Fibromuscular dysplasia

Last revised by Derek Smith on 13 Jun 2024

Fibromuscular dysplasia (FMD) is a heterogeneous group of vascular lesions characterized by idiopathic, non-inflammatory, and non-atherosclerotic angiopathy of small and medium-sized arteries.

The prevalence is unknown 7. It is most common in young women with a female to male ratio of 3:1, and is typically diagnosed between the ages of 30 and 50 4.

Fibromuscular dysplasia is frequently asymptomatic. Symptomatic patients commonly present with:

The exact cause is not well known. The underlying pathology is a fibrous or fibromuscular thickening of the arterial wall. Any layer of the vessel wall may be affected: intima, media or adventitia. Inflammatory cells are absent 1-4,7.

Fibromuscular dysplasia may affect any medium-sized artery in the body and is commonly multifocal and bilateral (up to 60% when involving the renal arteries). Fibromuscular dysplasia usually involves the mid-segment of vessels and spares origins. Some sites are more frequently involved 9:

Fibromuscular dysplasia is classified into five categories according to the vessel wall layer affected:

  • intima: 5% 

  • media: 90-95% 

    • medial dysplasia (70%, commonest type)

    • perimedial (subadventitial) fibroplasia (15-20%)

    • medial hyperplasia (8-10%)

  • adventitia: rare 

    • adventitial fibroplasia (1%) 8

The outcome is arterial stenoses. Fibromuscular dysplasia most commonly causes small stenoses along a vessel with intervening areas of dilatation (small aneurysms), creating a “string of beads” appearance. Less commonly the stenosis has a smooth tapered appearance. Fibromuscular dysplasia also weakens the vessel wall which predisposes to dissection.

Arterial imaging with CT angiography, MR angiography and digital subtraction angiography (DSA) may be used to visualize the lesions in fibromuscular dysplasia. 

Selective DSA is the gold standard because it allows the visualization of small or peripheral lesions. The characteristic finding, particularly in the more common medial subtype, is alternating stenoses and dilatations, causing a string of beads appearance 5

Less commonly in intimal and adventitial types, there is focal concentric, long-segment tubular stenosis or diverticular outpouching present (see carotid intimal fibromuscular dysplasia). Cross-sectional imaging (CT and MRI) allows the assessment of end-organ ischemic damage.

  • typical angiographic features include: vascular loops, fusiform vascular ectasia and a string of beads

  • less typical features include: arterial dissection, aneurysm and subarachnoid hemorrhage

Asymptomatic cases are only observed but if symptomatic then fibromuscular dysplasia responds well to angioplasty, with high long-term patency rates. A stent is generally not required.

The first case of fibromuscular dysplasia was initially described by Leadbetter and Burkland in 1938, while the first case of histologically proven carotid fibromuscular dysplasia was published by M C Connett and J M Lansche in 1965 7.

Imaging differential considerations include:

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