BACKGROUND:The management of catecholaminergic polymorphic ventricular tachycardia (CPVT) patients with drug refractory cardiac events (CEs) is challenging. OBJECTIVES:This study sought to assess the efficacy of left cardiac sympathetic denervation (LCSD) in 162 CPVT patients, focusing on those symptomatic without a high-risk genotype and compliant to therapy (main subanalysis, n = 118) of whom 41 had syncope on medical therapy. CEs included syncope, sudden cardiac arrest (SCA), sudden cardiac death (SCD), and appropriate implantable cardioverter-defibrillator (ICD) interventions. METHODS:A retrospective study including 162 CPVT patients (51% female, 80% probands, 79% RYR2 positive) who underwent LCSD worldwide. RESULTS:Most (n = 139; 85%) of the 162 patients experienced ≥1 CE before LCSD, 84 (52%) had CEs despite medical therapy, and 43 (27%) had previous SCA. Overall, 93% received a beta-blocker (nonselective in 85%), 53% both a beta-blocker and a class I antiarrhythmic drug, and 55% (89) had an ICD before LCSD. During a median of 48 months (Q1-Q3: 12-111 months) after LCSD, 28 of 162 patients (17%) had ≥1 CE, including 10 of 28 (36%) during noncompliance. Of the 118 patients (main subanalysis), 13% suffered CEs after LCSD, including 3 SCAs and 1 SCD despite an ICD (3%). Of the 41 with syncope on medical therapy, 6 (15%) experienced CEs after LCSD, including the SCD despite an ICD (3%). LCSD improved quality of life by reducing ICD shocks by 67% and electrical storms by 80%. CONCLUSIONS:Our data suggest that probably <5% of symptomatic CPVT patients compliant to medical therapy require an ICD after LCSD. ICDs do not reliably prevent SCD.

BACKGROUND AND AIMS:Modifier genes may cause different clinical phenotypes in patients with long QT syndrome (LQTS) carrying the same pathogenic variant. Variants in the MTMR4 gene have been previously associated, via patient-specific cardiomyocytes derived from induced pluripotent stem cells, with variable arrhythmic risk in a family with the p.Y111C-LQT1 mutation. This study aimed to evaluate the broader clinical impact of MTMR4 variants in patients with LQT1 and LQT2. METHODS:A total of 1192 LQTS patients were analysed: 638 with LQT1, 432 with LQT2, and 122 Swedish carriers of the p.Y111C-LQT1 variant. The association between MTMR4 variants and clinical severity was assessed by comparing patients with severe symptoms (cardiac arrest or syncope on beta-blockers) vs asymptomatic or mildly symptomatic individuals. ECG parameters, including Tpeak-Tend and T-wave heterogeneity, were also evaluated. RESULTS:In the LQT1 cohort, there was a significant decreasing pattern for cardiac events across MTMR4 genotypes (AA:15.9%, Aa:11.6%, aa:6.3%), while an opposite trend was apparent in the LQT2 cohort (AA:16.5%, Aa:18.2%, aa:27.6%). No pattern was apparent in the Swedish cohort. In the combined LQT1 cohort, the aa genotype was found in 15% of 702 mild/asymptomatic vs 1.7% of 58 with severe symptoms (P = .002). Vice versa, in LQT2, aa was more frequent in severe cases (24.1% vs 11.9%, P = .014). QTc was not associated with MTMR4, but the repolarization markers supported a gene-specific directionality of arrhythmic risk. CONCLUSIONS:The MTMR4 minor allele in homozygosis exerts a gene-specific and opposite impact on arrhythmic risk in LQTS. This finding should influence risk stratification in clinical practice.

BACKGROUND:Pathogenic/likely pathogenic variants in the KCNH2-encoded Kv11.1 potassium channel cause type 2 long QT syndrome (LQT2). Despite the updated 2015 American College of Medical Genetics (ACMG) variant interpretation guidelines, the burden of KCNH2 variants of uncertain significance (VUS) in patients evaluated for long QT syndrome (LQTS) remains ∼30%. Previously, we developed and validated phenotype-enhanced (PE) ACMG variant adjudication for type 1 long QT syndrome. OBJECTIVES:The purpose of this study was to determine whether a PE-ACMG variant classification approach can reduce the VUS burden in patients with clinically suspected LQT2. METHODS:Retrospective analysis was performed on 209 unique missense variants within KCNH2 from 2 LQTS specialty centers. Each variant was categorized based on the classification on the initial genetic test reports. Subsequently, all VUS were re-adjudicated with the use of a PE-ACMG framework that incorporates the patient's phenotype using the LQTS clinical diagnostic Schwartz score plus 2 LQT2-defining features: 1) biphasic/notches T waves; and 2) LQTS-triggered events during emotional stress or auditory stimuli. RESULTS:In total, 69/209 (33%) unique KCNH2 variants were classified as VUS based on their initial genetic test report. Mean Schwartz score for patients with a VUS was 3.6, and 41 patients (29%) had a score over 3.5. After PE-ACMG adjudication, 31/69 variants (45%) were upgraded to pathogenic, 18 (26%) to likely pathogenic, and 11 (16%) were downgraded to benign variants. Only 9 of 69 variants (13%) remained VUS. Overall, the VUS burden decreased from 69 of 209 (33%) to 9/209 (4%; P < 0.0001). CONCLUSIONS:Phenotype-guided variant adjudication significantly decreased the VUS burden of LQT2 case-derived KCNH2 missense variants from 2 LQTS specialty centers. There is clear value in incorporating LQT2-specific phenotype/clinical data to aid in the interpretation of KCNH2 missense variants identified during LQTS genetic testing, thereby facilitating prompt initiation of LQT2-guided therapy and cascade testing of appropriate relatives.

Aims The frequent occurrence of sudden death as the sentinel event in the long QT syndrome (LQTS) forces the search for still asymptomatic patients. We tested our hypothesis that complete fusion of the T and P waves (TP-fusion) at peak exercise might be a marker of likely LQTS. Methods and results A maximal exercise stress test, off-therapy, was performed by healthy athletes and genotype-positive LQTS patients. TP-fusion had to be complete in all precordial leads except V1. The study population (n = 578) included 310 healthy athletes (all with QTc <430 ms) and 268 LQTS patients. To deal only with clear-cut phenotypes, 64 athletes and 70 patients with incomplete TP-fusion were excluded. TP-fusion was present in 2% of controls and in 22% (P < 0.001) of the 198 LQTS patients. By limiting the analysis to subjects below age 25 (n = 262), the prevalence of TP-fusion increased to 3% in healthy athletes and to 32% in LQTS (P < 0.001). TP-fusion depends on the combination of QTc prolongation and fast heart rates; either alone is not sufficient. The appearance of TP-fusion predicted an 88% probability of being affected by LQTS. Importantly, of the 44 LQTS patients with TP-fusion, 24 (55%) had a borderline/normal baseline QTc (<460 ms). Conclusion TP-fusion, a qualitative trait more easily assessable at peak exercise than QTc prolongation, especially outside referral centres, unmasks a high probability of LQTS, even in subjects with normal baseline QTc. Further clinical assessment and genetic screening allow life-saving prophylactic therapy.

BACKGROUND:Congenital long QT syndrome (LQTS) is characterized by delayed ventricular repolarization, predisposing to potentially lethal ventricular arrhythmias. The variability in disease severity among patients remains largely unexplored, underscoring the limitations of current risk stratification methods. OBJECTIVE:We aimed to evaluate the potential utility of electrocardiographic markers from the exercise stress test (EST) in identifying patients with high-risk LQTS. METHODS:The study, which considered patients with LQTS type 1 and LQTS type 2, comprised a discovery cohort of 695 and a validation cohort of 635 patients. RESULTS:The change in corrected QT (QTc) interval between rest and recovery (between rest and 3-4 minutes into the recovery period, called recovery-rest ΔQTc) was consistently greater in symptomatic patients. Sensitivity analyses performed on EST data obtained on and off β-blockers as well as upon distinguishing between patients with a baseline QTc interval below and those above 470 ms demonstrated consistent findings. The association of recovery-rest ΔQTc with cardiac events remained significant in a subanalysis focusing on future events (ie, occurring after the EST). An optimal recovery-rest ΔQTc cutoff was determined for LQTS type 1 (35 ms) and LQTS type 2 (16 ms) separately and was shown to be significantly associated with cardiac events. CONCLUSION:Our findings suggest that in patients with LQTS, dynamic QT interval measures obtained during the EST are associated with lifetime arrhythmic events and events after the EST. Such measures can be helpful in identifying a higher-risk subset of patients with LQTS in order to optimize their management. Further research may confirm these findings in larger cohorts and explore the potential benefit of combining genetic and EST data for more precise risk stratification.

BACKGROUND:Approximately 15% to 20% of patients clinically diagnosed with long QT syndrome (LQTS) are genotype-negative (GEN-). Whether they have a different arrhythmic risk or should be managed differently remains unclear, often leading to incomplete treatment. OBJECTIVES:The purpose of this study was to compare clinical aspects of GEN- and genotype-positive (GEN+) LQTS patients. METHODS:We retrospectively evaluated 832 LQTS patients genetically screened in Japan (n = 347) and Italy (n = 485), including 698 with a disease-causing variant in the KCNQ1, KCNH2, and SCN5A genes (GEN+), and 134 without variants in these LQTS-related genes (GEN-). RESULTS:At diagnosis, the Japanese patients were more often probands (86% vs 60%), symptomatic (39% vs 18%), and of younger age than the Italian patients; conversely, they used less β-blockers (65% vs 95%), more rarely had a family history (FH) of LQTS (42% vs 73%), and had more cardiac events during follow-up (13% vs 4%) (P < 0.001 for all comparisons). Within the Japanese cohort, the GEN- had more cardiac arrests, used less β-blockers, and had much less FH for LQTS compared their GEN+ counterpart. The Italian cohort was more homogeneous, with just more LQTS FH among the GEN+. QTc shortening (close to 30 ms in all groups) during follow-up was similar between Japanese and Italian patients, irrespective of their being GEN+ or GEN-. In both cohorts, during an average follow-up of 6 and 7 years, respectively, GEN+ and GEN- patients showed a comparable clinical outcome. CONCLUSIONS:Arrhythmic risk is similar between GEN+ and GEN- LQTS patients; they should be managed and treated in the same way.

BACKGROUND:Despite major advances in the clinical management of long QT syndrome, some patients are not fully protected by beta-blocker therapy. Mexiletine is a well-known sodium channel blocker, with proven efficacy in patients with sodium channel-mediated long QT syndrome type 3. Our aim was to evaluate the efficacy of mexiletine in long QT syndrome type 2 (LQT2) using cardiomyocytes derived from patient-specific human induced pluripotent stem cells, a transgenic LQT2 rabbit model, and patients with LQT2. METHODS:Heart rate-corrected field potential duration, a surrogate for QTc, was measured in human induced pluripotent stem cells from 2 patients with LQT2 (KCNH2-p.A561V, KCNH2-p.R366X) before and after mexiletine using a multiwell multi-electrode array system. Action potential duration at 90% repolarization (APD90) was evaluated in cardiomyocytes isolated from transgenic LQT2 rabbits (KCNH2-p.G628S) at baseline and after mexiletine application. Mexiletine was given to 96 patients with LQT2. Patients were defined as responders in the presence of a QTc shortening ≥40 ms. Antiarrhythmic efficacy of mexiletine was evaluated by a Poisson regression model. RESULTS:After acute treatment with mexiletine, human induced pluripotent stem cells from both patients with LQT2 showed a significant shortening of heart rate-corrected field potential duration compared with dimethyl sulfoxide control. In cardiomyocytes isolated from LQT2 rabbits, acute mexiletine significantly shortened APD90 by 113 ms, indicating a strong mexiletine-mediated shortening across different LQT2 model systems. Mexiletine was given to 96 patients with LQT2 either chronically (n=60) or after the acute oral drug test (n=36): 65% of the patients taking mexiletine only chronically and 75% of the patients who performed the acute oral test were responders. There was a significant correlation between basal QTc and ∆QTc during the test (r= -0.8; P<0.001). The oral drug test correctly predicted long-term effect in 93% of the patients. Mexiletine reduced the mean yearly event rate from 0.10 (95% CI, 0.07-0.14) to 0.04 (95% CI, 0.02-0.08), with an incidence rate ratio of 0.40 (95% CI, 0.16-0.84), reflecting a 60% reduction in the event rate (P=0.01). CONCLUSIONS:Mexiletine significantly shortens cardiac repolarization in LQT2 human induced pluripotent stem cells, in the LQT2 rabbit model, and in the majority of patients with LQT2. Furthermore, mexiletine showed antiarrhythmic efficacy. Mexiletine should therefore be considered a valid therapeutic option to be added to conventional therapies in higher-risk patients with LQT2.

Background and Aims Risk scores are proposed for genetic arrhythmias. Having proposed in 2010 one such score (M-FACT) for the long QT syndrome (LQTS), this study aims to test whether adherence to its suggestions would be appropriate. Methods LQT1/2/3 and genotype-negative patients without aborted cardiac arrest (ACA) before diagnosis or cardiac events (CEs) below age 1 were included in the study, focusing on an M-FACT score >= 2 (intermediate/high risk), either at presentation (static) or during follow-up (dynamic), previously associated with 40% risk of implantable cardioverter defibrillator (ICD) shocks within 4 years. Results Overall, 946 patients (26 +/- 19 years at diagnosis, 51% female) were included. Beta-blocker (beta B) therapy in 94% of them reduced the rate of those with a QTc >= 500 ms from 18% to 12% (P < .001). During 7 +/- 6 years of follow-up, none died; 4% had CEs, including 0.4% with ACA. A static M-FACT >= 2 was present in 110 patients, of whom 106 received beta Bs. In 49/106 patients with persistent dynamic M-FACT >= 2, further therapeutic optimization (left cardiac sympathetic denervation in 55%, mexiletine in 31%, and ICD at 27%) resulted in just 7 (14%) patients with CEs (no ACA), with no CEs in the remaining 57. Additionally, 32 patients developed a dynamic M-FACT >= 2 but, after therapeutic optimization, only 3 (9%) had CEs. According to an M-FACT score >= 2, a total of 142 patients should have received an ICD, but only 22/142 (15%) were implanted, with shocks reported in 3. Conclusions Beta-blockers often shorten QTc, thus changing risk scores and ICD indications for primary prevention. Yearly risk reassessment with therapy optimization leads to fewer ICD implants (3%) without increasing life-threatening events.

BACKGROUND: Long QT syndrome is a lethal arrhythmia syndrome, frequently caused by rare loss-of-function variants in the potassium channel encoded by KCNH2. Variant classification is difficult, often because of lack of functional data. Moreover, variant-based risk stratification is also complicated by heterogenous clinical data and incomplete penetrance. Here we sought to test whether variant-specific information, primarily from high-throughput functional assays, could improve both classification and cardiac event risk stratification in a large, harmonized cohort of KCNH2 missense variant heterozygotes. METHODS: We quantified cell-surface trafficking of 18 796 variants in KCNH2 using a multiplexed assay of variant effect (MAVE). We recorded KCNH2 current density for 533 variants by automated patch clamping. We calibrated the strength of evidence of MAVE data according to ClinGen guidelines. We deeply phenotyped 1458 patients with KCNH2 missense variants, including QTc, cardiac event history, and mortality. We correlated variant functional data and Bayesian long QT syndrome penetrance estimates with cohort phenotypes and assessed hazard ratios for cardiac events. RESULTS: Variant MAVE trafficking scores and automated patch clamping peak tail currents were highly correlated (Spearman rank-order rho=0.69; n=433). The MAVE data were found to provide up to pathogenic very strong evidence for severe loss-of-function variants. In the cohort, both functional assays and Bayesian long QT syndrome penetrance estimates were significantly predictive of cardiac events when independently modeled with patient sex and corrected QT interval (QTc); however, MAVE data became nonsignificant when peak tail current and penetrance estimates were also available. The area under the receiver operator characteristic curve for 20-year event outcomes based on patient-specific sex and QTc (area under the curve, 0.80 [0.76-0.83]) was improved with prospectively available penetrance scores conditioned on MAVE (area under the curve, 0.86 [0.83-0.89]) or attainable automated patch clamping peak tail current data (area under the curve, 0.84 [0.81-0.88]). CONCLUSIONS: High-throughput KCNH2 variant MAVE data meaningfully contribute to variant classification at scale, whereas long QT syndrome penetrance estimates and automated patch clamping peak tail current measurements meaningfully contribute to risk stratification of cardiac events in patients with heterozygous KCNH2 missense variants.

Long QT syndrome (LQTS) is a lethal arrhythmia syndrome, frequently caused by rare loss-of-function variants in the potassium channel encoded by KCNH2. Variant-based risk stratification is complicated by heterogenous clinical data, incomplete penetrance, and low-throughput functional data. While variant-specific functional data can assist with variant classification, whether functional data can assist with prediction of outcomes is unknown.

BACKGROUND:Guideline-directed device therapy for long QT syndrome (LQTS) has evolved during the years, and indications for an implantable cardioverter-defibrillator (ICD) vary between professional cardiac societies. OBJECTIVE:We aimed to identify the subset of patients with LQTS who satisfied a class I or class II 2022 European Society of Cardiology guideline-based recommendation for an ICD and to determine the outcomes of those patients who received an ICD compared with those treated without an ICD. METHODS:Retrospective analysis was conducted of 2861 patients with LQT1, LQT2, or LQT3 to identify patients meeting contemporary recommendations for guideline-directed device therapy. Basic demographics, clinical characteristics, and frequency/type of breakthrough cardiac events (BCEs) were extracted, and outcomes/complications were compared between patients treated with an ICD and those treated without one. RESULTS:Of the 290 patients (approximately 10%) who met a guideline-based recommendation, 53 (18%) satisfied a class I/level B indication for an ICD; 56 (19%), a class I/level C indication; 19 (7%), a class IIa/level C indication; and 162 (56%), a class IIb/level B indication. However, most patients (156/290 [54%]) did not receive an ICD. Of those who received an ICD, 55 of 134 (41%) experienced ≥1 appropriate ventricular fibrillation-terminating ICD therapy, whereas ICD-related complications occurred in 13 patients (10%). Of those who were treated without an ICD, only 6 of 156 patients (4%) had nonlethal BCEs, which was significantly lower compared with the ICD group (P < .001). CONCLUSION:With >1200 years of combined follow-up, the experience and evidence from our 2 LQTS specialty centers suggest that many patients who satisfy a recommendation for an ICD based on the latest 2022 European Society of Cardiology guidelines may not need one. This is particularly true when the indication stemmed from a BCE while receiving beta blocker therapy or in asymptomatic patients with an increased 1-2-3-LQTS-Risk score.