Pathogenesis of Stress-Induced Cardiomyopathy by I-123 MIBG

Background: Since the initial Japanese description of Takotsubo's cardiomyopathy in 1991 as a transient systolic dysfunction of the apical or mid left ventricular segments in the absence of obstructive coronary artery disease, stress induced cardiomyopathy has been increasingly recognized in the Unites States. Takotsubo's cardiomyopathy accounts for 1.2 to 2.2 percent of all cases of acute coronary syndrome. The current American Heart Association Statistical Update estimates that approximately 1.2 million Americans will experience an acute coronary event in 2010. Based on this estimate, between 15 and 26 thousand Americans will have stress induced cardiomyopathy annually. Takotsubo's cardiomyopathy has also been described in stroke and critically ill patients. Post menopausal women are disproportionally affected, accounting for 80 to 100 percent of the patient population. These patients classically present with signs of acute heart failure or acute coronary syndrome after a severe emotional stress. The presentation may include chest pain, shortness of breath, elevated troponin enzymes, ST segment elevations, deep T-wave inversions, ventricular arrhythmias, pulmonary edema or elevated biomarkers. Cardiac catheterization reveals angiographically normal coronary arteries while the ventriculogram and the echocardiogram shows apical ballooning with basal hyperkinesis. While the majority of patients recover complete function within few days to two weeks, up to eight percent of the patients will die from the acute heart failure.

The etiology of stress-induced cardiomyopathy remains speculative. Catecholamine excess leading to microvascular dysfunction or direct cardiomyocyte toxicity is hypothesized as the most likely etiology. This hypothesis is supported by the fact that most patients with Takotsubo's cardiomyopathy experience an intense physical or emotional stress. Furthermore, several other observations support this hypothesis. First, catecholamines levels are elevated in patients with stress induced cardiomyopathy at presentation when compared to patients with acute coronary syndrome. Second, multi-vessel coronary vasospasm and transient myocardial perfusion defects have been identified repeatedly in this population. Third, myocardial biopsies show myocarditis, interstitial fibrosis and mononuclear infiltrates, signs consistent with catecholamine toxicity. Fourth, in a mouse model, elevated epinephrine levels cause a switch from beta-2 adrenoreceptor mediated Gs protein signaling to Gi protein signaling, which is negatively inotropic. These findings all support the theory that there is altered sympathetic activity in patients with stress induced cardiomyopathy.

Thus, based on the existing knowledge base of this intriguing disease, an imaging approach that specifically evaluates the sympathetic activation state of the myocardium would appear to be ideally suited to further explore pathophysiology. I-123 radiolabeled metaiodobenzylguanidine, (mIBG) imaging allows for direct analysis of cardiac sympathetic function because it is structurally similar to norepinephrine (NE), and is transported into the cardiac sympathetic neurons by human norepinephrine transporter 1 ( hNET1), in the synaptic cleft. Unlike NE, mIBG is not metabolized by monoamine oxidase or catechol-o-methyl transferase. mIBG requires an intact myocardial sympathetic nervous system for uptake, is stored in the presynaptic vesicles and is released by stimulation with acetylcholine. Experimental manipulation of cardiac sympathetic function alters mIBG uptake and distribution. Planar imaging acquisition enables evaluation of sympathetic activation, while SPECT characterizes regional abnormalities. Measurement of the heart to mediastinal ratio during early and delayed planimetry assesses the initial uptake and washout of the tracer. mIBG uptake follows one of three general patterns: good uptake and retention, good uptake with washout or poor uptake. The different patterns likely represent the level of sympathetic activation, increase in sympathetic tone and heart failure-induced damage to the myocardial sympathetic nervous system. mIBG uptake is altered in patient with diabetic neuropathy, congestive heart failure, myocardial infarction. The uptake and washout patterns correlate with severity of neuropathy, severity of congestive heart failure, congestive heart failure treatment response, improvement in ejection fraction, cardiac death and ventricular arrhythmogenic potential.

Preliminary data in patients with Takotsubo's cardiomyopathy has shown decrease in mIBG uptake with an increased washout in the acute phase, with improved retention after left ventricular functional recovery. Furthermore, regional decrease in tracer uptake corresponds to the regional wall motion abnormalities. However, a systematic exploration of mIBG uptake patterns in consecutive patients with Takotsubo's cardiomyopathy has not been performed. Of note, PET imaging with 11C Hydroxyephedrine has described similar sympathetic dysfunction in Takotsubo's cardiomyopathy.