Renal artery stenosis, the most common cause of secondary hypertension, is predominantly caused by the atherosclerotic renovascular disease. Fibromuscular dysplasia (FMD) is a rare systemic vascular disease, affecting younger women and accounting for 10% to 20% of the cases of renal artery stenosis. FMD is an idiopathic, non-inflammatory, non-atherosclerotic disease commonly involving renal and carotid arteries; however, it can affect any arterial bed. FMD classically presents as renovascular hypertension but can also manifest as stroke in young adults. Early diagnosis and treatment are important for long-term prognosis.
Etiology of FMD is unclear despite extensive research. Environmental and genetic factors have been associated with FMD. Biggazi et al. reported bilateral FMD in identical twins, raising the possibility of inheritance. In a retrospective analysis of 104 patients with renal FMD, Pannier-Moreau et al. reported an 11% prevalence of familial cases where at least one sibling showed angiographic evidence of renal FMD. Bofinger et al. described an association between the polymorphisms of the renin-angiotensin gene. Plouin et al. described an association between HLA-DRw6 histocompatibility antigen and FMD.
Environmental influences include smoking, with a retrospective analysis showing more smokers among FMD patients. Mechanical factors like renal mobility have been suggested, given higher vulnerability of right kidney over left. Female predilection raised the role of estrogens as a causative factor; however, the number of pregnancies did not affect the risk for FMD per Sang Et al. Despite all the possible explanations, etiology of FMD remains elusive.
FMD may be associated with other disorders like Marfan syndrome, tuberous sclerosis, Alport's syndrome, medullary sponge kidney, Pheochromocytoma, Collagen 3 glomerulopathy, cystic medial necrosis, coarctation of the aorta, Alpha-1 antitrypsin deficiency, Ehlers-Danlos syndrome, neurofibromatosis type 1, and Williams syndrome.
Leadbetter and Burkland first described FMD of renal arteries in 1938. Palubinskas and Ripley reported FMD of craniocervical arteries in 1946. The exact prevalence of FMD remains unknown. The prevalence of renal FMD is 0.4 %, whereas the estimated prevalence of craniocervical FMD is much lower at 0.1%. Autopsy series from Mayo clinic reported 1.1% prevalence of renal FMD, while prevalence among kidney donors who underwent angiography varied between 2 % and 6.6%. Amongst older hypertensive patients who underwent renal angiography, a prevalence of 5.8 % was noted from the Cardiovascular Outcomes in Renal Atherosclerotic Lesions (CORAL) trial. Evidence from the above studies, interestingly, note higher prevalence, and many of cases may be asymptomatic and thereby often under-diagnosed.
FMD occurs predominantly in females below the age of 50 years. Female to male ratio reported in most studies is 3:1, however, data from the US and French registry supported 9:1 female preponderance. The median age for FMD is 48 years, though cases have been reported in pediatric as well as geriatric populations. There is no evidence to support racial or ethnic propensity, but Caucasians are more likely to be affected than blacks. Bilateral involvement is seen in 15 % of the patients.
McCormack and Colleagues first used the term "Fibromuscular hyperplasia" in the setting of hypertension and renal artery stenosis in 1958. It was in 1965 that Hunt et al. introduced the term as they observed dysplasia rather than hyperplasia alone.
FMD can affect any arteries. Mettinger and Ericson reported that renal arteries are involved in 58% of cases; craniocervical arteries, 32%; and other arterial beds, 10%. FMD is a segmental, non-atherosclerotic disease affecting the musculature of the small and medium-sized arterial walls and causing stenosis, aneurysms or dissections of the vascular territories. Macroaneurysms of the carotid and renal vasculature are more common in FMD, predisposing to rupture and leading to intracerebral hemorrhage and renal infarcts. Arteriovenous fistulas and thrombosis also can occur. Hypertension in renal FMD is hypothesized to be from secondary activation of the renin-angiotensin-aldosterone system, though some studies argue against this theory.
Harrison and McCormack proposed a histopathological classification of FMD based on the arterial layer affected, which was later revised by Stanley. FMD is mainly classified into three types: intimal, medial, and peri-adventitial fibroplasia. These three variants are not mutually exclusive and can co-exist.
This is the most frequent type, occurring in 85 % of the cases with alternating ridges of collagen and loss of elastic membrane but preserved internal elastic lamina. The classical “string of beads” appearance on angiography is due to alternating areas of stenosis and dilatations of the renal artery. In contrast to the atherosclerotic renal artery stenosis, FMD rarely affects the proximal or ostial section of renal artery.
Medial FMD is further divided into:
This is the second most common, accounting for 5% to 10 % of the cases with circumforaneous deposition of collagen in the intima, resulting in fragmentation of internal elastic lamina. Angiographic appearance is that of a concentric smooth stenosis or narrowing.
This is the least common, occurring in less than 1% of the cases with dense deposition of collagen in the adventitia resulting in smooth narrowing appearance on angiography.
Kincaid et al. proposed four angiographic variants of FMD:
The American Heart Association further simplified the classification into focal and multifocal.
Clinical manifestation of FMD principally depends on the vascular territory affected. Family history may be positive for stroke and hypertension at the young age.
Hypertension, specifically resistant hypertension, is the most common presentation of renal FMD. Other features of renal FMD include:
Diagnosis of renal artery FMD should prompt screening for associated carotid artery lesions to prevent catastrophic complications. Cranio-cervical FMD may be asymptomatic and found incidentally on imaging studies. Clinical manifestations include:
Lower Limb FMD
Vascular imaging is the primary modality to diagnose FMD.
Doppler Ultrasound remains the first line screening test for FMD. Duplex ultrasound revealing turbulence, tortuosity, elevated velocities, and resistive indices in the mid and distal artery are suggestive of FMD. However, duplex ultrasound is operator dependent, and resolution is impaired by obesity, renal mobility during respiration, and bowel gas.
Computed Tomography Angiography
Computed tomography angiography (CTA) is one of the best tools available to diagnose FMD. It is non-invasive, accurate, reliable, and can generate high-resolution spatial images. Classical CTA findings include the "string-of-beads" appearance of medial FMD. Renal aneurysms and infarcts can also be visualized.
Magnetic Resonance Angiography
Magnetic resonance angiography (MRA) has comparable sensitivity and specificity to that of CTA but without the risk of radiation exposure and contrast nephropathy; however, it is limited by it may not be used in patients with advanced renal failure. Willoteaux et al. recently showed that MRA had sensitivity and specificity of 97 % and 93 % respectively when compared to conventional angiography.
Conventional angiography remains the gold standard to diagnose FMD and can also measure pressure gradient across the stenotic lesions. A pressure gradient of more than 10 % is considered to be hemodynamically significant stenosis. Catheter-based angiography is not only superior when compared to other imaging modalities but also offers the opportunity for endovascular therapy at the same time. Novel diagnostics like intravascular ultrasound may further enhance the angiography and aid in evaluating restenosis following angioplasty.
Major therapeutic goals of FMD include controlling risk factors, controlling blood pressure, and preventing ischemic events.
Controlling hypertension is the cornerstone in the treatment of renal FMD. Angiotensin-converting enzyme inhibitor (ACE-I) or angiotensin receptor blocker (ARB) is the drug of choice, given activation of the renin-angiotensin-aldosterone system in renal FMD. Renal function should be monitored, as acute kidney injury may complicate the treatment, especially in bilateral renal artery stenosis or solitary kidney. Other anti-hypertensive agents like diuretics, calcium channel blockers, and beta blockers are added as needed to control the blood pressure.
Smoking cessation should be advocated as smokers tend to have more aggressive course than non-smokers. Role of statins is uncertain; Davies et al. demonstrated no benefit of statins on restenosis rates following angioplasty.
Anti-platelet therapy with aspirin is reasonable for all FMD patients with an ischemic insult like stroke or renal infarct. Anticoagulation is reserved for patients with renal or extracranial dissection.
Revascularization is indicated in young adults with resistant hypertension, to prevent ischemic nephropathy in severe stenosis with the goal of curing hypertension. Revascularization also may be indicated in an aneurysm or dissection of the involved artery. Revascularization of carotid artery FMD is reserved for patients with recurrent cerebral ischemia, pseudoaneurysm, and intolerance to medical therapy.
Percutaneous transluminal angioplasty (PTA) is the first-line modality for revascularization, with cure rates reaching 60% to 80% and technical success rate up to 100%. There is no indication for stent placement in FMD under most circumstances per the American Heart Association. Angioplasty with stent placement is considered for dissection or perforation of the involved artery. The hypertensive cure is achieved in 50 % of the cases. The meta-analysis by Trinquart and his colleagues revealed a 45.7 % cure rate for blood pressure; however, when adjusted for patients not on antihypertensives, it was only 35.8%. Negative prognostic factors include age greater than 50 years, longer duration of hypertension and co-morbid factors like diabetes, hyperlipidemia, smoking, and atherosclerosis. Hematoma, hemorrhage, dissection, and thrombo-embolic complications can complicate PTA in 6% of the patients.
There are no randomized control trials comparing PTA with surgical revascularization. PTA is the preferred choice over surgery because it is less invasive, lower cost, and has fewer complications. Surgical revascularization is recommended for patients with smaller renal arteries (less than 4mm), branch renal artery, severe intimal fibroplasia, associated aneurysms, and failure of the PTA. Surveillance with duplex ultrasonography is recommended post-revascularization at the first post-operative visit, bi-annually for two years, and then yearly.
Atherosclerotic Renal Artery Stenosis
Long term data regarding prognosis in FMD is lacking. The American Heart Association states that progression in FMD in unusual, and moreover, it is challenging to accurately assess progressive stenosis.