Fibromuscular Dysplasia

Description

Fibromuscular dysplasia (FMD) is an idiopathic, non-inflammatory, non-atheroscloertic focal irregular thickening in medium and large muscular arteries, classically causing stenosis of renal and carotid vessels. It accounts for 10 – 20% of cases of renal artery stenosis.

Epidemiology

FMD is a rare systemic vascular disease affecting younger women (age 15 – 50 years) and accounting for 10 – 20% of renal artery stenosis.

Clinical Features

Renal

Resistant hypertension is the most common presentation. Other renal manifestations include:

• Renal artery dissection
• Flank pain
• Haematuria
• Renal infarct
• Hypokalaemia from secondary hyperaldosteronism, but ischaemic nephropathy with renal failure is uncommon
• Abdominal bruit

Cranio-cervical

Often asymptomatic and detected incidentally on imaging. Diagnosis of renal artery FMD should prompt screening for associated carotid artery lesions to prevent catastrophic complications such as stroke. Cranio-cervical manifestations include:

• Headache
• Pulsatile tinnitus
• Dizziness
• Cervical artery dissection
• Neck pain
• Stroke
• Horner syndrome – ipsilateral enophthalmos, blepharoptosis, pupillary miosis and facial anhydrosis
• Subarachnoid hemorrhage
• Carotid bruit

Other

Mesenteric

• Weight loss
• Fatigue
• Postprandial abdominal pain
• Epigastric bruit

Subclavian

• Upper arm weakness
• Paresthesias
• Claudication
• Steal syndrome

Lower Limb

• Claudication
• Cold legs
• Ischaemic feet

Coronary

• Chest pain
• Shortness of breath
• Acute coronary syndrome

Complications

• Spontaenous dissections
• Renal artery aneurysms
• Intracranial aneurysms; subarachnoid haemorrhage
• Arteriovenous fistula
• Distal emoblisation of thrombus formed in aneurysm

Pathological Features

FMD is an idiopathic, non-inflammatory, nonatherosclerotic vasculopathy of medium to large arteries, characterised by segmental areas of collagen deposition and smooth muscle overgrowth. This may cause stenosis, dissection, aneurysms.

• Renal artery: Most commonly involved
• Carotid artery: 2nd most commonly involved

Can also affect iliac arteries, and vertebral arteries and less commonly the ceoliac trunk, mesenteric arteries, and subclavian arteries

Classified based on involved layer of arterial wall:

• Intimal fibroplasia (< 10%) – Focal concentric or long-segment tubular stenosis. Internal elastic lamina may be disrupted
• Medial fibroplasia (3 subtypes)
  • Medial dysplasia (80%): Multiple stenoses and aneurysms involving mid to distal renal artery causing classic string of beads appearance
  • Perimedial fibroplasia (10-15%): May also cause string of beads appearance; beads have smaller diameter than parent vessel
  • Medial hyperplasia (1-2%): Smooth stenosis
• Adventitial fibroplasia (< 1%) – Similar appearance to intimal fibroplasia

Radiological Features

General

• Characteristic string of beads appearance – alternating stenosis or diverticular outpouching – dilated arterial segments (beads) often larger in diameter than parent vessel
• DSA is gold standard for diagnosis
• Involves mid to distal renal arteries (60-75%). Can also affect accessory renal arteries (less commonly intrarenal branches). Bilateral (up to 60% of cases).

CT

With contrast

• Renal artery stenosis: corticomedullary thinning/renal atrophy, delayed parenchymal enhancement/excretion

CT Angiography

• Multifocal stenoses and aneurysms (string of beads) 
• Ring-like or long-segment tubular stenosis may be seen if FMD involves layers other than media
• Best seen on MIP and 3D reformats

MR

T1WI C+

• Signs of renal artery stenosis

MRA

• Findings similar to those seen on CTA
• Best seen on MIP reformats

US

Spectral Doppler

• Pulsus parvus et tardus waveform – slow-rising
• Decrease renal arterial resistive index

Colour Doppler

• Turbulent post-stenotic flow.
• Elevated vascular flow in renal arteries. Peaked Systolic Velocity (PSV) >180-200cm/s
• Irregularity of renal vasculature
• Intravascular ultrasound (IVUS)
• Eccentric ridges or spiral folds
• Elevated pressure gradients (> 20-25 mm Hg) along affected length of renal artery

DSA

• Gold standard for FMD diagnosis
• Can obtain pressure gradients across renal artery
• Medial fibroplasia subtype: Multifocal stenoses/aneurysms in mid to distal renal artery
• Causes string of beads appearance, vascular loops, fusiform vascular ectasia.
• Eccentric ridges/spiral folds
• Intimal or adventitial subtypes: Focal, concentric, or long-segment tubular stenosis

NM

Tc-99m MAG-3 or Tc-99m DTPA captopril renography

• Positive if captopril administration results in reduced glomerular filtration rate (eGFR) by > 40%, delayed isotope uptake &/or excretion. Equivocal if low baseline eGFR or bilateral renal artery involvement

Differential Diagnoses

• Atherosclerosis renovascular disease: Most common caused for secondary hypertension. Usually at the origin of the artery, multiregion and older patient cohort
• Vasculitides: does not typically cause string of beads, affects multiple vascular beds, acute phase reactants such as ESR elevated +/- fever. 
• Traumatic/iatrogenic vascular injury
• Segmental arterial mediolysis – presents as spontaneous intra-abdominal hemorrhage in middle-aged and elderly due to fusiform aneurysms, stenoses, dissections and occlusions within splanchnic arterial branches.
• Neurofibromatosis, Marfan Syndrome, or Ehlers-Danlos Syndrome Type IV