Sandbox: SCAD

Spontaneous coronary artery dissection

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Arzu Kalayci, M.D. [2]

Synonyms and keywords: SCAD

MECHANISM AND PATHOPHYSIOLOGY

The arterial dissection with SCAD can occur within or between any of the 3 layers (intima , media, or adventitia) of the coronary artery wall. Two potential mechanisms for the initiation of arterial wall separation have been proposed. The first is the intimal tear hypothesis, in which a primary disruption in the intimal luminal interface creates an entry point for intramural hematoma (IMH) accumulation inside the false lumen, leading to separation of the arterial wall. The second is the medial hemorrhage hypothesis, in which a hemorrhage into the arterial wall is the primary mechanism, perhaps due to spontaneous rupture from the increased density of the vasa vasorum. It is possible for the latter process, with increased pressure from IMH, to cause a rupture into the true lumen, resulting in a “reverse” intimal rupture. Intuitively, the pathognomonic angiographic appearance of multiple lumens would require the presence of visible intimal disruption. However, the presence of intimal disruption does not imply a specific mechanism for genesis of the tear.

Overview

Spontaneous coronary artery dissection (SCAD) is becoming recognised as an important cause of myocardial infarction (MI) particularly among young women.

Spontaneous coronary artery dissection (SCAD) is an infrequent cause of coronary artery disease. Whilst a recognized disease entity since 1930s, its pathogenesis remains unclear and various hypotheses have been postulated from clinical and clinic-pathological association. Whilst angiography alone has traditionally been perceived to significantly under diagnose this condition, recent development in intravascular imaging, particularly optical coherence tomography (OCT), have seen significant advances in the understanding of SCAD. From a histopathological perspective, OCT has shown that there may be distinct pathological subsets in SCAD in reference to a dissection flap and an intramural hematoma. Early data also suggests that OCT may be important in optimizing PCI results in SCAD. The future is particularly promising if a collaborative approach is undertaken to utilize OCT as an in vivo microscope to shed more light in this poorly understood disease entity.

PATHOLOGY AND PATHOPHYSIOLOGY

The underlying mechanism of non-atherosclerotic spontaneous coronary artery dissection (NA-SCAD) is not fully understood, but an intimal tear or bleeding of vasa vasorum with intramedial hemorrhage has been proposed [1]. Both result in creation of a false lumen filled with intramural hematoma [2]. Pressure-driven expansion of the false lumen by an enlarging hematoma may lead to luminal encroachment and subsequent myocardial ischemia and infarction. Atherosclerotic SCAD is a mechanistically distinct variant of SCAD and is typically limited in extent by medial atrophy and scarring [3]. NA-SCAD, on the other hand, can result in extensive dissection lengths, especially in the presence of arterial fragility from predisposing arteriopathies, and intracoronary imaging studies clearly show the absence of atherosclerosis in these cases [4,5].

Intramural hematoma involving the outer two-thirds of the media is common. Histologically, an inflammatory reaction (eg, eosinophilic infiltrates) in the adventitia has been described, suggestive of periarteritis that may breakdown the medial-adventitial layer predisposing the artery to dissection. However, this inflammatory response may be reactive rather than causative [6].

One retrospective study has proposed that coronary artery tortuosity may be a marker for or a potential mechanism for SCAD [8]. In this study, the coronary angiograms of 246 patients with SCAD were compared with 313 controls. Tortuosity, as defined by the presence of ≥3 consecutive curvatures of 90 to 180 degrees measured at end-diastole in a major epicardial coronary artery ≥2 mm in diameter, was found in 78 and 17 percent, respectively. However, the presence of coronary tortuosity was also associated with extracoronary vasculopathy (eg, fibromuscular dysplasia) [9,10]; as such, it is more likely that the tortuosity, similar to dissection, is a manifestation of the underlying predisposing vasculopathy.

Although intimal tear or bleeding of vasa vasorum with intramedial hemorrhage seems to be most probable reason, the exact underlying mechanism of non-atherosclerotic spontaneous coronary artery dissection (NA-SCAD) is still unknown [1]. Consequently intramural hematoma creates a false lumen [2]. Progressive expansion of the false lumen may cause subsequent myocardial ischemia and infarction.

In pregnancy or early postpartum period, dissection may be occur because of the physiological responses to the increased hemodynamic stresses and having more fragile coronary arterial wall due to the hormonal effects [6,7]. Multiparity raises the risk of SCAD further. Dissection in the other arterial beds are also more common in pregnancy [7]. Intramural hematoma is typically placed in the outer two-thirds of the media. An inflammatory reaction consisting predominantly eosinophilic infiltrates has been described in the adventitial layer making the artery more fragile. On the other hand it seems more likely a result than a reason [6].

Clinical-Angiographic Score System for SCAD Faster Diagnosis
Clinical Characteristics
Connective tissue disorder (Marfan syndrome, Ehler-Danlos syndrome, cystic medial necrosis) / Fibromuscular Dysplasia	+2
Youth/<50 year	+1
Estroprogestinic therapy	+1
No classical coronary risk factors	+1
Pregnancy (peri-partum, history of multiple pregnancy)/ Female gender	+1
History of coronary artery spasm/ previous SCAD	+3
Amphetamines/Cocaine/Vasospastic drug abuse	+1
Systematic inflammation (SLE, Chron disease, sarcoidosis, polyarteritis nodosa, Behcet's syndrome	+2
Emotional/ Physical stress	+1
Angiographic Characteristics
One vessel disease (no typical atherosclerotic lesions in order coronary arteries)	+1
Long/tortuose suspected lesion	+1
Diffuse, typically smooth arterial narrowing	+1
At least 3 points: Indication to perform endovascular imaging (OCT or, if not available, IVUS)
In the presence of a score of at least three points in a patient presenting with chest pain, ECG/Echo abnormalities or troponin rise/fall, perform OCT/IVUS analysis for suspected SCAD.

Algorithm

Spontaneous Coronary Artery Dissection (SCAD)

Stable,no high-risk feature

High-risk feature present

Left main dissection

Ongoing/recurrent ischemia

Ventricular tachycardia or fibrillation

Cardiogenic shock

Hemodynamically unstable?

No

Yes

Consider: Intra-aortic balloon pump, Extra corporeal membrane oxygenation, Left ventricular assist device, Implantable cardioverter defibrillator

Isolated Left Main

Involving Left Anterior Descending (LAD +/or circumflex

Is percutaneous coronary intervention (PCI) feasible?

No

Yes

Ostial LAD or ≥ 2 proximal SCAD?

Conservative therapy

PCI

Coronary artery bypassing