48-year-old woman is evaluated for newly diagnosed hypertension in the absence of atherosclerotic risk factors. A systolic abdominal bruit is discovered on clinical examination. Neither carotid nor femoral bruits are auscultated, and her remaining examination is otherwise unremarkable.

Laboratory investigation for secondary causes of hypertension is unrevealing. However, a duplex arterial ultrasound of her mid renal arteries illustrates 60% to 99% bilateral renal artery stenosis in the absence of plaque. A peculiar beaded appearance exists along the arterial walls. A diagnosis of fibromuscular dysplasia (FMD) is subsequently made.

FMD is a noninflammatory, nonatherosclerotic arterial disease that most commonly affects the renal and internal carotid arteries (ICAs) but has been observed in almost every artery in the body. It is characterized by smooth muscle cell and fibrous tissue overgrowth in one or more layers of the renal arterial wall.

The renal arteries compromise 65% to 75% of cases of FMD, occurring bilaterally in 35% of cases. The carotid and vertebral arteries are involved in 25% to 30% of cases. Multivessel involvement is common.

FMD occurs most frequently in women aged between 20 and

60 years, but may also be seen in men, older individuals, and in the pediatric population.^1,2 It is estimated to affect anywhere between 5.8 and 8.6 million women in the United States.

Hypertension, headache, and pulsatile tinnitus are the most common presenting manifestations. Other signs and symptoms include dizziness, cervical and abdominal bruits, neck pain, flank or abdominal pain, nonpulsatile tinnitus, transient ischemic attack (TIA), and stroke. The clinical presentation is dictated by the affected arteries.

FMD can be asymptomatic and incidentally identified on an imaging study performed for a different clinical indication. In addition to arterial stenosis, patients with FMD can present with aneurysms and dissections. A history of FMD in first- or second-degree relatives is rare and occurs in around 7% of patients.

FMD is classified into three categories related to the pathologic layer of the arterial wall that is affected—intima, media, and adventitia (perimedial).

Medial FMD

It is by far the most common type and is further subdivided into medial fibroplasia, perimedial fibroplasia, and medial hyperplasia.

Medial fibroplasia accounts for 80% to 90% of all types of FMD. This subtype is defined histologically by alternating areas of thinned media and thickened collagen-containing medial ridges. Multiple stenotic webs cause arterial stenosis and poststenotic dilation, often displaying the typical “string of beads” appearance on angiography (Figures 44-1 and 44-2). The bead component is often larger and weaker than the normal arterial lumen, and in a subset of patients with FMD, aneurysms are present that may require treatment.

Perimedial fibroplasia (less numerous and smaller beads than medial fibroplasia) is quite uncommon and usually occurs in young females aged between 5 and 15 years who present with hypertension and renal impairment.

Medial hyperplasia is extremely rare and requires a pathologic specimen for diagnosis.