What is fibromuscular dysplasia?

Fibromuscular Dysplasia (FMD)

Fibromuscular dysplasia is a nonatherosclerotic, noninflammatory vascular disease that primarily affects medium-sized arteries, most commonly the renal and carotid arteries, characterized by abnormal cell development in the arterial wall leading to stenosis, aneurysms, or dissections.

Definition and Pathophysiology

Fibromuscular dysplasia (FMD) is characterized by:

• A nonatherosclerotic, segmental, noninflammatory angiopathy 1
• Abnormal development of cells in the arterial wall causing stenosis, dissection, or aneurysmal dilation
• Most commonly affects the renal arteries and extracranial segment of the internal carotid artery (ICA) 1
• Histologically classified into several subtypes, with medial fibroplasia being the most common (80% of cases) 1, 2

Epidemiology

• Most commonly affects women aged 25-50 years, though it can occur in both genders at any age 1, 2
• Estimated prevalence of symptomatic renal FMD is approximately 4/1000 3
• Prevalence of cervicocranial FMD is roughly half that of renal FMD 3
• Approximately 10% of cases appear to be familial 3
• May affect as many as 4% of adult women 4

Clinical Presentation

FMD can present with various symptoms depending on the affected vascular bed:

Renal Artery Involvement (Most Common)

• Renovascular hypertension 1, 2
• Typically affects the middle and distal two-thirds of the main renal artery 1
• May involve renal artery branches 1

Cerebrovascular Involvement

• Stroke or transient ischemic attack (TIA) 1
• Headache, particularly migraine-type 2
• Pulsatile tinnitus ("swishing" sound in ears) 4
• Horner syndrome 3
• Cranial nerve palsies 1
• Subarachnoid hemorrhage (due to associated aneurysms) 1

Other Manifestations

• Cervical bruits 2
• Arterial dissection 1, 5
• Arterial aneurysms 5
• Arterial tortuosity 5
• Limb ischemia (when peripheral arteries are involved) 6, 7

Diagnostic Approach

Initial Evaluation

• Comprehensive clinical assessment focusing on hypertension, bruits, and neurological symptoms 2
• Basic metabolic panel and screening for primary aldosteronism in hypertensive patients 2

Imaging Modalities (in order of preference)

1. Duplex Ultrasonography (Class I recommendation, Level of Evidence B)

   • First-line screening test 2
   • Non-invasive, no radiation exposure
   • Limited sensitivity (28%) compared to angiography 2

2. Computed Tomographic Angiography (CTA)

   • Recommended for patients with normal renal function 2
   • Highest sensitivity (84.2%) among non-invasive modalities 2
   • Superior to MRA for visualizing vascular lesions in FMD 2

3. Magnetic Resonance Angiography (MRA)

   • Alternative screening test (Class I recommendation, Level of Evidence B) 2
   • Better sensitivity (62.5%) than ultrasound 2
   • Preferred in patients with renal impairment 2

4. Catheter Angiography

   • Gold standard for diagnosis 2, 3
   • Reserved for inconclusive cases or when intervention is planned 2
   • Most accurate for detecting the classic "string of beads" appearance 3

Angiographic Classification

1. Multifocal FMD (>80% of cases)

   • Classic "string of beads" appearance 2, 3
   • Related to medial fibroplasia 3
   • Alternating areas of stenosis and dilation 1

2. Unifocal FMD

   • Single focal or tubular stenosis 2
   • Less common than multifocal type

Management

Medical Management

• For all FMD patients:

  • Antiplatelet therapy is recommended to prevent thromboembolism 1
  • Blood pressure control in hypertensive patients 1
  • Lifestyle modification 1

• For patients with FMD and history of ischemic stroke/TIA:

  • Antiplatelet therapy is strongly recommended (Class 1, Level of Evidence C-LD) 1

Revascularization

• For symptomatic carotid FMD:

  • Carotid angioplasty with or without stenting is reasonable for patients with retinal or hemispheric cerebral ischemic symptoms (Class IIa, Level of Evidence C) 1
  • Consider revascularization in patients with recurrent ischemic stroke despite optimal medical management (Class IIb, Level of Evidence C-LD) 1

• For renal artery FMD with hypertension:

  • Percutaneous angioplasty for severe stenoses 3
  • Reconstructive surgery for complex FMD extending to segmental arteries 3

• For asymptomatic FMD:

  • Revascularization is not recommended regardless of stenosis severity (Class III: No Benefit, Level of Evidence C) 1

Surveillance

• Annual noninvasive imaging is reasonable initially to detect changes in disease extent or severity 1, 2
• Studies may be repeated less frequently once stability is confirmed 1
• More frequent imaging may be needed in the first few years after diagnosis 2

Complications and Prognosis

• Arterial dissection (12% of patients in the US FMD registry) 1
• Aneurysm formation 3, 5
• Stroke (9.8% in the US FMD registry) 1
• Progression of renal artery FMD is typically slow 3
• Rarely leads to ischemic renal failure 3

Differential Diagnosis

• Atherosclerotic stenosis 3
• Vascular Ehlers-Danlos syndrome 3
• Williams syndrome 3
• Type 1 neurofibromatosis 3
• Takayasu's arteritis 1
• Segmental arterial mediolysis 5

FMD should be considered in the differential diagnosis of a young person (especially women) with a cervical bruit, pulsatile tinnitus, TIA/stroke, arterial dissection, or early-onset hypertension (≤35 years) 4.