High-rate atrial pacing as an innovative bridging therapy in a neonate with congenital long QT syndrome

Long QT syndrome in neonates: conduction disorders associated with HERG mutations and sinus bradycardia with KCNQ1 mutations

OBJECTIVES

We hypothesized that neonatal long QT syndrome (LQTS) with 2:1 atrioventricular block (AVB) could be related to HERG mutations.

BACKGROUND

Early onset of LQTS is rare but carries a high risk of life-threatening events such as ventricular arrhythmias and conduction disorders. There are no data on possible gene specificity.

METHODS

We analyzed the characteristics and outcomes of 23 neonate probands from our LQTS population. Samples of DNA were available in 18 cases.

RESULTS

Long QT syndrome was diagnosed because of corrected QT interval (QTc) prolongation (mean QTc of 558 +/- 62 ms) and neonatal bradycardia attributable to sinus bradycardia (n = 8) or 2:1 AVB (n = 15). Symptoms included syncope (n = 2), torsades de pointes (n = 7), and hemodynamic failure (n = 6). Three infants with 2:1 AVB died during the first month of life. During the neonatal period, all living patients received beta-blockers (BB) and 13 had a combination of BB and permanent cardiac pacing. Under treatment, patients remained asymptomatic, with a mean follow-up of seven years. Mutations were identified in HERG (n = 8) and KCNQ1 (n = 8), and one child had three mutations (HERG, KCNQ1, and SCN5A). Conduction disorders were associated with LQT2, whereas sinus bradycardia was associated with LQT1.

CONCLUSIONS

Two-to-one AVB seems preferentially associated with HERG mutations, either isolated or combined. Long QT syndrome with relative bradycardia attributable to 2:1 AVB has a poor prognosis during the first month of life. In contrast, sinus bradycardia seems to be associated with KCNQ1 mutations, with a good short-term prognosis under BB therapy.

Evaluation and treatment of syncope in infants

Syncope in the infant and newborn occurs as a loss of consciousness due to a variety of etiologies. Because syncope at this age may be a harbinger of sudden infant death, the symptom provokes anxiety and challenges clinicians to identify those babies with an increased risk for life threatening events. Recently introduced diagnostic tests and advances in molecular biology offer promising potential, but the population at risk remains unknown. Controversy surrounds: many potential risk factors; the value of home monitoring; and appropriate preventive and therapeutic strategies. This article reviews the differential diagnosis of syncope in children less than 18 months of age, with particular attention to those diagnoses and problems specific to the evaluation and treatment in this age group. Recommendations are presented for an efficient evaluation, which must include a careful history, complete physical examination and thorough investigation of the family history and home environment. In addition, specific diagnostic tests and a practical approach to treatment are suggested.

Homozygous SCN5A mutation in long-QT syndrome with functional two-to-one atrioventricular block

Heterozygous mutations in genes encoding cardiac ionic channel subunits KCNQ1, HERG, SCN5A, KCNE1, and KCNE2 are causally involved in the dominant form of long-QT syndrome (LQTS) while homozygous mutations in KCNQ1 and KCNE1 cause LQTS with or without congenital deafness. In addition, two homozygous HERG mutations have been associated with severe LQTS with functional atrioventricular conduction anomalies in young children. A 2:1 atrioventricular block (AVB) with a major QTc prolongation (526 ms) was evidenced in a 5-year-old boy referred for syncope and seizure. LQTS was diagnosed and beta-blocking therapy initiated leading to normal atrioventricular conduction. Electrophysiological study provided support that location of the AVB was infra-Hisian. DNA analysis was performed in the proband and in asymptomatic family members. A novel missense mutation, V1777M, in the early C-terminal domain of SCN5A was identified. The proband was homozygous while the parents and two siblings were heterozygous carriers. Homozygote and heterozygote expression of the mutant channels in tsA201 mammalian cells resulted in a persistent inward sodium current of 3.96+/-0.83% and 1.49+/-0.47% at -30 mV, respectively, which was dramatically reduced in the presence of tetrodotoxin. This study provides the first evidence for a homozygous missense mutation in SCN5A and suggests that LQTS with functional 2:1 AVB in young children, a severe phenotype associated with bad prognosis, may be caused by homozygous or heterozygous compound mutations not only in HERG but also in SCN5A. The full text of this article is available at http://www.circresaha.org.

Radiofrequency ablation of right-sided accessory pathways in pediatric patients

Right free-wall and septal accessory pathways encompass the full spectrum of accessory pathway electrophysiology and are situated in complex anatomical arrangements. Understanding this diversity of physiology is necessary for the successful and safe elimination of these connections with transcatheter radiofrequency ablation. When radiofrequency catheter ablation of these pathways is attempted in children, anatomical relationships often become more complex, and spatial constraints require more adaptive techniques than in adults. It is clear that considerable progress has been made with radiofrequency catheter ablation, such that it is now first-line therapy for most children who have been diagnosed with one of the broad spectrum of clinical manifestations that result from the presence of these accessory connections. This review will discuss how accessory pathway electrophysiology and anatomy impact the clinical syndromes observed in children, and how these factors, as well as others particular to children, determine the approach, results and potential long-term consequences of radiofrequency catheter ablation of right-sided accessory pathways in the pediatric population.

Arrhythmias in the congenital long QT syndrome: how often is torsade de pointes pause dependent?

OBJECTIVE

To determine the frequency and predictors of pause dependent torsade de pointes among patients with the congenital long QT syndrome and spontaneous ventricular tachyarrhythmias.

DESIGN

The literature on the "congenital long QT" was reviewed. Articles with illustrations demonstrating the onset of spontaneous polymorphic ventricular arrhythmias in the absence of arrhythmogenic drugs were included.

RESULTS

Illustrations of 62 spontaneous episodes of torsade de pointes among patients with congenital long QT syndrome were found in the literature. The majority (74%) of documented arrhythmias were "pause dependent"; 82% of these pauses were longer than the basic cycle length by > 100 ms. Age and sex correlated with the mode of arrhythmia initiation. Arrhythmias in infants (</= 3 years old) were not pause dependent, while female sex correlated with pause dependent torsade. Using multivariate analysis, age was the only independent predictor of the mode of onset of torsade de pointes.

CONCLUSION

Available data suggest that the majority of spontaneous arrhythmias in the congenital long QT syndrome are pause dependent. Torsade de pointes that is not preceded by pauses appears to be limited to patient subgroups with severe forms of the disease, like symptomatic infants. These findings have important implications regarding the use of cardiac pacing for arrhythmia prevention.

Cardiac pacing in the long QT syndrome: review of available data and practical recommendations

A review of published data on cardiac pacing in the long QT syndrome (LQTS) is presented, in the hope that optimization of patient selection and pacemaker programming will prevent arrhythmic death. LQT3 patients may derive particular benefit from pacing because the dispersion of repolarization worsens steeply during bradycardia in this genotype. However, concluding that other genotypes will not benefit from pacing is premature. Pacing may be especially beneficial for patients with "pause-dependent" arrhythmias. Programming should include a sufficiently fast lower rate limit. Features that allow heart rate slowing beyond the lower rate limit or that may trigger pauses must be programmed "off" because pauses are proarrhythmic in this population. Pause-prevention pacing algorithms may be beneficial.

Use of an implantable cardioverter defibrillator in an eight-month-old infant with ventricular fibrillation arising from a myocardial fibroma

We report our experience with use of a ICD in a 7-month-old infant who presented with VF. We utilized an epicardial patch and active generator in the abdomen. Development of mediastinitis required explantation and eventual replacement with a subcutaneous patch and active generator in the abdomen.

Two-to-one AV block associated with the congenital long QT syndrome

INTRODUCTION

Conduction abnormalities associated with long QT syndrome (LQTS) have been reported as "pseudo 2:1 AV block" due to sinus intervals shorter than ventricular refractoriness.

METHOD AND RESULTS

We report the electrophysiologic characteristics of a patient suffering from congenital LQTS with episodes of true 2:1 AV block. Induction of 2:1 infra-Hisian blocks and return to 1:1 conduction were observed using single atrial and ventricular extrastimuli. The block was located in the Purkinje network but not in the myocardium.

CONCLUSION

The His-Purkinje system of our LQTS patient displayed dynamic properties with a strong increase in refractoriness for short-long sequences and a decrease for long-short sequences that triggered intermittent 2:1 AV blocks.