Fibromuscular dysplasia (FMD)

Created 14/03/2022, last revision 07/11/2023

mostly sporadic, rarely hereditary, noninflammatory disease of unknown etiology affecting medium-sized arteries
localization (usually sparing the origin of the vessel):
- an extracranial portion of the ICA / vertebral arteries
- renal arteries (most common)
- splanchnic arteries (rare)
- subclavian, axillary, and iliac arteries (rare)
usually occurs in young to middle-aged women
- F:M ratio of 3:1
- onset typically between 30 and 50 years of age
involved arteries show segmental stenoses and enlargements; ~ 1/3 of patients have multiple lesions
FMD can lead to serious complications ( TIA/stroke, spontaneous dissection, aneurysm formation, myocardial infarction)

Pathology

unknown etiology
- identification of YY1AP1 mutations (typical for Grange syndrome) as a cause of FMD indicates that this condition may result from various genetic disorders (Guo, 2017)
fibrous or fibromuscular thickening of the arterial wall (intima, media, and adventitia) leading to segmental stenoses and microaneurysms
- intima is involved in < 5% of cases (⇒ intimal fibroplasia, also called carotid web)
- media involvement dominates (90-95% of cases)
- adventitia (rare, < 1%)
histology
- accumulations of dysplastic muscle cells + disruption of the internal elastic membrane
- no inflammatory cells are present
carotid lesions are typically localized in the middle and distal segments of the extracranial internal carotid artery, not rarely extending to the skull base
FMD predisposes to the development of aneurysms (extra- and intracranial) and arterial dissections
- ⇒ it is advisable to examine the renal arteries in all patients with dissection

carotid artery lesions are often asymptomatic but can cause TIA/stroke due to:
- distal thromboembolism (e.g., from the aneurysm)
- arterial dissection
renal artery involvement leads to renovascular hypertension (by activating RAAS) or renal impairment
coronary artery involvement can lead to angina pectoris, myocardial infarction, and cardiac death

FMD is diagnosed by a typical appearance on vascular imaging (CTA, MRA, DSA, ultrasound)
- involved arteries usually have a rosary-like appearance, caused by concentric luminal narrowing alternating with areas of mural dilatation (a string of beads/pearls appearance)
  - this finding is present in up to 90% of patients with FMD
- less commonly, concentric long-segment tubular stenosis is present
FMD may present with carotid dissection (best seen on fat-saturated T1 imaging)
less typical features include intracranial aneurysms that can cause subarachnoid hemorrhage (SAH)

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asymptomatic carotid lesions – a conservative approach is preferred; antiplatelet therapy is not recommended
stroke patients with FMD:
- the strict control of blood pressure and other vascular risk factors
- antiplatelet therapy (AHA/ASA 2021 I/C-LD)
  - ASA is most commonly used (based on expert opinion, no studies available)
  - statins are not routinely used in FMD unless otherwise indicated
- for stroke caused by carotid dissection without thrombosis, antiplatelet therapy is usually used (AHA/ASA 2021 2a/C-EO)
- if medical treatment fails or significant carotid artery stenosis develops, consider angioplasty (AHA/ASA 2021 2b/C-LD) (Tekieli, 2015)
  - angioplasty in FMD has high long-term patency rates; stenting is not always required
  - robust data on the clinical benefit of CAS over conservative therapy is lacking

renal artery angioplasty with stenting is indicated in proven renovascular hypertension
- RAA system provides adequate glomerular pressure by vasoconstriction
- ACE inhibitors can cause renal hypoperfusion