Fibromuscular dysplasia (FMD) is a non-atherosclerotic, non-inflammatory vascular disease affecting the intima, media, or adventitia layers of medium-sized arteries. The renal arteries (60-75%) and internal carotid/vertebral arteries (25-30%) are most commonly affected. Medial fibroplasia is the most common subtype, causing the classic 'string of beads' appearance. It occurs in women 9 times more frequently than men, typically between ages 20-60. It can cause renovascular hypertension and cerebrovascular events (dissection, aneurysm). CT/MR angiography is the primary imaging modality for diagnosis; digital subtraction angiography (DSA) is the gold standard and allows simultaneous percutaneous transluminal angioplasty (PTA) treatment.
Age Range
20-60
Peak Age
40
Gender
Female predominant
Prevalence
Uncommon
The exact etiology of FMD is unknown; multiple factors including genetic predisposition (familial clustering 7-11%), hormonal factors (estrogen effect — female predominance), and mechanical stress are thought to play a role. In the medial fibroplasia type, smooth muscle cells in the arterial media layer are replaced by fibrous tissue with collagen accumulation and elastic fiber loss. This process leads to alternating segments of stenosis and dilation — forming the basis of the 'string of beads' pattern on imaging. In stenotic areas, medial fibrosis narrows the vessel lumen, while in dilated areas, medial weakening causes microaneurysm formation. In renal artery involvement, decreased perfusion distal to stenosis activates the juxtaglomerular apparatus → renin-angiotensin-aldosterone system (RAAS) activation → renovascular hypertension. On CT/MR angiography, the alternating stenosis-dilation areas become conspicuous due to contrast agent demonstrating different flow dynamics related to luminal diameter changes.
The characteristic 'string of beads' pattern formed by alternating stenosis and dilation areas in the mid-to-distal arterial segment is the pathognomonic finding of fibromuscular dysplasia (medial fibroplasia type). The diameter of dilation areas exceeds the normal arterial diameter, and these bead-like outpouchings represent microaneurysms resulting from medial weakening. This finding reliably distinguishes FMD from atherosclerosis and vasculitides.
The 'string of beads' pattern on CT angiography, formed by alternating areas of stenosis and dilation in the mid-to-distal segment of the renal or carotid artery. Dilated areas exceed the normal diameter of the artery. This pattern is specific to the medial fibroplasia type and is most commonly seen in the distal two-thirds of the renal artery. Best evaluated with multiplanar reformatting (MPR) and 3D volume rendering (VR) reconstruction. Luminal narrowing in stenotic segments ranges from 50-90%.
Report Sentence
A 'string of beads' pattern with alternating areas of stenosis and dilation in the mid-to-distal renal artery segment is observed, consistent with fibromuscular dysplasia (medial fibroplasia type).
String of beads pattern on contrast-enhanced MR angiography (CE-MRA) or time-of-flight (TOF) MRA in renal/carotid artery. CE-MRA with gadolinium-based contrast demonstrates FMD with accuracy comparable to CT angiography. TOF MRA provides flow-based imaging without contrast but may exaggerate stenosis degree due to turbulence-related signal loss. Phase-contrast MRA can measure renal artery flow velocities and evaluate hemodynamically significant stenosis.
Report Sentence
MR angiography demonstrates luminal irregularity and alternating stenosis-dilation areas in the mid-to-distal renal artery segment, consistent with fibromuscular dysplasia.
Elevated peak systolic velocity (PSV) >180-200 cm/s and renal-aortic ratio (RAR) >3.5 at the renal artery stenosis segment on Doppler ultrasound. Tardus-parvus waveform distal to stenosis (low resistance, slow systolic acceleration time >80 ms, low acceleration index). Turbulent flow pattern may be observed in dilation areas between segmental stenoses. Bilateral renal artery examination is mandatory. Elevated RAR without PSV elevation alone is significant as it normalizes for cardiac output variations.
Report Sentence
Doppler examination demonstrates elevated peak systolic velocity and tardus-parvus waveform distal to stenosis, consistent with hemodynamically significant renal artery stenosis.
Renal artery aneurysm or dissection developing in the setting of FMD. Aneurysms may be fusiform or saccular and are typically located at distal renal artery branching points. Dissection is characterized by intimal flap and double lumen appearance. CT angiography can evaluate aneurysm size, location, and false lumen-true lumen differentiation in dissection. Aneurysms >15 mm in diameter or those showing size increase require treatment.
Report Sentence
FMD-associated fusiform/saccular aneurysm is identified in the distal renal artery segment, requiring evaluation of size and morphology.
FMD findings in the internal carotid artery (ICA) or pre-cerebellar segment of the vertebral artery: string of beads pattern, focal stenosis, web-like lesion, or dissection. Carotid FMD typically involves the mid-to-distal ICA (C1-C2 vertebral level) segment — unlike atherosclerosis, the proximal bulb is not involved. Vertebral artery FMD is seen in V2-V3 segment (within and exiting the foramen transversarium). Intracranial aneurysm may coexist (in 7-13% of patients).
Report Sentence
Luminal irregularity and focal stenosis consistent with FMD are observed in the mid-to-distal internal carotid artery segment; cerebral angiography is recommended to evaluate for intracranial aneurysm.
Delayed and prolonged nephrogram on the side with hemodynamically significant renal artery stenosis. The affected kidney shows delayed enhancement compared to the contralateral side and retains contrast longer in the late phase. Affected kidney size may be smaller than the contralateral side (>1.5 cm difference is significant). This asymmetry reflects the perfusion difference due to stenosis and is an indirect indicator of hemodynamic significance. In acute dissection, wedge-shaped cortical perfusion defect (renal infarction) may be seen.
Report Sentence
Delayed and prolonged nephrogram in the left/right kidney compared to the contralateral side in the late phase, this asymmetry is consistent with hemodynamically significant renal artery stenosis.
Criteria
Most common type (70-80%). Smooth muscle hyperplasia and fibrous tissue accumulation in the media layer. Creates classic 'string of beads' pattern. Dilation areas exceed normal arterial diameter.
Distinct Features
String of beads appearance is pathognomonic. Usually in distal two-thirds of renal artery. Bilateral involvement is common. Responds well to PTA. Best prognosis.
Criteria
Rare type (5-10%). Concentric fibrous thickening in the intima layer. Usually creates long-segment focal stenosis — string of beads pattern not seen. Relatively more common in children and young males.
Distinct Features
Smooth contoured, concentric long-segment stenosis. Differs from atherosclerosis: no calcification, young age. No string of beads. May show progressive course.
Criteria
Rare type (5-15%). Dense collagen accumulation at the adventitia-media junction. Creates smooth or irregular focal stenosis. Higher risk of dissection and thrombosis.
Distinct Features
Arterial wall thickening may be visible on contrast CT/MR. Stenosis is usually short segment. Dissection complication is more frequent. Surgical revascularization may be needed.
Criteria
FMD involvement in more than one arterial bed. 28-66% of patients have more than one arterial bed affected. Most common combination is renal + carotid/vertebral.
Distinct Features
Head-to-pelvis complete body angiography screening is recommended. Intracranial aneurysm screening is mandatory. Each arterial bed should be independently evaluated and treatment plan adjusted accordingly.
Distinguishing Feature
Takayasu arteritis affects large arteries (aorta and primary branches) with concentric wall thickening; FMD affects medium-sized arteries without wall thickening and creates string of beads pattern. Takayasu is accompanied by systemic inflammation markers (elevated ESR/CRP); FMD has no inflammation signs.
Distinguishing Feature
Polyarteritis nodosa (PAN) creates multiple aneurysms and stenoses in small-medium arteries but does not show string of beads pattern; aneurysms are irregularly shaped and smaller. Multiple microaneurysms in renal, hepatic, and mesenteric arteries are typical in PAN. PAN is a systemic vasculitis accompanied by fever, weight loss, arthralgia; FMD is not inflammatory.
Distinguishing Feature
Atherosclerotic renal artery stenosis involves the proximal renal artery orifice and ostium; FMD involves the mid-to-distal segment. Atherosclerosis is accompanied by calcified plaque, wall irregularity, and aortic atheromatous changes; FMD has no calcification. Atherosclerosis is common in older, hyperlipidemic, diabetic males; FMD is typical in young-to-middle-aged women.
Distinguishing Feature
Giant cell arteritis affects temporal and cranial arteries while FMD does not involve these arteries. MRI shows arterial wall edema and enhancement in giant cell arteritis; FMD has no wall enhancement. Giant cell arteritis occurs in patients >50 years and may be accompanied by polymyalgia rheumatica; FMD starts at younger age.
Urgency
urgentManagement
interventionalBiopsy
Not NeededFollow-up
6-monthWhen FMD is diagnosed, screening of all arterial beds (head-to-pelvis CT/MR angiography) and intracranial aneurysm evaluation is recommended. For renal FMD with hemodynamically significant stenosis, percutaneous transluminal angioplasty (PTA) is the primary treatment (80-90% technical success, 50-70% hypertension improvement). Stenting is generally not needed. Aneurysms >15 mm or growing require embolization/surgery. Antiplatelet therapy and blood pressure control are applied in cerebrovascular FMD. Anticoagulation is given if dissection develops. Regular clinical and imaging follow-up is required.
Renal FMD is treated with angioplasty (stent usually not needed) — unlike atherosclerotic stenosis, response rates are excellent (80-100% cure). Carotid/vertebral FMD carries dissection risk. Screening of all vascular beds is recommended (renal + carotid + intracranial). US-guided FMD registries guide current treatment decisions.