Fibromuscular dysplasia

Abstract

Fibromuscular dysplasia (FMD), formerly called fibromuscular fibroplasia, is a group of nonatherosclerotic, noninflammatory arterial diseases that most commonly involve the renal and carotid arteries. The prevalence of symptomatic renal artery FMD is about 4/1000 and the prevalence of cervicocranial FMD is probably half that. Histological classification discriminates three main subtypes, intimal, medial and perimedial, which may be associated in a single patient. Angiographic classification includes the multifocal type, with multiple stenoses and the 'string-of-beads' appearance that is related to medial FMD, and tubular and focal types, which are not clearly related to specific histological lesions. Renovascular hypertension is the most common manifestation of renal artery FMD. Multifocal stenoses with the 'string-of-beads' appearance are observed at angiography in more than 80% of cases, mostly in women aged between 30 and 50 years; they generally involve the middle and distal two-thirds of the main renal artery and in some case also renal artery branches. Cervicocranial FMD can be complicated by dissection with headache, Horner's syndrome or stroke, or can be associated with intracerebral aneurysms with a risk of subarachnoid or intracerebral hemorrhage. The etiology of FMD is unknown, although various hormonal and mechanical factors have been suggested. Subclinical lesions are found at arterial sites distant from the stenotic arteries, and this suggests that FMD is a systemic arterial disease. It appears to be familial in 10% of cases. Noninvasive diagnostic tests include, in increasing order of accuracy, ultrasonography, magnetic resonance angiography and computed tomography angiography. The gold standard for diagnosing FMD is catheter angiography, but this invasive procedure is only used for patients in whom it is clinically pertinent to proceed with revascularization during the same procedure. Differential diagnosis include atherosclerotic stenoses and stenoses associated with vascular Ehlers-Danlos and Williams' syndromes, and type 1 neurofibromatosis. Management of cases with renovascular hypertension includes antihypertensive therapy, percutaneous angioplasty of severe stenoses, and reconstructive surgery in cases with complex FMD that extends to segmental arteries. The therapeutic options for securing ruptured intracerebral aneurysms are microvascular neurosurgical clipping and endovascular coiling. Stenosis progression in renal artery FMD is slow and rarely leads to ischemic renal failure.

Figure 1

The "string-of-beads" feature in medial fibromuscular dysplasia. The sign is caused by areas of relative stenoses alternating with small aneurysms. The diameters of the aneurysms exceed the normal diameter of the artery. The sign is characteristic of medial FMD. A similar bead appearance may be seen in perimedial FMD, but the diameters of the beads do not exceed the normal diameter of the artery [10]. Note the involvement of branch renal arteries.

Figure 2

Unifocal (left) and tubular (right) stenoses. Unifocal stenoses are typically seen in intimal FMD. Tubular stenoses are not specifically associated with any one of the three dominant FMD types (intimal, medial or perimedial)

Figure 3

Percentage of women by type of fibromuscular dyplasia. The blue columns indicate the percentages of patients who are women according to a binary pathological classification (adapted from [4]), and the green columns the percentages of patients who are women according to a binary angiographical classification (adapted from [12]). Note that women make up the majority of cases of the most prevalent type, medial FMD, which is associated with the "string-of-beads" feature on angiography.

Figure 4

Computed tomography angiography in a patient with medial dibromuscular dysplasia.