What is fibromuscular dysplasia?

Fibromuscular dysplasia (FMD) is a rare medical condition. Patients with FMD have abnormal cellular growth in the walls of their medium and large arteries. This can cause the arteries with the abnormal growth to look beaded. The arteries may also become narrow (stenosis).

Most cases of FMD affect the carotid and renal arteries. The carotid arteries are in the neck and connect the heart and the brain. The renal arteries are the blood vessels that carry blood from the aorta to the kidneys.

Fibromuscular dysplasia can also affect the arteries to the intestines (the mesenteric arteries), the arteries to the legs or arms, the coronary arteries (arteries that supply blood to the heart), and arteries in other parts of the body, although this is less common. Many times, arteries in more than one location are affected by FMD.

Who is affected by FMD?

Fibromuscular dysplasia is most common in women between the ages of 40 of and 60, but the condition can also occur in children and the elderly. The majority (more than 90%) of patients with FMD are women. However, men can also have FMD, and those who do have a higher risk of complications such as aneurysms (bulging) or dissections (tears) in the arteries.

Types of fibromuscular dysplasia

Medial Fibroplasia, Fibromuscular Dysplasia

Figure 1: Fibromuscular dysplasia of the right renal artery. The classic “beads on a string” appearance is typical of multifocal fibromuscular dysplasia, the most common type of FMD.

Intimal Fibroplasia, Fibromuscular Dysplasia

Figure 2: Fibromuscular dysplasia of the right renal artery. The smooth, concentric narrowing (arrow) has the typical appearance of focal FMD. In this case, there is severe narrowing of the artery, and the patient was treated with balloon angioplasty.

Artery walls have three layers:

  • Tunica Intima (the inside layer)
  • Tunica Media (the middle layer)
  • Adventitia (the outside layer)

In the past, FMD was classified according to the layer of the artery that was affected and by the lesions a patient had. There were five different types of FMD — medial fibroplasia, intimal fibroplasia, perimedial fibroplasia, medial hyperplasia, and periarterial hyperplasia. However, the only way to absolutely know which of these types of FMD a patient has is to look at the artery wall under a microscope, such as after a biopsy or surgical procedure, and this is rarely done. The majority of patients with FMD are diagnosed using imaging studies, such as angiography. As a result, the types of FMD were recently simplified to match the appearance of FMD on these studies. The condition is now classified as either multifocal or focal FMD.

Multifocal FMD

  • Affects approximately 90% of patients with FMD
  • Patients have multiple lesions; most often appears as a “string of beads,” which is caused by alternating areas of widening and narrowing along the artery
  • Includes medial fibroplasia and perimedial fibroplasia types of FMD
  • Medial fibroplasia is the most common type of FMD in this category

Focal FMD

  • Affects less than 10% of patients with FMD
  • Patients have distinct focal lesions or tubular narrowing (stenosis)
  • Includes intimal fibroplasia, periarterial fibroplasia, and medial hyperplasia types of FMD
  • Intimal fibroplasia is the most common type of FMD in this category

What are the symptoms of FMD?

Some people with FMD do not have any symptoms, but symptoms can occur if the stenosis restricts blood flow through the affected artery.

Symptoms of FMD in the carotid or vertebral arteries (that supply blood to the back of the brain) may include headaches (especially migraine type headaches), a pulsatile “swooshing” noise in the ears, neck pain and lightheadedness. If FMD affects the carotid arteries, the doctor will hear a swooshing noise in the neck. This is called a bruit and means there is abnormal blood flow to the area. More advanced cases of FMD can cause a transient ischemic attack (TIA) or stroke.

FMD of the carotid or vertebral arteries can lead to a tear in the artery. This is called a dissection. Symptoms of dissection include headache, sudden neck pain, drooping of one of the eyes or unequal pupils, and, in severe cases, symptoms of stroke or TIA.

FMD of the renal arteries frequently causes high blood pressure in these arteries (renovascular hypertension) and/or poor kidney function (renal insufficiency). FMD usually does not lead to kidney failure.

FMD of the mesenteric arteries (arteries to the intestines) may cause abdominal pain after eating and weight loss, but patients may not have any symptoms.

FMD of the extremities may cause pain in the affected area during exercise (claudication), or much less commonly, acute limb ischemia.

FMD of the coronary arteries has recently been associated with spontaneous coronary artery dissection (SCAD). Coronary artery dissection usually causes chest pain and heart attack.

What causes FMD?

Despite a great deal of research, it is still not clear what causes FMD. It is very likely that FMD has multiple underlying causes. Some of the factors that may play a role include:

  • Hormonal influences: The disease occurs most commonly in women.
  • Genetics: About 7-11% of cases are inherited. Some patients with FMD also have genetic abnormalities that affect the blood vessels.
  • Internal mechanical stress, including trauma to the artery walls or mechanical forces on the vessel.
  • Loss of oxygen supply to the blood vessel wall: This occurs when the tiny blood vessels in the artery walls that supply them with oxygen-rich blood get blocked by fibrous lesions.

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