# Fibromuscular Dysplasia: Multibed Vascular Involvement

## Introduction

Fibromuscular dysplasia (FMD) is a non-atherosclerotic and non-inflammatory arterial disease characterized by stenosis, aneurysm formation, dissection, and occlusion of small- and medium-sized arteries. It predominantly affects women of childbearing age and often involves multiple vascular beds simultaneously. Because of this frequent multivessel involvement, a comprehensive vascular survey is essential at the time of diagnosis to assess the full extent of disease.

## Epidemiology and Risk Factors

The prevalence of FMD is estimated to be between 3% and 6% in the general population based on angiographic studies. There is a strong female predominance, with a female-to-male ratio of approximately 9:1. The mean age at diagnosis ranges from 40 to 60 years, although pediatric cases have also been reported. There appears to be a genetic predisposition, as first-degree relatives of affected individuals have a higher incidence of FMD. Among modifiable risk factors, smoking is the only consistently identified contributor. Despite historical concerns, there is no clear association between FMD and oral contraceptive use.

## Pathological Classification

Medial fibroplasia is the most common pathological subtype of FMD, accounting for about 80% of cases. It is characterized by alternating areas of thinned arterial media and thickened fibromuscular ridges, producing the classic "string of beads" appearance on angiography. These beads are larger than the normal arterial diameter and represent areas of aneurysmal dilatation alternating with stenosis.

Intimal fibroplasia, which accounts for approximately 5% of cases, involves circumferential or eccentric collagen deposition within the intima. This subtype typically appears as a focal, concentric stenosis on imaging and is more commonly seen in children.

Perimedial or adventitial fibroplasia comprises about 10-15% of cases and is characterized by collagen deposition in the outer media extending into the adventitia. This subtype produces severe stenosis but lacks the classic bead pattern seen in medial fibroplasia.

| Subtype | Frequency | Layer Affected | Angiographic Appearance | Demographics |
|---------|-----------|---------------|------------------------|--------------|
| Medial fibroplasia | ~80% | Media | "String of beads" (beads larger than normal diameter) | Most common; typical adult FMD |
| Intimal fibroplasia | ~5% | Intima | Focal, concentric stenosis | More common in children |
| Perimedial/adventitial fibroplasia | 10–15% | Outer media/adventitia | Severe stenosis without classic beading | Adults; severe stenosis |

![Angiographic appearance of the "string of beads" sign in renal artery FMD](images/fmd-string-of-beads.jpg)

## Vascular Beds Involved

The renal arteries are the most commonly affected vascular territory in FMD, involved in 60-75% of cases. Patients often present with renovascular hypertension that is frequently refractory to medical therapy. Unlike atherosclerotic disease, FMD typically affects the mid-to-distal segments of the renal artery and its branches.

The extracranial carotid and vertebral arteries are the second most commonly involved vessels, affected in 25-30% of patients. Involvement of these arteries carries a risk of spontaneous dissection, transient ischemic attack, and stroke. The disease usually targets the mid-to-distal internal carotid artery in this region.

Other arterial beds can also be involved, although often subclinically. Iliac and mesenteric arteries may be affected without overt symptoms. Upper extremity arteries such as the brachial and radial arteries have been described as sites of involvement. Coronary artery involvement is rare but increasingly recognized as a cause of spontaneous coronary artery dissection (SCAD). Data from the US FMD Registry indicate that multivessel involvement occurs in more than 60% of patients.

![Cross-sectional histology of medial fibroplasia showing alternating thick and thin segments](images/fmd-histology.jpg)

## Diagnostic Evaluation

Duplex ultrasound serves as the first-line screening modality, allowing assessment of mid-to-distal renal artery velocities and evaluation of the carotid arteries. Computed tomography angiography (CTA) provides excellent visualization of the characteristic beading, aneurysms, and dissections associated with FMD. Magnetic resonance angiography (MRA) is useful for avoiding radiation exposure but has limited spatial resolution for distal vessels. Catheter-based angiography remains the gold standard for diagnosis and offers the opportunity for simultaneous intervention. Given the systemic nature of FMD, a full vascular survey at diagnosis is recommended, including imaging of the renal, carotid, vertebral, iliac, and mesenteric arteries.

## Management Strategies

Medical therapy for FMD includes antiplatelet treatment, with aspirin 81 mg daily recommended for all patients to reduce thrombotic risk. Antihypertensive medications are used to control blood pressure pending any interventional procedures. In cases of acute arterial dissection, anticoagulation is often employed, although the optimal duration is debated; typically, treatment lasts between three and six months.

Percutaneous transluminal angioplasty (PTA) is the first-line intervention for symptomatic renal artery FMD. This procedure results in cure or improvement of hypertension in 60-80% of cases. Stenting is generally not required and is reserved for complications such as flow-limiting dissection following angioplasty. PTA demonstrates excellent long-term durability compared to interventions for atherosclerotic disease.

Surgical revascularization is reserved for complex anatomical situations, including branch vessel disease, aneurysm formation, or failed PTA. Surgical options include aortorenal bypass, ex-vivo reconstruction, and autotransplantation. For carotid FMD, surgical repair may be considered in cases of aneurysm or recurrent dissection.

![CT angiogram demonstrating bilateral renal artery FMD with multifocal stenoses](images/fmd-cta-renal.jpg)

## Surveillance and Long-Term Follow-Up

Long-term follow-up involves annual duplex ultrasound of affected and at-risk vascular beds to monitor disease progression. Cross-sectional imaging with CTA or MRA is recommended every three to five years or sooner if new symptoms arise. Regular monitoring of blood pressure and renal function is essential. Screening of first-degree relatives with non-invasive imaging should be considered if they are symptomatic.

## Key Clinical Pearls

Fibromuscular dysplasia is a systemic arteriopathy, so a comprehensive head-to-pelvis vascular survey should always be performed at diagnosis. The "string of beads" sign on angiography is pathognomonic for medial fibroplasia, the most common subtype. Unlike atherosclerotic renal artery stenosis, FMD responds well to balloon angioplasty alone without the need for stenting. Spontaneous arterial dissection in a young woman should prompt evaluation for underlying FMD. Optimal management requires multidisciplinary collaboration among nephrology, neurology, and cardiology specialists.

## References

1. Olin JW, Gornik HL, Bacharach JM, et al. Fibromuscular dysplasia: state of the science and critical unanswered questions. *Circulation*. 2014;129(9):1048-1078.  
2. Gornik HL, Persu A, Adlam D, et al. First International Consensus on the diagnosis and management of fibromuscular dysplasia. *Vasc Med*. 2019;24(2):164-189.  
3. Plouin PF, Baguet JP, Thony F, et al. High prevalence of multiple arterial bed lesions in patients with fibromuscular dysplasia. *J Hypertens*. 2017;35(6):1325-1331.  
4. Weinberg I, Defined A, Engel A. FMD Registry data on multibed involvement and clinical outcomes. *J Vasc Surg*. 2020;72(3):1045-1053.