EDITORIAL COMMENT
Cardiovascular Magnetic Resonance for Risk Stratification of Arrhythmia in Hypertrophic Cardiomyopathy*
Saman Nazarian, MD* and
João A.C. Lima, MD, FACC
Johns Hopkins Hospital, Baltimore, Maryland.
* Reprint requests and correspondence: Dr. Saman Nazarian, Division of Cardiac Arrhythmia, Johns Hopkins Hospital, Carnegie 592C, 600 North Wolfe Street, Baltimore, Marlyand 21287. (Email: snazarian{at}jhmi.edu).
Hypertrophic cardiomyopathy (HCM) is an autosomal dominant disease of the cardiac sarcomere characterized clinically by generalized or localized hypertrophy of the left ventricle. Clinical manifestations of the disease vary, ranging from no symptoms to heart failure, syncope, or sudden cardiac death (SCD). Regarding the latter, clinical decision making is driven by the effectiveness of implantable defibrillators at ending arrhythmia (1), and the devastating potential outcome of a sustained ventricular arrhythmia event (2–4). However, defibrillator implantation can be associated with complications and low quality of life (5,6), and is not feasible for all patients with HCM. To identify high-risk candidates, risk factors including family history of SCD (7), syncope (8), nonsustained ventricular tachycardia (NSVT) (9–11), abnormal blood pressure response to exercise (12), and severe left ventricular hypertrophy (13) have been proposed. Despite the application of such risk factors to patient selection, a high proportion of current defibrillator recipients never need a shock, and SCD continues to occur in many undiagnosed patients (14).
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Rationale for Using Cardiovascular Magnetic Resonance
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Cardiovascular magnetic resonance (CMR) recently has been shown to predict induciblity of ventricular arrhythmia in ischemic (15,16) and nonischemic (17) cardiomyopathies. There is also accumulating evidence that CMR may predict spontaneous clinical arrhythmia and SCD (18,19). Pathology studies in which hearts of patients with HCM were examined after death or transplantation have shown greater extent of fibrosis in patients with NSVT (20). Therefore, it is plausible that the presence and extent of scar on CMR would predict arrhythmia risk.
In the current issue of the Journal, Adabag et al. (21) present results that suggest the utility of CMR for identification of HCM patients with increased susceptibility to ventricular arrhythmia. The authors correlated arrhythmic events on 24-h Holter monitoring of 177 HCM patients to the presence of scar on CMR. Patients with any pattern of scar were found to have more premature ventricular contractions and NSVT. The authors conclude that CMR may identify patients with increased susceptibility to ventricular tachyarrhythmia. These results are in agreement with earlier studies. Similar to the findings reported by Adabag et al. (21), Teraoka et al. (22) found higher occurrence of scar and a significantly larger extent of scar in HCM patients with NSVT. Dimitrow et al. (23) also reported lower likelihood of scar in patients without NSVT compared with those with NSVT.
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Implications for Risk Stratification
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The use of surrogates of SCD as end points for studies of risk stratification methodology is suboptimal and may lead to incorrect estimates of association. However, until long-term survival data have been collected, these findings may provide additional data for selection of high-risk patients. To apply these results to clinical patient care, an estimate of the magnitude of increased risk for SCD in HCM patients with scar on CMR may be beneficial.
Probability theory provides a conceptual framework to use gathered evidence toward the probability estimation of dependent events. In the absence of long-term survival data, we can mathematically approximate the probability of SCD given the presence of scar on CMR (P (SCD|CMR+)) by the following formula:
Similarly, the probability of SCD without the presence of scar on CMR (P(SCD|CMR–)) can be approximated by this formula:
In HCM patients, the 5-year probability of SCD given NSVT (P(SCD|NSVT+)) and the probability of SCD in the absence of NSVT (P(SCD|NSVT–) have been previously reported to be 22.4% and 5.9%, respectively (10). The current study reports the probability of NSVT given scar on CMR (P(NSVT+|CMR+)) and the probability of NSVT given lack of scar on CMR (P(NSVT+|CMR–)) to be 28% and 4%, respectively. The probability of lack of NSVT despite scar on CMR (P(NSVT–|CMR+)) is then 100% – 28% = 72%. Similarly, the probability of lack of NSVT given lack of scar on CMR (P(NSVT–|CMR–)) is 100% – 4% = 96%. So, the 5-year probability of SCD given the presence of scar on CMR is approximately:
And the 5-year probability of SCD without the presence of scar on CMR is approximately:
Therefore, in the absence of long-term data, we would approximate the 5-year probability of sudden death to be 1.6-fold higher if scar is noted on CMR of a patient with HCM. Extrapolation to the prevalence of HCM in the general population (2 out of 1,000 young adults) (24), suggests that identification of high-risk patients with CMR may have important consequences. Because CMR can accurately diagnose and characterize HCM, its added potential for arrhythmic risk stratification may suggest its use in place of echocardiography for the initial evaluation of patients with suspected disease and preserved renal function (estimated glomerular filtration rate >30 ml/min/1.73 m2).
Cardiovascular magnetic resonance appears to be a promising tool for identification of the substrate underlying ventricular arrhythmia in structural heart disease (15–19). This issue of the journal presents evidence in support of using CMR for risk stratification of arrhythmia in patients with another high-risk structural heart disease. Future studies of long-term survival in HCM patients stratified by CMR characteristics are warranted.
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Footnotes
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* Editorials published in the Journal of the American College of Cardiology reflect the views of the authors and do not necessarily represent the views of JACC or the American College of Cardiology. 
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References
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