The SCAD-FMD Connection: Why One Diagnosis Should Trigger a Search for the Other

SCAD is rarely the whole story. In the majority of patients, it is a signpost to a systemic vascular condition that has been hiding in arteries no one has looked at yet.

Fibromuscular dysplasia (FMD) is present in 50-80% of SCAD patients across major registry series, a figure established prominently in Hayes SN et al., “Spontaneous Coronary Artery Dissection: Current State of the Science,” Circulation 2018. That figure is not a statistical footnote. It is a clinical mandate: every SCAD diagnosis requires a vascular map that extends beyond the coronary arteries. The coronary angiogram done during the acute presentation sees only the coronary circulation. It says nothing about what is happening in the renal arteries, the carotid arteries, or the vertebral arteries of the same patient.

This article explains what FMD is, how it connects mechanistically to SCAD, what the post-SCAD vascular screening requires, and what happens when FMD is found in vascular beds outside the heart.

The Mechanism

FMD is a non-atherosclerotic, non-inflammatory disease of the arterial wall. It is not caused by cholesterol. It is not caused by inflammation. It is a primary structural disease of the arterial media and fibrous components of the arterial wall, causing architectural abnormalities that weaken the vessel from within.

The classic angiographic appearance is the “string of pearls” or beaded pattern in affected arteries: alternating areas of narrowing and dilation along the vessel. This pattern is diagnostic when present. To understand why the beading looks the way it does, the histology matters. In the most common subtype, medial fibroplasia, abnormal fibromuscular ridges develop within the tunica media. The ridges are thickened bands of fibrous tissue and disorganized smooth muscle cells. Between the ridges, the medial layer is thinned, the smooth muscle cells are reduced in number, and elastin fibers are fragmented or absent. The thinned zones dilate outward under arterial pressure, creating the outpouchings visible on angiography. The fibromuscular ridges produce the relative narrowings between them. The result is the characteristic alternating bead-and-furrow appearance along the length of the artery.

Less classic forms include unifocal stenosis (a single focal narrowing without the beaded pattern) and tubular stenosis, which are less common but within the FMD spectrum. The multifocal beaded type accounts for approximately 80% of cases and is the pattern most reliably identified on CT angiography.

FMD predominantly affects medium-sized arteries. The most commonly involved vessels:

• Renal arteries: causing renovascular hypertension through renin-angiotensin system activation from reduced renal perfusion pressure
• Internal carotid and vertebral arteries: causing arterial dissection, stroke risk, and cervical bruits
• Mesenteric arteries: less common, can cause intestinal angina
• Coronary arteries: directly contributing to SCAD through medial weakness

The demographics are striking: FMD is overwhelmingly a disease of women. In the US Registry for FMD, approximately 91% of enrolled patients are female, with a mean age at diagnosis in the mid-50s. The registry, established through collaboration of major vascular disease centers and coordinated by the Society for Vascular Medicine, has enrolled hundreds of patients and characterized the natural history and vascular distribution of FMD across multiple arterial beds (Olin JW et al., Circulation 2012).

Why FMD predominantly affects women remains incompletely understood. Hormonal influences on arterial wall connective tissue are proposed: estrogen and progesterone both affect the composition of the arterial media. Relaxin, produced during pregnancy, remodels collagen throughout the body and may alter the structural integrity of already-weakened medial zones. The female predominance tracks with SCAD’s female predominance, suggesting shared underlying biology that is not yet fully characterized at a molecular level.

The precise mechanism by which FMD predisposes to coronary artery dissection is incompletely established, but the association is robust and the biological plausibility is high.

FMD alters the architecture of the arterial media, the middle layer of the arterial wall responsible for tensile strength and structural integrity. In a healthy coronary artery, the media provides resistance to the mechanical stresses of pulsatile blood flow, elevated heart rate during exertion, and hormonal changes during pregnancy. The smooth muscle cells and connective tissue (elastin, collagen fibrils) within the media are organized in a way that distributes mechanical stress evenly.

In FMD, this organization is disrupted. Areas of medial fibroplasia alternate with areas where the media is thinned and weakened. Within the thinned zones, the arterial wall has substantially reduced resistance to mechanical stress.

The intimal or medial tear that initiates SCAD begins at these weakened planes. When coronary artery pressure rises with exertion, emotional stress, or the hemodynamic changes of childbirth, the arterial wall at FMD-affected sites may give way. Blood enters the space between the layers, the false lumen forms, and it expands under arterial pressure, compressing the true lumen. FMD is not the only pathway to SCAD: approximately 20-50% of SCAD patients have no identified FMD. But in the majority of cases it appears to be the pre-existing structural substrate, as described by Hayes SN et al. in Circulation 2018.

The peripartum association reinforces this mechanism. Pregnancy increases cardiac output by 40-50%, exposes coronary arteries to sustained elevated flows and pressures, and relaxin remodels connective tissue throughout the body (including arterial walls) to allow pelvic expansion for delivery. In a woman with FMD-weakened coronary arteries, this combination of mechanical stress and connective tissue remodeling creates the conditions for dissection. The 20-25% of SCAD cases that occur in the peripartum period reflect this convergence.

What the Evidence Shows

The quantitative data on the SCAD-FMD association come from several converging sources.

Hayes SN et al., Circulation 2018: This AHA Scientific Statement on SCAD drew on multiple case series and the nascent SCAD registry literature to establish the 50-80% FMD prevalence in SCAD populations as the figure guiding clinical practice. The statement also formalized the recommendation for post-SCAD vascular screening of non-coronary beds. It remains the primary document cited when the post-SCAD imaging protocol is discussed in cardiology.

Saw J et al., Circulation: Cardiovascular Interventions 2017: The largest prospective SCAD cohort at that time found FMD in 72% of SCAD patients who underwent dedicated vascular imaging. This study was significant because it required that all enrolled patients actually receive the imaging, rather than relying on incidental prior imaging, and still found the prevalence above 70%.

Olin JW et al., Circulation 2012: The foundational US Registry for FMD publication characterized the vascular distribution of FMD in 447 patients. Renal artery involvement was present in 79.6% of patients, carotid/vertebral artery involvement in 75.5%, and multivessel involvement in the majority. This registry established that FMD is a multivessel disease in most patients, not an isolated single-vessel finding.

Lather HD et al., JAMA Internal Medicine 2019: Analysis of the SCAD Research Consortium registry found that patients with SCAD and concurrent FMD had different clinical profiles than SCAD patients without FMD, including higher rates of recurrent SCAD, supporting the argument that FMD is not merely an incidental co-finding but is mechanistically relevant.

The intracranial aneurysm data is narrower. Registry-based estimates place the prevalence of intracranial aneurysms in FMD patients at approximately 7-12%, substantially higher than the 2-3% prevalence in the general population (Perdu J et al., Annals of Internal Medicine 2007). The mechanism is analogous: FMD-related medial weakness in intracranial arteries allows aneurysmal dilation at branch points where hemodynamic stress concentrates.

What to Do This Week

1. Ask your cardiologist directly whether FMD vascular screening has been completed. The specific question is: “Have I had CT angiography of my renal arteries and carotid/vertebral arteries for fibromuscular dysplasia?” If it has not been done, request the referral at that visit. Do not defer to a future appointment without a scheduled date. This imaging is recommended in the Hayes SN et al. Circulation 2018 AHA Scientific Statement and is not optional.

2. If carotid or vertebral FMD is confirmed, tell every provider who offers manual therapy involving your neck about the diagnosis. Chiropractic cervical manipulation carries risk of arterial dissection in patients with carotid or vertebral FMD. This includes massage therapists using deep cervical techniques and physical therapists performing cervical mobilization. The FMD diagnosis changes what is safe for your cervical vessels.

3. If FMD is found in any vascular bed, request a referral to a vascular medicine specialist or to an FMD center of excellence. The US Registry for FMD at fmdregistry.org lists participating centers. A general cardiologist managing isolated coronary disease is not the appropriate long-term specialist for multivessel FMD, and the decisions around renal artery intervention, antiplatelet therapy, and aneurysm surveillance require FMD-specific expertise.

4. If you have been told your SCAD was an isolated event and no further imaging was recommended beyond cardiac follow-up, that guidance is incomplete by current published standards. The Hayes 2018 AHA Scientific Statement on SCAD explicitly recommends vascular screening, and this recommendation exists because FMD in non-coronary beds is both common and clinically consequential. Return to your cardiologist with this information and ask specifically about the imaging protocol.

5. If you have new or worsening blood pressure control after a prior SCAD and prior negative renal imaging, raise the possibility of interval development or progression of renal artery FMD with your physician. FMD can progress over time. Treatment-resistant hypertension in a patient with a history of SCAD and prior negative renal CTA is a signal for repeat imaging, not simply a prompt to add a fourth antihypertensive.

FMD as a Systemic Arterial Disease

The clinical implication of the SCAD-FMD connection is that SCAD in the coronary artery is not a localized event in the majority of patients. It is a presentation of a systemic arterial wall disease that has manifested at one point but is anatomically distributed across the arterial tree.

This reframes the clinical conversation substantially: a SCAD survivor is not simply a patient who had a cardiac event and whose coronary artery healed. She is a patient with systemic FMD whose first clinical manifestation was a coronary dissection. The cardiac event is the starting point of a longitudinal vascular surveillance program, not the entirety of her diagnosis.

The US Registry for FMD data show that the majority of patients have involvement of more than one vascular bed when systematically screened, confirming the systemic nature of the disease (Olin JW et al., Circulation 2012). A patient found to have renal FMD should be considered at risk for carotid FMD, vertebral FMD, and potentially intracranial aneurysm, not simply at risk for renovascular hypertension in isolation.

The surveillance program after FMD diagnosis requires annual blood pressure monitoring with particular attention to treatment-resistant hypertension as a signal for renal artery involvement not previously apparent. Repeat vascular imaging at intervals determined by the severity of initial findings and vascular beds involved: patients with carotid FMD may need carotid duplex surveillance annually to detect progressive stenosis or new aneurysmal dilation. Patients with intracranial aneurysms need a surveillance imaging protocol determined by aneurysm size. Small aneurysms below 5-7mm are typically observed with annual or biennial MRA. Larger aneurysms require neurosurgical or neurointerventional evaluation.

Antiplatelet therapy with aspirin is standard for FMD management in most practice patterns, based on analogy with other arterial diseases and the logic of reducing thrombosis risk at sites of abnormal arterial architecture. The evidence base for aspirin specifically in FMD is observational rather than from randomized trials. In patients who had SCAD and are already on antiplatelet therapy for post-SCAD management, the antiplatelet decision is typically already addressed for the combined indication.

Related Reading

For the SCAD mechanism and clinical presentation: SCAD: The Heart Attack That Tears the Artery Wall.

For SCAD in the postpartum setting: SCAD and Pregnancy.

For the full picture of non-obstructive coronary disease in women: Your Angiogram Was Normal. You Are Not Fine..