually involves imaging tests such as ultrasound, CT, or MRI scans. Treatment focuses on managing blood pressure and cholesterol, making lifestyle changes like exercising and quitting smoking, and in some cases, surgical procedures such as angioplasty or stenting to restore blood flow. With regular monitoring and proper medical care, many patients with RAS can maintain good kidney function and a better quality of life.