Fibromuscular dysplasia (FMD) is a vascular condition characterized by abnormal cell growth within artery walls. Unlike other vascular diseases, FMD is not caused by plaque buildup (atherosclerosis) or inflammation (vasculitis). This article explores the factors contributing to FMD’s development.
Understanding Fibromuscular Dysplasia
FMD involves abnormal cell development within arterial walls, affecting their structure and function. This cellular disorganization can lead to various changes in blood vessels, including narrowing (stenosis), bulging (aneurysms), or tearing (dissections). The characteristic “string of beads” appearance, often seen on imaging, results from alternating areas of stenosis and dilation in the affected arteries. While FMD can occur in almost any artery, it most commonly affects the renal arteries (supplying blood to the kidneys) and the carotid and vertebral arteries (supplying the brain).
Genetic and Environmental Influences
Evidence suggests a genetic predisposition for FMD, though it does not typically follow a simple inheritance pattern. Family history is reported in a minority of cases (1.9% to 7.3%). Research has identified specific genetic variants, such as a common variant in the PHACTR1 gene on chromosome 6, which can increase FMD risk by about 40%. The COL5A1 gene is also a potential factor in some cases.
Hormonal factors are influential, given the higher prevalence of FMD in women, accounting for over 90% of adult cases. It is often diagnosed in women of childbearing age, suggesting a role for female hormones like estrogen. Studies have indicated higher progesterone receptor expression in arterial tissue from FMD patients.
Environmental factors, such as smoking, are also being investigated. Smoking has been identified as an exacerbating factor, with some studies showing an association between current smoking and an earlier diagnosis of FMD and hypertension. Individuals with FMD who smoke may also have a higher rate of aneurysms. While not a direct cause, smoking appears to influence the disease’s presentation and progression.
Cellular Changes in Arteries
FMD is characterized by abnormal fibromuscular thickening within the arterial wall. Arteries are composed of three layers: the tunica intima (innermost), tunica media (middle), and tunica adventitia (outermost). FMD involves the disorganization or abnormal growth of smooth muscle cells and fibroblasts within these layers.
Different types of FMD are classified based on the arterial layer primarily affected. Medial fibroplasia, the most common type, involves the middle layer and causes the classic “string of beads” appearance due to alternating narrowing and bulging. Intimal fibroplasia affects the innermost layer, leading to focal or smooth narrowing. Less common types, such as adventitial fibroplasia, impact the outermost layer. These changes can impair blood flow and weaken arterial walls.
The Multifactorial Nature of FMD
The exact cause of fibromuscular dysplasia is often not fully understood, leading it to be described as idiopathic. It is considered a multifactorial condition, meaning its development likely results from a complex interplay of various elements. Genetic predispositions, hormonal influences, and environmental factors like smoking all appear to contribute to its onset and progression. Ongoing research continues to shed light on this vascular disorder.