What is renovascular fibromuscular dysplasia?

Fibromuscular dysplasia (FMD) is a non-inflammatory vascular disease that causes non-atherosclerotic stenosis of small and medium-sized arteries, mainly the carotid and renal arteries. This means that there is thinning of the blood vessels, causing high blood pressure through these vessels, the thinning being ‘non-atherosclerotic’ means that it is not caused by a buildup of fatty plaques in arteries like most common cardiovascular disease, instead there is weakening of the muscles in blood vessels causing narrowing (stenosis), and raising blood pressure.¹

Renovascular means that the FMD affects the arteries of the kidneys (renal system), leading to high blood pressure and kidney function progressively worsening.² 

People are generally diagnosed with FMD in their 50s, with women being more likely to have it than men, at a 9:1 ratio.³ 

Types of fibromuscular dysplasia

FMD can be categorised based on the arterial layer that the disease affects. These types are not mutually exclusive and can occur in unison. These types are:

Medial FMD

There are alternating ridges of collagen and loss of the elastic membrane, which gives a significant ‘string of beads' appearance on angiograms, caused by alternating areas of stenosis (narrowing) and dilation of the renal artery. This occurs in the middle layer of the blood vessels.

Intimal FMD

Deposition of collagen occurs in the inner layer of the blood vessels, angiograms give a view of concentric areas of stenosis.

Adventitial FMD

Dense deposits of collagen in the outermost layer of arteries.

Medial is the most common, accounting for roughly 85-90% of cases, while intimal is second with 5-10% of cases and adventitial is the least common, accounting for only around 1% of FMD cases.⁴ 

Symptoms of renovascular fibromuscular dysplasia

Renovascular FMD symptoms would be mainly related to kidney function; the most common are:

• Renal artery dissection (tear in artery wall)
• Side pain
• Blood in urine
• Low potassium levels
• Abdominal bruits (audible blood flow, around the kidneys)

There may also be other symptoms related to high blood pressure, such as headaches, dizziness and potential strokes.⁴ 

Causes of renovascular FMD

There is no known cause of FMD, although there is thought to be some genetic element contributing to those who are more at risk. With women being more likely to suffer from FMD, the presence of female hormones may play a key role in the FMD disease occurring.¹ 

Diagnosis

In order to diagnose Renovascular Fibromuscular Dysplasia, a number of imaging techniques may be used to view potentially affected blood vessels in those presenting with symptoms. The techniques that may be used are:

Duplex ultrasonography 

This is the first line screening test for fibromuscular dysplasia, and can show suggestive markers for FMD. This can be limited as a diagnostic tool due to the resolution of the image being altered by several factors, such as obesity or the operator’s capability.

Computed tomography angiography (CTA) 

One of the best tools for diagnosis, creating a clear, high-resolution image, whilst being non-invasive. Renal aneurysms can be detected with this imaging technique, which is a CT scan.

Magnetic resonance angiography 

This is an MRI scan, which has advantages over CTA. It does not require radiation or contrast material to create an image. It is sensitive but cannot be used to create an accurate image to diagnose FMD in those with advanced renal failure.

Arteriography

A catheter is used to create a picture of the arteries, particularly the renal artery. This can also detect the pressure across the artery and in the areas of narrowing (stenosis).

Arteriography is considered the best possible tool for diagnosing FMD, but all these imaging techniques may be of use in diagnosis and understanding the progression of the disease.⁴

Treatment

Treatment for Renovascular fibromuscular dysplasia is dependent on the level of stenosis that has occurred. 

Revascularization

An effective form of treatment for those who suffer significant stenosis is revascularisation, where areas of damaged tissue are given better blood flow with more blood vessels, similar to a heart bypass. This is the most common form of treatment in younger individuals and has a high percentage of success, and seems to be effective at curing renal FMD.

Percutaneous transluminal angioplasty 

A catheter is inserted into the artery, through which a balloon can then be inflated to widen the areas of stenosis. A stent can then be placed in these weakened passages of the blood vessels to keep them from narrowing.

These are the most effective treatments that can be used for those who are suffering from renovascular fibromuscular dysplasia. These treatments focus on restoring shape and structure to the arteries of the kidneys, so that normal blood flow can be reestablished and reduce high blood pressure or any further damage.⁵ 

Common medications used to treat hypertension (high blood pressure), such as ACE inhibitors or beta blockers, may be used in treatment to reduce blood pressure and therefore the chance of more damage to the kidneys and the blood vessels around them. Although some of these medications can have a negative impact on kidney function so continued monitoring and changes to treatment are required.¹ 

Renovascular fibromuscular dysplasia moving forward

Research moving forward will focus on fully understanding the mechanism of the disease, its causes, and how renal vasculature can remain intact while hypertension occurs.

Renal FMD is underdiagnosed; thus, improvements in screening methods are required to assist in the identification and treatment of renal FMD, which can be managed effectively.⁶ 

Summary

Renovascular Fibromuscular Dysplasia (Renal FMD) is a non-inflammatory vascular disease characterised by stenosis (narrowing) of the renal artery. This leads to increased blood pressure and less effective function of the kidneys. The narrowing of the blood vessels is caused by weakening of the muscles in the blood vessels, causing them to lose structure. Women are the most common sufferers of FMD, although it is not known why exactly this is. There is a theory that it may be related to female hormones.FMD can be divided into different types based upon the layer of the blood vessel that is affected: inner, middle or outer. The most common is the middle (medial) layer being affected.

The main symptoms caused are:

• Blood in urine.
• Side pain (kidney area).
• Low potassium levels.
• Tear in artery walls.
• Bruits (audible blood flow caused by high pressure).

Other symptoms would be typical of high blood pressure, such as dizziness and headaches.

The main diagnostic tools for renal FMD diagnosis are imaging techniques, which are:

• Duplex Ultrasonography
• Computed Tomography angiography
• Magnetic Resonance Angiography
• Arteriography

These imaging techniques can give a full picture of the structure of the renal artery and the level of blood flow through the vessel; with this information, a definitive diagnosis of renal FMD can be given. 

When treating FMD, the most successful method, particularly in younger patients, is revascularisation, where more vessels are added to increase blood flow and reduce blood pressure, similar to a heart bypass. This is an effective cure for renal FMD.

Another effective treatment is Percutaneous Transluminal Angioplasty, which involves the insertion of a catheter and balloon that is inflated in the narrowed region of the artery. A stent can then be inserted to keep the artery open and reduce blood pressure.

Blood pressure medication may also be used to treat those with FMD, but these can often have a detrimental effect on kidney function, so this must be monitored closely.