Fibromuscular dysplasia classification

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Mohsen Basiri M.D.

Overview

The classification system for fibromuscular dysplasia (FMD) was first according to the arterial layer involved. (intima, media, or adventitia) [18-20]. However, with use of TPA for treatment and its preference rather than surgery, the obtaining of pathological specimens are restricted.Thus,today, FMD is a disease diagnosed radiographically and histopathological classification has been replaced by an arteriographicfindings [3]. 3. Olin JW, Gornik HL, Bacharach JM, et al. Fibromuscular dysplasia: state of the science and critical unanswered questions: a scienti􀉹c statement from the American Heart Association. Circulation 2014; 129:1048. 20. Lüscher TF, Lie JT, Stanson AW, et al. Arterial 􀉹bromuscular dysplasia. Mayo Clin Proc 1987; 62:931

Classification

arteriographic classification two angiographic subtypes of FMD: ●Multifocal FMD (more common) (image 1) has the angiographic appearance of a "string of beads." Multifocal FMD corresponds pathologically to medial fibroplasia, the most common histologic type, and to perimedial fibroplasia, which is less common. ●Focal FMD (less common) (image 2) has the angiographic appearance of a "circumferential or tubular stenosis" and corresponds pathologically to intimal fibroplasia. Medial hyperplasia and periarterial hyperplasia are histologic types that may also have a focal appearance.

It has been shown that these two different angiographic subtypes of FMD (multifocal and focal) have different phenotypic presentations and natural history [21]. 21. Savard S, Steichen O, Azarine A, et al. Association between 2 angiographic subtypes of renal artery 􀉹bromuscular dysplasia and clinical characteristics.Circulation 2012; 126:3062. This has led some investigators to question whether FMD is, in fact, a single disease [22]. Olin JW. Is 􀉹bromuscular dysplasia a single disease? Circulation 2012;126:2925.

Histologic classification

— FMD is not usually classified histologically because pathological specimens are rarely obtained. In the past, fibrous lesions were classified according to the arterial layer affected 

●Medial fibroplasia represents the most common dysplastic lesion, accounting for more than 80 percent of fibromuscular lesions [18,19]. Angiographically, medial fibroplasia is characterized by the classic "string of beads" appearance (image 1). This appearance is due to alternating fibromuscular webs and aneurysmal dilatation. In areas of dilatation, the internal elastic lamina is absent, which is possibly the primary defect. Total occlusion is uncommon. 18. Stanley JC, Gewertz BL, Bove EL, et al. Arterial 􀉹brodysplasia. Histopathologic character and current etiologic concepts. Arch Surg 1975;110:561. 19. Harrison EG Jr, McCormack LJ. Pathologic classi􀉹cation of renal arterialdisease in renovascular hypertension. Mayo Clin Proc 1971; 46:161.

●Intimal fibroplasia (which accounts for approximately 10 percent of FMD) is caused by circumferential or eccentric deposition of collagen in the intima (image 2). The internal elastic lamina may be intact, fragmented, or duplicated, the latter especially in the childhood form. There is no inflammatory or lipid component.

●In perimedial fibroplasia, which occurs predominately in children, large parts of the media (in particular, the outer zone) are replaced by collagen, with irregular thickening of the media. Total occlusion may occur with development of collateral vasculature. "Beading" is present in this type, but the "beads" are less numerous and are smaller than in medial fibroplasia. Aneurysm formation is uncommon.

●Medial hyperplasia, a rare manifestation, is caused by smooth muscle cell hyperplasia without fibrosis. ●Periarterial hyperplasia, also a rare manifestation, is caused by expansion of the fibrous adventitia; collagen extends into the periarterial fat, with accompanying inflammation