104. Nuclear and Multimodality Imaging: Anomalous Coronary Arteries & Myocardial Bridges

CardioNerd Amit Goyal is joined by Dr. Erika Hutt (Cleveland Clinic general cardiology fellow), Dr. Aldo Schenone (Brigham and Women’s advanced cardiovascular imaging fellow), and Dr. Wael Jaber (Cleveland Clinic cardiovascular imaging staff and co-founder of Cardiac Imaging Agora) to discuss nuclear and complimentary multimodality cardiovascular imaging for the evaluation of abnormal coronary anatomy including anomalous coronary arteries and myocardial bridges. Show notes were created by Dr. Hussain Khalid (University of Florida general cardiology fellow and CardioNerds Academy fellow in House Thomas). To learn more about multimodality cardiovascular imaging, check out Cardiac Imaging Agora! Collect free CME/MOC credit just for enjoying this episode! CardioNerds Multimodality Cardiovascular Imaging PageCardioNerds Episode PageCardioNerds AcademyCardionerds Healy Honor Roll Subscribe to The Heartbeat Newsletter!Check out CardioNerds SWAG!Become a CardioNerds Patron! Show Notes & Take Home Pearls Five Take Home Pearls Anomalous coronaries are present in 1-6% of the general population and predominantly involve origins of the right coronary artery (RCA). Anomalous origination of the left coronary artery from the right sinus, although less common, is consistently associated with sudden cardiac death, especially if there is an intramural course. Sudden cardiac death can occur due to several proposed mechanisms: (1) intramural segments pass between the aorta and pulmonary artery making them susceptible to compression as the great vessels dilate during strenuous exercise; (2) an acute angle takeoff of the anomalous coronary can create a “slit-like” ostium making it vulnerable to closure. Anomalous left circumflex arteries are virtually always benign because the path taken behind the great vessels to reach the lateral wall prevents vessel compression.Myocardial bridging (MB) is a congenital anomaly in which a segment of the coronary artery (most commonly, the mid-left anterior descending artery [LAD]) takes an intramuscular course and is “tunneled” under a “bridge” of overlying myocardium. In the vast majority of cases, these are benign. However, a MB >2 mm in depth, >20 mm in length, and a vessel that is totally encased under the myocardium are more likely to be of clinical significance, especially if there is myocardial oxygen supply-demand mismatch such as with tachycardia (reduced diastolic filling time), decreased transmural perfusion gradient (e.g. in myocardial hypertrophy and/or diastolic dysfunction), and endothelial dysfunction resulting in vasospasm.PET offers many benefits over SPECT in functional assessment of MB including the ability to acquire images at peak stress when using dobutamine stress-PET, enhanced spatial resolution, and quantification of absolute myocardial blood flow. For pharmacologic stress in evaluation of MB, we should preferentially use dobutamine over vasodilator stress. Its inotropic and chronotropic effects enhance systolic compression of the vessel, better targeting the pathological mechanisms in pearl 2 above that predispose a MB to being clinically significant.CCTA can help better define the anatomy of MB as well as anomalous origination of the coronary artery from the opposite sinus (ACAOS), help with risk stratification, and assist with surgical planning.Instantaneous wave-free ratio (iFR) measures intracoronary pressure of MB during the diastolic “wave-free” period – the period in the cardiac cycle when microvascular resistance is stable and minimized allowing the highest blood flow. This allows a more accurate assessment of a functionally significant dynamic stenosis than fractional flow reserve (FFR) – which can be falsely normal due to systolic overshooting. Detailed Show Notes What are some examples of abnormal coronary anatomies and how often do they lead to clinical events?Abnormal coronary anatomy can relate to the origin (e.g. anomalous origination of coronary artery from the opposite sinus [ACAOS]), course (e.g. myocardial bridging [MB]), intrinsic properties (e.g. aneurysm or hypoplasia), or termination (e.g. fistula) of the coronary artery. In this episode and in these notes, we examine MB and ACAOS in more detail. For an excellent case discussion of anomalous left coronary artery from the pulmonary artery (ALCAPA) by the team from Massachusetts General Hospital, listen to CardioNerds Podcast Episode 81!MB –Myocardial BridgingMB is a congenital anomaly in which a segment of the coronary artery (most commonly, the mid-left anterior descending artery [LAD]) takes an intramuscular course and is “tunneled” under a “bridge” of overlying myocardium.MB was originally identified at autopsy by Reyman in his dissertation, “Disertatio de vasis cordis propriis “ in 1737. In the largest subsequent autopsy study by Risse et al. involving 1056 patients, MB was demonstrated in 26% of patients.Because it is so prevalent, it is difficult to determine its clinical significance. In most patients, MB is an incidental finding with an excellent survival rate (97% at 5 years); however, there are associations with myocardial ischemia, infarction, stress cardiomyopathy, arrhythmia, and sudden cardiac death (SCD).MB can generally be classified into two subtypes: a “superficial” variant which represents 75% of cases and a “deep” variant in which the LAD deviates towards the right ventricle (RV) and dives into the intraventricular septum. The overlying muscle bundle in the deep variant is typically at an oblique or transverse angle resulting in twisting of the tunneled segment and more commonly compromised coronary flow.One of the longest MB usually occurs in association with ACAOS! In this case, the left coronary artery comes off the right coronary cusp. The Left Main (LM) is around 3-4x longer in this instance and dives into the interventricular septum and takes a trans-septal course behind the pulmonary artery before emerging on the other side.There is increased prevalence in certain patient populations: hypertrophic cardiomyopathy (HCM), patients with spontaneous coronary artery dissection (SCAD) +/- fibromuscular dysplasia (FMD), and heart transplant recipientsACAOS - anomalous origination of coronary artery from the opposite sinusAnomalous coronaries are present in 1-6% of the general population and predominantly involve the origin of the right coronary artery (RCA)Anomalous origination of the left coronary artery from the right sinus, although less common, is consistently related to SCD. Separate studies have shown the incidence of SCD may be as high as 23% or 59% of cases in athletes under the age of 20 years.In a large Armed Forces Institute of Pathology (AFIP) study of 6.3 million military recruits, the autopsies of recruits who suffered nontraumatic deaths over a 25-year period were reviewed and ACAOS was found to be the most common cause. It accounted for 33% (64 of 126) of nontraumatic deaths and all cases involved a left coronary artery with an interarterial course. What features predispose MB or ACAOS to become clinically significant? What is the pathophysiology behind development of ischemia in those with clinically significant MB or ACAOS?MB – myocardial bridgingGiven the majority of MB is benign, correlating MB as causative in myocardial ischemia and its consequences has been a diagnostic challenge.In systole, the portion of the artery that is tunneled under the MB (bridge segment) is compressed. This can manifest clinically as angina, acute coronary syndrome, left ventricular (LV) dysfunction, arrhythmias, and SCD. However, the majority of myocardial perfusion occurs in diastole which is why MB is usually benign. Nonetheless, certain conditions in patients with MB can set up an oxygen supply-demand mismatch severe enough to lead to myocardial ischemia:Exercise-related stress leads to tachycardia which can decrease diastolic filling time for the coronary arteries and lead to more of the cardiac cycle to be spent in systoleMyocardial hypertrophy and diastolic dysfunction can affect the transmural perfusion gradient increasing supply-demand mismatch. Furthermore, LV hypertrophy can compress the microvasculature and reduce the microvascular reserve.Endothelial dysfunction (driven by metabolic changes secondary to hypoxia) can contribute to coronary compression and lead to the development of accelerated atherosclerosis and/or coronary vasospasm (leading to compression of the epicardial coronary artery throughout the cardiac cycle, not just during systole)There has been a recognized multiplier-effect described by Klues et al. in which the greater the degree of systolic narrowing of the MB, the greater the reduction in diastolic vessel diameter. This is also associated with increased retrograde flow in the coronary artery (which not only reduces perfusion but can introduce shear wall stress and predispose to plaque formation) and reduced flow reserve.Myocardial ischemia can also occur due to “branch steal.” The LAD may have septal perforators that arise from the tunneled segment. When there is compression of the vessel under the MB, there can be “steal” from these septal branches due to the Venturi effect. The septal branches are essentially depressurized because as the vessel narrows, velocity increases but the fluid (coronary blood flow) exerts less pressure. Thus, mild to moderate MB severity typically demonstrates septal ischemia (due to branch steal) rather than distal ischemia downstream from the compression.The vessel segment proximal to the bridge appears to develop atherosclerosis at increased rates approaching 90% -- likely as the sequela of shear stress. In contrast,