Fibromuscular dysplasia (FMD) is a heterogeneous group of vascular lesions characterised by an idiopathic, non-inflammatory, and non-atherosclerotic angiopathy of small and medium-sized arteries.
The prevalence is unknown 7. It is most common in young women with a female to male ratio of 3:1, and is typically diagnosed between the ages of 30 and 50 4.
FMD is frequently asymptomatic. Symptomatic patients commonly present with:
- hypertension or less commonly renal impairment due to renal artery stenosis
- CNS symptoms (e.g. headache, neck pain, pulsatile tinnitus, Horner syndrome) from transient ischaemic attack, stroke, dissection, due to carotid and vertebral artery involvement 11
- angina, myocardial infarction or sudden cardiac death due to coronary artery involvement 10
- symptoms of mesenteric ischaemia (mesenteric infarction is rare due to formation of collateral supply) 11
The exact cause is not well known. The underlying pathology is fibrous or fibromuscular thickening of the arterial wall. Any layer of the vessel wall may be affected: intima, media or adventitia. There is absence of inflammatory cells 1-4,7.
FMD is classified into five categories according to the vessel wall layer affected:
- intima: 5%
- intimal fibroplasia
- media: 90-95%
- medial dysplasia (70%, commonest type)
- perimedial (subadventitial) fibroplasia (15-20%)
- medial hyperplasia (8-10%)
- adventitia: rare
- adventitial fibroplasia (1%) 8
The outcome is arterial stenoses. FMD most commonly causes small stenoses along a vessel with intervening areas of dilatation (small aneurysms), creating a “string of beads” appearance. Less commonly the stenosis has a smooth tapered appearance. FMD also weakens the vessel wall which predisposes to dissection.
FMD may affect any medium sized artery in the body, and is commonly multifocal and bilateral (up to 60% when involving the renal arteries). FMD usually involves mid segment of the vessels and spares origins. Some sites are very frequently involved 9:
- renal arteries (most common)
- extracranial internal carotid arteries
- vertebral arteries
- iliac arteries
- coeliac trunk and mesenteric arteries
- subclavian and axillary arteries
- spontaneous dissection
- distal embolisation (of thrombus formed in aneurysm)
- arteriovenous fistula
Arterial imaging with CT angiography, MR angiography and digital subtraction angiography (DSA) may be used to visualise the lesions in FMD. Selective DSA is the gold standard because it allows visualisation of small or peripheral lesions. The characteristic finding, particularly in more common intimal subtype, is alternating stenoses and dilatations, causing a string of beads appearance 5. Less commonly in intimal and adventitial type, there is focal concentric, long-segment tubular stenosis or diverticular outpouching present. Cross-sectional imaging (CT and MRI) allows assessment of end-organ ischaemic damage.
- typical angiographic features include: vascular loops, fusiform vascular ectasia and a string of beads
- less typical features include: arterial dissection, aneurysm and subarachnoid haemorrhage
Treatment and prognosis
Asymptomatic cases are only observed but if symptomatic then FMD responds well to angioplasty, with high long-term patency rates. A stent is generally not required.
Imaging differential considerations include:
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