Fibromuscular Dysplasia (FMD)

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Definition & General Features

• Idiopathic, non-atherosclerotic, non-inflammatory arteriopathy.

• Affects small- and medium-sized muscular arteries (elastic arteries rarely).

• Characterized by abnormal proliferation of fibrous and muscular tissue within vessel wall → stenosis, aneurysm, dissection, or thromboembolism.

• Classic angiographic hallmark: “string of beads” (due to alternating stenosis and aneurysmal dilatation).

• Key distinction: FMD differs from vasculitis (no inflammation) and atherosclerosis (younger patients, distal involvement, female predominance).

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Epidemiology

• Age: Peak 30–50 years; can occur at any age.

• Sex: ♀ : ♂ = 8 : 1 (in adults); equal in children.

• Ethnicity: More common in whites.

• Second most common cause of renal artery stenosis:

  • Atherosclerosis = most common (elderly men).

  • FMD = commonest in young women.

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Pathophysiology & Histology

• Histological subtypes (based on arterial wall layer):

  1. Medial fibroplasia (70–80% cases): string of beads.

  2. Intimal fibroplasia.

  3. Adventitial/perimedial fibroplasia (rare).

• Mechanisms of ischemia/complications:

  • Fixed stenosis → ↓ perfusion.

  • Aneurysm formation → rupture.

  • Arterial dissection → acute occlusion.

  • Intravascular thrombus → embolization.

• Renal artery FMD → ↓ perfusion → RAAS activation → secondary hypertension.

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Localization & Frequency

• Renal arteries (75–80%) → usually bilateral.

• Carotid & vertebral arteries (65–75%) → often bilateral; risk of cerebrovascular events.

• Less common: mesenteric, iliac, brachial arteries.

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Clinical Manifestations

Renal FMD

• Secondary hypertension (often resistant).

• Abdominal / flank bruit.

• Chronic kidney disease (ischemic nephropathy).

• Flank or abdominal pain.

Cerebrovascular FMD

• Headache, neck pain, pulsatile tinnitus.

• TIA, amaurosis fugax, ischemic stroke.

• Horner’s syndrome (if sympathetic fibers involved).

• Cervical bruit.

Other (rare)

• Mesenteric ischemia → postprandial pain, weight loss.

• Peripheral artery disease (limb claudication, bruits).

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Diagnosis

Imaging

• Initial (renal): Duplex ultrasonography, CT angiography, MR angiography.

  
  
  

• Initial (carotid/vertebral): CTA or MRA.

• Gold standard: Digital Subtraction Angiography (DSA).

  • String of beads = alternating stenosis/dilatation.

  • Less common: smooth/tubular stenosis.

Laboratory

• Serum creatinine → renal function.

• Rule out vasculitis (inflammatory markers usually normal).

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Differential Diagnosis

• Atherosclerosis

  • Older, male, risk factors (smoking, DM, dyslipidemia).

  • Lesions at proximal/ostial artery segments.

  • FMD → distal/mid-artery involvement.

• Vasculitis (e.g., GCA, PAN) → systemic inflammation, ↑ ESR/CRP.

• Takayasu arteritis → younger women, inflammatory signs, arch vessel involvement.

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Treatment

General Principles

• Asymptomatic patients → observation + risk factor modification.

• Symptomatic → medical therapy ± intervention.

Medical Management

• Renal FMD:

  • ACE inhibitors / ARBs = first line (counteract RAAS activation).

• Cerebrovascular FMD:

  • Antiplatelet therapy (low-dose aspirin) for stroke prevention.

Definitive Therapy

• Percutaneous transluminal balloon angioplasty (PTA)

  • Preferred for renal FMD.

  • Usually without stenting (stents only if dissection or elastic recoil).

• Surgical revascularization → rarely, if angioplasty fails.

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Complications

• Hypertension (often resistant).

• Chronic kidney disease.

• Cerebral ischemic events (TIA, stroke).

• Aneurysm rupture.

• Arterial dissection.

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High-Yield Pearls (Exam Focus)

• Young woman + resistant hypertension + abdominal bruit → think FMD.

• String of beads on angiography = diagnostic.

• ACEi/ARB for renal disease, aspirin for cerebrovascular disease.

• Balloon angioplasty (without stent) = definitive treatment.

• Always differentiate from atherosclerosis (older, proximal disease) and vasculitis (inflammatory).