The Influence of Genotype on the Phenotype, Clinical Course, and Risk of Adverse Events in Children with Hypertrophic Cardiomyopathy.

Genetic testing in children with hypertrophic cardiomyopathy (HCM) can modify clinical management and lifestyle counseling. However, predicting long-term outcome and response to management in individual patients remains challenging, because of the peculiar genetic heterogeneity of the disease in the pediatric age range. Children with HCM secondary to an inborn error of metabolism or malformation syndromes tend to have a worse outcome compared with those with the classic sarcomeric form. Among the latter, adverse genetic features are represented by the identification of a pathogenic variant in MYH7, often associated with severe hypertrophy, a complex genotype, or a de novo variant.

Diagnosis and Management of Cardiovascular Involvement in Friedreich Ataxia.

Friedreich ataxia (FRDA) is an autosomal recessive neurodegenerative disorder caused by a homozygous GAA triplet repeat expansion in the frataxin gene. Cardiac involvement, usually manifesting as hypertrophic cardiomyopathy, can range from asymptomatic cases to severe cardiomyopathy with progressive deterioration of the left ventricular ejection fraction and chronic heart failure. The management of cardiac involvement is directed to prevent disease progression and cardiovascular complications. However, direct-disease therapies are not currently available for FRDA. The present review aims to describe the current state of knowledge regarding cardiovascular involvement of FRDA, focusing on clinical-instrumental features and management of cardiac manifestation.

Aortic Arch Interruption and Persistent Fifth Aortic Arch in Phace Syndrome: Prenatal Diagnosis and Postnatal Course.

PHACE is a rare congenital neurocutaneous syndrome where posterior fossa malformations, hemangiomas, cerebrovascular anomalies, aortic arch anomalies, cardiac defects, and eye abnormalities are variably associated. We describe the prenatal detection and the postnatal course of a child with PHACE syndrome with a unique type of aortic arch anomaly consisting of proximal interruption of the aortic arch and persistence of the fifth aortic arch. The fifth aortic arch represented in this case a vital systemic-to-systemic connection between the ascending aorta and the transverse portion of the aortic arch allowing adequate forward flow through the aortic arch without surgical treatment.

Aortic Dilatation in Pediatric Patients with Bicuspid Aortic Valve: How the Choice of Nomograms May Change Prevalence.

BACKGROUND: Aortic dilation (AoD) is commonly reported in patients with bicuspid aortic valve (BAV) and has been related to flow abnormalities and genetic predisposition. AoD-related complications are reported to be extremely rare in children. Conversely, an overestimate of AoD related to body size may lead to excess diagnoses and negatively impact quality of life and an active lifestyle. In the present study, we compared the diagnosis performance of the newly introduced Q-score (based on a machine-learning algorithm) versus the traditional Z-score in a large consecutive pediatric cohort with BAV. MATERIALS AND METHODS: Prevalence and progression of AoD were evaluated in 281 pediatric patients ages > 5 and < 18 years at first observation, 249 of whom had isolated BAV and 32 had BAV associated with aortic coarctation (CoA-BAV). An additional group of 24 pediatric patients with isolated CoA was considered. Measurements were made at the level of the aortic annulus, Valsalva sinuses, sinotubular aorta, and proximal ascending aorta. Both Z-scores using traditional nomograms and the new Q-score were calculated at baseline and at followup (mean 4.5 years). RESULTS: A dilation of the proximal ascending aorta was suggested by traditional nomograms (Z-score > 2) in 31.2% of patients with isolated BAV and 18.5% with CoA-BAV at baseline and in 40.7% and 33.3%, respectively, at followup. No significant dilation was found in patients with isolated CoA. Using the new Q-score calculator, ascending aorta dilation was detected in 15.4% of patients with BAV and 18.5% with CoA-BAV at baseline and in 15.8% and 3.7%, respectively, at followup. AoD was significantly related to the presence and degree of aortic stenosis (AS) but not to aortic regurgitation (AR). No AoD-related complications occurred during the followup. CONCLUSIONS: Our data confirm the presence of ascending aorta dilation in a consistent subgroup of pediatric patients with isolated BAV, with progression during followup, while AoD was less common when CoA was associated with BAV. A positive correlation was found with the prevalence and degree of AS, but not with AR. Finally, the nomograms used may significantly influence the prevalence of AoD, especially in children, with a possible overestimation by traditional nomograms. This concept requires prospective validation in long-term followup.

Cardiac Involvement in Classical Organic Acidurias: Clinical Profile and Outcome in a Pediatric Cohort.

BACKGROUND: Cardiac involvement is reported in a significant proportion of patients with classical organic acidurias (OAs), contributing to disability and premature death. Different cardiac phenotypes have been described, among which dilated cardiomyopathy (DCM) is predominant. Despite recent progress in diagnosis and treatment, the natural history of patients with OAs remains unresolved, specifically with regard to the impact of cardiac complications. We therefore performed a retrospective study to address this issue at our Referral Center for Pediatric Inherited Errors of Metabolism. METHODS: Sixty patients with OAs (propionic (PA), methylmalonic (MMA) and isovaleric acidemias and maple syrup urine disease) diagnosed from 2000 to 2022 were systematically assessed at baseline and at follow-up. RESULTS: Cardiac anomalies were found in 23/60 OA patients, all with PA or MMA, represented by DCM (17/23 patients) and/or acquired long QT syndrome (3/23 patients). The presence of DCM was associated with the worst prognosis. The rate of occurrence of major adverse cardiac events (MACEs) at 5 years was 55% in PA with cardiomyopathy; 35% in MMA with cardiomyopathy; and 23% in MMA without cardiomyopathy. Liver transplantation was performed in seven patients (12%), all with PA or MMA, due to worsening cardiac impairment, and led to the stabilization of metabolic status and cardiac function. CONCLUSIONS: Cardiac involvement was documented in about one third of children diagnosed with classical OAs, confined to PA and MMA, and was often associated with poor outcome in over 50%. Etiological diagnosis of OAs is essential in guiding management and risk stratification.

Genetic Testing and Counselling in Hypertrophic Cardiomyopathy: Frequently Asked Questions.

Genetic counselling and genetic testing in hypertrophic cardiomyopathy (HCM) represent an integral part of the diagnostic algorithm to confirm the diagnosis, distinguish it from phenocopies, and suggest tailored therapeutic intervention strategies. Additionally, they enable cascade genetic testing in the family. With the implementation of Next Generation Sequencing technologies (NGS), the interpretation of genetic data has become more complex. In this regard, cardiologists play a central role, aiding geneticists to correctly evaluate the pathogenicity of the identified genetic alterations. In the ideal setting, geneticists and cardiologists must work side by side to diagnose HCM as well as convey the correct information to patients in response to their many questions and concerns. After a brief overview of the role of genetics in the diagnosis of HCM, we present and discuss the frequently asked questions by HCM patients throughout our 20-year genetic counselling experience. Appropriate communication between the team and the families is key to the goal of delivering the full potential of genetic testing to our patients.

Real-World Use and Predictors of Response to Disopyramide in Patients with Obstructive Hypertrophic Cardiomyopathy.

Background: Although disopyramide has been widely used to reduce left ventricular outflow obstruction (LVOTO) and to improve symptoms in patients with obstructive hypertrophic cardiomyopathy (oHCM), its use in real world as well as patient characteristics associated with a positive treatment response are still unclear. Methods: From 1980 to 2021, 1527 patients with HCM were evaluated and 372 (23%) had a LVOTO with active follow-up. The efficacy and safety of disopyramide were assessed systematically during 12 months (2-, 6-, and 12-month visits). Responders were patients with a final NYHA = I and a LVOTO < 30 mmHg; incomplete responders were those patients with NYHA > I and a LVOTO < 30 mmHg; and non-responders were symptomatic patients with no change in functional class NYHA and a LVOT gradient > 30 mmHg. Results: Two-hundred-fifty-four (66%) patients were in functional class NYHA I/II and 118 (34%) in NYHA III/IV. A total of 118/372 (32%, 55 ± 16 years) underwent disopyramide therapy. Twenty-eight (24%) patients responded to therapy, 39 (33%) were incomplete responders, and 51 (43%) did not respond. Responder were mainly patients in functional NYHA class I/II (24/28, 86%), whereas incomplete responders and non-responders were more often in functional NYHA class III/IV (50/54 (93%)). An independent predictor of response to disopyramide treatment was the presence of NYHA I/II at the initiation of therapy (HR 1.5 (95% CI 1.1-4.5), p = 0.03). No major life-threatening arrhythmic events or syncope occurred, despite 19 (16%) patients showing reduced QTc from baseline, 19 (16%) having no difference, while 80 (69%) patients had prolonged QTc interval. Thirty-one (26%) patients experienced side effects, in particular, 29 of the anticholinergic type. Conclusions: Disopyramide was underused in oHCM but effective in reducing LVOTO gradients and symptoms in slightly symptomatic patients with less severe disease phenotype with a safe pro-arrhythmic profile.

Safety and efficacy of ranolazine in hypertrophic cardiomyopathy: Real-world experience in a National Referral Center.

OBJECTIVES: We assessed the efficacy and safety of ranolazine in real-world patients with hypertrophic cardiomyopathy (HCM). BACKGROUND: Ranolazine is an anti-anginal drug that inhibits the late phase of the inward sodium current. In a small prospective trial, ranolazine reduced the arrhythmic burden and improved biomarker profile in HCM patients. However, systematic reports reflecting real-world use in this setting are lacking. METHODS: Changes in clinical and instrumental features, symptoms and arrhythmic burden were evaluated in 119 patients with HCM before and during treatment with ranolazine at a national referral centre for HCM. RESULTS: Patients were treated with ranolazine for 2 [1-4] years; 83 (70%) achieved a dosage ≥1000 mg per day. Treatment interruption was necessary in 24 patients (20%) due to side effects (n = 10, 8%) or disopyramide initiation (n = 8, 7%). Seventy patients (59%) were treated with ranolazine for relief of angina. Among them, 51 (73%) had total symptomatic relief and 47 patients (67%) showed ≥2 Canadian Cardiovascular society (CCS) angina grade improvement. Sixteen patients (13%) were treated for recurrent ventricular arrhythmias, including 4 with a clear ischemic trigger, who experienced no further arrhythmic episodes while on ranolazine. Finally, 33 patients (28%) were treated for heart failure associated with severe diastolic dysfunction: no symptomatic benefit could be observed in this group. CONCLUSION: Ranolazine was safe and well tolerated in patients with HCM. The use of ranolazine may be considered in patients with HCM and microvascular angina.

Focus on Paediatric Restrictive Cardiomyopathy: Frequently Asked Questions.

Restrictive cardiomyopathy (RCM) is characterized by restrictive ventricular pathophysiology determined by increased myocardial stiffness. While suspicion of RCM is initially raised by clinical evaluation and supported by electrocardiographic and echocardiographic findings, invasive hemodynamic evaluation is often required for diagnosis and management of patients during follow-up. RCM is commonly associated with a poor prognosis and a high incidence of heart failure, and PH is reported in paediatric patients with RCM. Currently, only a few therapies are available for specific RCM aetiologies. Early referral to centres for advanced heart failure treatment is often necessary. The aim of this review is to address questions frequently asked when facing paediatric patients with RCM, including issues related to aetiologies, clinical presentation, diagnostic process and prognosis.

[Clinical pathway on pediatric cardiomyopathies: a genetic testing strategy proposed by the Italian Society of Pediatric Cardiology]. / Indicazioni all'esecuzione del test genetico nella diagnosi delle cardiomiopatie ad esordio pediatrico: percorso clinico della Società Italiana di Cardiologia Pediatrica.

Pediatric cardiomyopathies are rare diseases, heterogeneous in clinical presentation, etiology and prognosis. Etiological diagnosis, where genetic analysis plays a key role, is of fundamental importance for defining diagnostic and therapeutic pathways. Furthermore, the identification of the genetic substrate represents a prerequisite for cascade screening in the proband's family members and to allow conscious reproductive choices. To date, genetic testing is performed with the analysis of gene panels (targeted panels) or with the study of the entire exome (whole exome sequencing) using next generation sequencing (NGS) technology. The great genetic heterogeneity and the temporal variability of the clinical manifestations lead to unique problems for pediatric cardiomyopathies, distinct from those of the adult, such as the possible indications for access to the test, the type of test to be used (exome or panel of genes), the importance of analyzing parents, especially in cases with neonatal onset; moreover, the correct execution of bioinformatics analysis and the interpretation of NGS data play a crucial role in the impact of the results on clinical management.

The role of the electrocardiographic phenotype in risk stratification for sudden cardiac death in childhood hypertrophic cardiomyopathy.

AIMS: The 12-lead electrocardiogram (ECG) is routinely performed in children with hypertrophic cardiomyopathy (HCM). An ECG risk score has been suggested as a useful tool for risk stratification, but this has not been independently validated. This aim of this study was to describe the ECG phenotype of childhood HCM in a large, international, multi-centre cohort and investigate its role in risk prediction for arrhythmic events. METHODS AND RESULTS: Data from 356 childhood HCM patients with a mean age of 10.1 years (±4.5) were collected from a retrospective, multi-centre international cohort. Three hundred and forty-seven (97.5%) patients had ECG abnormalities at baseline, most commonly repolarization abnormalities (n = 277, 77.8%); left ventricular hypertrophy (n = 240, 67.7%); abnormal QRS axis (n = 126, 35.4%); or QT prolongation (n = 131, 36.8%). Over a median follow-up of 3.9 years (interquartile range 2.0-7.7), 25 (7%) had an arrhythmic event, with an overall annual event rate of 1.38 (95% CI 0.93-2.04). No ECG variables were associated with 5-year arrhythmic event on univariable or multivariable analysis. The ECG risk score threshold of >5 had modest discriminatory ability [C-index 0.60 (95% CI 0.484-0.715)], with corresponding negative and positive predictive values of 96.7% and 6.7. CONCLUSION: In a large, international, multi-centre cohort of childhood HCM, ECG abnormalities were common and varied. No ECG characteristic, either in isolation or combined in the previously described ECG risk score, was associated with 5-year sudden cardiac death risk. This suggests that the role of baseline ECG phenotype in improving risk stratification in childhood HCM is limited.

[Bicuspid aortic valve and associated aortopathy: from clinical examination to advanced multimodality imaging]. / Valvola aortica bicuspide e aortopatia associata: dall'esame clinico all'imaging multimodale avanzato.

Bicuspid aortic valve (BAV) is the most common congenital heart disease, affecting 0.5-2% of the general population. It is often associated with aortopathy with aneurysmal dilation of the ascending aorta (AA). Up to 75% of patients with coarctation of the aorta (CoA) present with BAV and its diagnosis is crucial because it can increase the risk for aortic complications. CoA is a cardiac malformation that can be undiagnosed until old age with only hypertension as a marker of its presence.We present the case of an asymptomatic 17-year-old boy who underwent regular visit for competitive sports activity. During a treadmill test he showed a hypertensive response to effort and the echocardiogram diagnosed a normally functioning BAV associated with AA dilation. The clinical suspicion of CoA was confirmed by a complete echocardiogram, and the patient underwent catheter-based invasive aortography and the CoA was treated with angioplasty and stenting. Few years after the interventional procedure, he performed a thoracic computed tomography that showed an increased aortic dilation at the level of the Valsalva sinus and the AA. The patient underwent surgical aortic repair using the David technique. At present, the patient continues a close follow-up with advanced cardiac imaging (including cardiac magnetic resonance) for BAV surveillance and accurate detection of all aortic measures.

Clinical presentation and long-term outcomes of infantile hypertrophic cardiomyopathy: a European multicentre study.

AIMS: Children presenting with hypertrophic cardiomyopathy (HCM) in infancy are reported to have a poor prognosis, but this heterogeneous group has not been systematically characterized. This study aimed to describe the aetiology, phenotype, and outcomes of infantile HCM in a well-characterized multicentre European cohort. METHODS AND RESULTS: Of 301 children diagnosed with infantile HCM between 1987 and 2019 presenting to 17 European centres [male n = 187 (62.1%)], underlying aetiology was non-syndromic (n = 138, 45.6%), RASopathy (n = 101, 33.6%), or inborn error of metabolism (IEM) (n = 49, 16.3%). The most common reasons for presentation were symptoms (n = 77, 29.3%), which were more prevalent in those with syndromic disease (n = 62, 61.4%, P < 0.001), and an isolated murmur (n = 75, 28.5%). One hundred and sixty-one (53.5%) had one or more co-morbidities. Genetic testing was performed in 163 (54.2%) patients, with a disease-causing variant identified in 115 (70.6%). Over median follow-up of 4.1 years, 50 (16.6%) underwent one or more surgical interventions; 15 (5.0%) had an arrhythmic event (6 in the first year of life); and 48 (15.9%) died, with an overall 5 year survival of 85%. Predictors of all-cause mortality were an underlying diagnosis of IEM [hazard ratio (HR) 4.4, P = 0.070], cardiac symptoms (HR 3.2, P = 0.005), and impaired left ventricular systolic function (HR 3.0, P = 0.028). CONCLUSIONS: This large, multicentre study of infantile HCM describes a complex cohort of patients with a diverse phenotypic spectrum and clinical course. Although overall outcomes were poor, this was largely related to underlying aetiology emphasizing the importance of comprehensive aetiological investigations, including genetic testing, in infantile HCM.

Impact of cardiovascular involvement on the clinical course of paediatric mitochondrial disorders.

BACKGROUND: Primary mitochondrial disorders (PMD) are rare conditions resulting in progressive multi-organ failure. Cardiovascular involvement (CVI) has been reported in paediatric patients. However, its age-related prevalence, clinical presentation and prognostic impact are unresolved. We detailed CVI in a cohort of children diagnosed with PMD over two decades at a tertiary referral centre. RESULTS: We enrolled 86 PMD patients (M/F = 30/56; mean age 6.4 ± 8.58 years). CVI was detected in 31 patients (36%), with mean age at onset of 5.7 ± 7.8 years including the pre- and neonatal phase in 14, often representing the first sign of PMD (42% of those with CVI). Heart disease resulted more common in males and in children with specific aetiologies (Barth, TMEM70 and MELAS syndromes). Hypertrophic, non-compaction and dilated cardiomyopathies were the prevalent disorders, although pulmonary arterial hypertension was also found. Adverse cardiac events (heart failure, resuscitated cardiac arrest, ICD/PM implantation, sudden death) occurred in 19% of children with CVI over a follow-up period of 5.4 ± 4.3 years. All-cause mortality was higher in patients with CVI compared to those without CVI (45.1% vs 21.8%; p < 0.01); female sex, age at onset < 5 years, acute heart failure at presentation and diabetes also proved independent predictors of outcome. CONCLUSION: Cardiovascular involvement occurred in over one-third of children diagnosed with PMD, often at a very early age, and was associated with adverse prognosis. Final outcome of PMD-related CVI was influenced by the specific underlying aetiology, suggesting the need for tailored management of heart failure and sudden death prevention.

A rare case of pediatric cardiomyopathy: Alström syndrome identified by gene panel analysis.

Genetic investigation of early-onset Dilatative cardiomyopathy phenotype, including molecular autopsy, is the key to appropriate recognition and management of rare etiologies and atypical presentations and to offer genetic counseling to the family.

[Clinical pathway for cardiomyopathies: a genetic testing strategy proposed by ANMCO in Tuscany]. / Percorso clinico proposto dall'ANMCO Toscana per la diagnosi genetica delle cardiomiopatie in un sistema assistenziale in rete.

Hereditary cardiomyopathies, hypertrophic cardiomyopathy, dilated cardiomyopathy, arrhythmogenic cardiomyopathy, restrictive cardiomyopathy and left ventricular noncompaction, are clinically and genetically very heterogeneous diseases, and they represent a frequent cause of cardiac arrest and sudden death. To date, over 100 genes are known to be associated with the onset of cardiomyopathies. Genetic testing is performed by next generation sequencing, a technology that has made it possible to analyze hundreds of genes in many patients simultaneously, shortening costs and execution times. However, with the use of this technology, new problems have arisen regarding the indications for access to the test, the interpretation of the data and the clinical implications of the results.This document aims to represent an operational support tool for hospital cardiologists to make the use of genetic testing more accessible and appropriate for their patients with suspected or ascertained hereditary cardiomyopathy.

Sex-related differences in exercise performance and outcome of patients with hypertrophic cardiomyopathy.

AIMS: Exercise performance is known to predict outcome in hypertrophic cardiomyopathy (HCM), but whether sex-related differences exist is unresolved. We explored whether functional impairment, assessed by exercise echocardiography, has comparable predictive accuracy in females and males with HCM. METHODS: We retrospectively evaluated 292 HCM patients (46 ± 16 years, 72% males), consecutively referred for exercise echocardiography; 242 were followed for 5.9 ± 4.2 years. RESULTS: Peak exercise capacity was 6.5 ± 1.6 metabolic equivalents (METs). Sixty patients (21%) showed impaired exercise capacity (≤5 METs). Exercise performance was reduced in females, compared with males (5.6 ± 1.6 vs 6.9 ± 1.5 METs, p < 0.001; peak METs ≤ 5 in 40% vs 13%, p < 0.001), largely driven by a worse performance in women >50 years of age. At multivariable analysis, female sex was independently associated with impaired exercise capacity (odds ratio: 4.67; 95% confidence interval (CI): 1.83-11.90; p = 0.001). During follow-up, 24 patients (10%) met the primary endpoint (a combination of cardiac death, heart failure requiring hospitalization, sustained ventricular tachycardia, appropriate implantable cardioverter defibrillator discharge, resuscitated sudden cardiac death and cardioembolic stroke). Event-free survival was reduced in females (p = 0.035 vs males). Peak METs were inversely related to outcome in males (hazard ratio (HR) per unit increase: 0.57; 95% CI: 0.39-0.84; p = 0.004) but not in females (HR: 1.22; 95% CI: 0.66-2.24; p = 0.53). CONCLUSIONS: Female patients with HCM showed significant age-related impairment in functional capacity compared with males, particularly evident in post-menopausal age groups. While women were at greater risk of HCM-related complications and death, impaired exercise capacity predicted adverse outcome only in men. These findings suggest the need for sex-specific management strategies in HCM.

Timing of invasive septal reduction therapies and outcome of patients with obstructive hypertrophic cardiomyopathy.

BACKGROUND: Whether early vs. delayed referral to septal reduction therapies (SRT, alcohol septal ablation or surgical myectomy) bears prognostic relevance in hypertrophic obstructive cardiomyopathy (HOCM) is unresolved. We analyzed the impact of SRT timing on the outcome of HOCM patients. METHODS: We followed 126 patients for 5 ±â€¯4 years after SRT (mean age 53 ±â€¯15 years; 55 post-ASA and 71 post-SM). Based on time-to-treatment (TTT; from HOCM diagnosis to SRT), patients were divided into three groups: "<3" years, N = 50; "3-5" years, N = 25; ">5" years, N = 51. RESULTS: Patients with TTT > 5 years were younger at diagnosis and more often had atrial fibrillation (AF). Left ventricular outflow tract (LVOT) gradients were comparable in the 3 TTT groups. Two patients died peri-operatively, all with TTT > 5. Long-term, 8 patients died (3 suddenly and 5 due to heart failure). Mortality increased progressively with TTT (2% vs. 4% vs. 12% for TTT "<3", "3-5", and ">5" years, p for trend = 0.039). Independent predictors of disease progression (new-onset AF, worsening to NYHA III/IV symptoms, re-intervention or death) were TTT ("3-5" vs. "<3" years: HR: 4.988, 95%CI: 1.394-17.843; ">5" vs. "<3" years: HR: 3.420, 95%CI: 1.258-9.293, overall p-value = 0.025), AF at baseline (HR: 1.896, 95%CI: 1.002-3.589, p = 0.036) and LVOT gradient (HR per mm Hg increase: 1.022, 95%CI: 1.007-1.024, p = 0.023). CONCLUSIONS: Delay in SRT referral has significant impact on long-term outcome of patients with HOCM, particularly when >5 years from first detection of gradient, even when successful relief of symptoms and gradient is achieved. Earlier interventions are associated with lower complication rates and better prognosis, suggesting the importance of timely SRT to maximize treatment benefit and prevent late HOCM-related complications.

Long-term Outcomes of Pediatric-Onset Hypertrophic Cardiomyopathy and Age-Specific Risk Factors for Lethal Arrhythmic Events.

Importance: Predictors of lethal arrhythmic events (LAEs) after a pediatric diagnosis of hypertrophic cardiomyopathy (HCM) are unresolved. Existing algorithms for risk stratification are limited to patients older than 16 years because of a lack of data on younger individuals. Objective: To describe the long-term outcome of pediatric-onset HCM and identify age-specific arrhythmic risk factors. Design, Setting, and Participants: This study assessed patients with pediatric-onset hypertrophic cardiomyopathy diagnosed from 1974 to 2016 in 2 national referral centers for cardiomyopathies in Florence, Italy. Patients with metabolic and syndromic disease were excluded. Exposures: Patients were assessed at 1-year intervals, or more often, if their clinical condition required. Main Outcomes and Measures: Lethal arrhythmic events (LAEs) and death related to heart failure. Results: Of 1644 patients with HCM, 100 (6.1%) were 1 to 16 years old at diagnosis (median [interquartile range], 12.2 [7.3-14.1] years). Of these, 63 (63.0%) were boys. Forty-two of the 100 patients (42.0%) were symptomatic (defined as an New York Heart Association classification higher than 1 or a Ross score greater than 2). The yield of sarcomere gene testing was 55 of 70 patients (79%). During a median of 9.2 years during which a mean of 1229 patients were treated per year, 24 of 100 patients (24.0%) experienced cardiac events (1.9% per year), including 19 LAEs and 5 heart failure-related events (3 deaths and 2 heart transplants). Lethal arrhythmic events occurred at a mean (SD) age of 23.1 (11.5) years. Two survivors of LAEs with symptoms of heart failure experienced recurrent cardiac arrest despite an implantable cardioverter defibrillator. Risk of LAE was associated with symptoms at onset (hazard ratio [HR], 8.2; 95% CI, 1.5-68.4; P = .02) and Troponin I or Troponin T gene mutations (HR, 4.1; 95% CI, 0.9-36.5; P = .06). Adult HCM risk predictors performed poorly in this population. Data analysis occurred from December 2016 to October 2017. Conclusions and Relevance: Pediatric-onset HCM is rare and associated with adverse outcomes driven mainly by arrhythmic events. Risk extends well beyond adolescence, which calls for unchanged clinical surveillance into adulthood. In this study, predictors of adverse outcomes differ from those of adult populations with HCM. In secondary prevention, the implantable cardioverter defibrillator did not confer absolute protection in the presence of limiting symptoms of heart failure.