A diagnostic detour: Myocarditis-associated junctional ectopic tachycardia in an infant

What We Learned.

• ▪Although rare, consider junctional ectopic tachycardia within the differential diagnosis of infants presenting with myocarditis and a usual complex tachycardia.
• ▪Management of the underlying myocardial disease can result in spontaneous resolution of the arrhythmia and allow discontinuation of antiarrhythmic therapy.
• ▪Ivabradine is an effective antiarrhythmic medication for junctional ectopic tachycardia that can help avoid toxicities related to amiodarone exposure in children.

Introduction

Junctional ectopic tachycardia (JET) is typically encountered in the postoperative setting. However, rare cases of nonoperative JET do occur and can be difficult to recognize given their rarity. The following case of nonoperative JET in an infant provides an opportunity to illustrate the diagnostic approach and key management principles of this tachyarrhythmia.

Case presentation

A previously well 5-week-old infant presented to a peripheral hospital with fever and underwent a septic workup. He was initiated on empiric antibiotics pending cultures that later returned negative results. On his third day of admission, he developed a usual complex tachycardia fluctuating between a rate of 220 to 270 beats/min that did not terminate with vagal maneuvers or adenosine. Esmolol infusion was initiated with the presumptive diagnosis of ectopic atrial tachycardia and increased to 200 μg/(kg⋅min) without effect.

Subsequently, the electrophysiology service was consulted and a higher dose of adenosine was administered that confirmed ventriculoatrial (VA) dissociation and no change in QRS morphology with sinus capture and junctional escape beats (Figure 1). This was quickly followed by recrudescence of the arrhythmia. As such, the diagnosis was changed to JET. His treatment was switched to amiodarone with a bolus of 1 mg/kg followed by an infusion at 5 μg/(kg⋅min), causing rapid conversion to sinus rhythm. His baseline electrocardiogram obtained during sinus rhythm was unremarkable. His echocardiogram obtained during sinus rhythm showed a structurally normal heart with moderate left ventricular systolic dysfunction (ejection fraction 31%), severe mitral regurgitation, and left atrial dilation. His peak troponin I level was 12,800 ng/mL, and the β-natriuretic peptide level was more than 35,000 ng/mL. A myocarditis workup was completed, and his nasopharyngeal viral swab returned positive for enterovirus. He required transient support with continuous positive airway pressure and milrinone.

Figure 1.

Usual complex tachycardia with ventriculoatrial dissociation. Adenosine administration (arrow) confirms ventriculoatrial dissociation and no change in QRS morphology with sinus capture and junctional escape beats. Atrial activity is marked by blue circles and ventricular activity by red lines.

His clinical status quickly stabilized with downtrending troponin and cessation of supportive therapies. Given the diagnosis of JET, he was transitioned from amiodarone to low-dose ivabradine after 2 days and remained in sinus rhythm for the remainder of his admission. On review, there was no significant family history of cardiac disease or arrhythmias, but a broad arrhythmia genetic panel was sent, which returned positive only for an unrelated variant of unknown significance in OBSCN. He was discharged from hospital after a 1-week stay on sole ivabradine therapy at 0.05 mg/kg per dose twice daily. During follow-up, his ventricular function normalized within 2 weeks. Given the lack of arrhythmia recurrence, his ivabradine dose was not adjusted and was discontinued 3 months later, without relapse of JET seen on subsequent surveillance Holter monitoring during medium-term follow-up of 9 months.

Discussion

JET results from abnormal automaticity or triggered activity in the atrioventricular node or His bundle.¹ Typical of automatic rhythms, it will demonstrate gradual onset and offset and rate variability. The QRS complex will be identical to baseline, which can be confirmed during sinus capture beats. Either VA dissociation with a greater number of ventricular than atrial complexes or 1:1 VA association can be seen. Adenosine administration into JET with resultant VA dissociation will not terminate the arrhythmia and can demonstrate that the atria are not required to sustain the tachycardia.

Based on etiology, JET can be divided into postoperative and nonoperative forms. Postoperative JET is more common, occurring after surgical repair of congenital heart disease and is typically self-limiting.¹ It is associated with several congenital heart disease repairs and other risk factors and can be effectively treated with antiarrhythmic medications with low rates of morbidity and mortality in the contemporary era.

Nonoperative JET is a much rarer disease that is usually congenital and mostly associated with autosomal dominant inheritance of TNNI3K variants.^1,2 A few case reports have previously associated JET with cardiac tumors, myocarditis, and congenital heart block. Congenital JET, with presentation at less than 6 months of age, has traditionally been associated with high morbidity and mortality and a refractoriness to antiarrhythmic therapies. Complete suppression of JET with antiarrhythmic medication is uncommon, and typically 2 or more medications are required to achieve some level of control.³ While amiodarone was historically most commonly used, ivabradine has more recently been shown to be effective in the treatment of congenital JET with a more favorable safety profile.⁴ The incessant nature and medical refractoriness of congenital JET leading to tachycardia-induced cardiomyopathy necessitates ablation in some children, which is an effective and safe therapy.³ While spontaneous remission of JET can occur, in the absence of curative ablation, congenital JET often requires long-term medical therapy.³

In our patient, several factors led to the presumption of this being an unusual case of enterovirus myocarditis associated with JET. His laboratory and echocardiographic findings were consistent with acute myocarditis. The resolution of his tachyarrhythmia with normalization of his ventricular function, the rapid response to antiarrhythmic monotherapy, and his ability to wean off ivabradine in short-term follow-up pointed toward an acute inciting event causing JET rather than the incessant congenital JET typically seen within this age group. The lack of a TNNI3K variant on genetic testing similarly provided further support. Importantly, consistent with recent literature, ivabradine was effective and avoided exposure to the potential toxicities of amiodarone therapy. We anticipate his discharge from further ambulatory follow-up if he remains symptom-free.