The Fontan procedure, first described in the early 1970s, transformed outcomes for infants born with a single functioning ventricle (single ventricle heart disease, SVHD). This condition accounts for barely 1% of congenital heart disease (CHD) cases, yet it is responsible for a disproportionate share of CHD-related mortality and morbidity, particularly during early life. Today, owing to surgical advances, roughly 85% of those who undergo the operation survive into adulthood, and an estimated more than 70000 people worldwide are now living with a Fontan circulation.

That survival comes at a cost: a demanding physiology defined by chronically elevated systemic venous pressure, reduced ventricular preload, and the absence of pulsatile pulmonary flow. Over time, this predisposes patients to hepatic fibrosis, renal dysfunction, arrhythmia, thromboembolic events, and neuropsychological difficulties. Until now, most available evidence came from single-center series or cohorts limited to a particular surgical era, and this contemporary, multicenter snapshot was precisely what was missing.

The authors conducted a cross-sectional analysis of the first 1121 patients enrolled in the FON (Fontan Outcomes Network) registry between August 2022 and August 2024, drawn from 16 of the 38 participating centers that had already completed data quality review. Cohort characteristics were examined descriptively and stratified into three age groups — under 12 years, 12 to 18 years, and adults — which ultimately accounted for 38.3%, 25.8%, and 36.0% of the sample, respectively.

Mean age in the study was 16.3 ± 10.2 years, and 42% of participants were female. The cohort was predominantly White (78.5%), with 7.3% Black patients and 14% of Hispanic or Latino origin. Hypoplastic left heart syndrome (HLHS) was by far the most frequent diagnosis (38.5%), followed by tricuspid atresia in 16% of patients. Slightly more than half of patients had a right ventricular dominant anatomy (54.9%), and the extracardiac conduit was the most commonly used Fontan technique (66.8%, versus 23.5% for the lateral tunnel, the latter considerably more frequent among adults operated on in earlier surgical eras). Around 59% had experienced at least one typical Fontan-related complication — chylous pleural effusion (9.6%), thrombosis (7.1%), non-perioperative stroke (6.5%) — and 41.3% had a history of arrhythmia. Nearly all patients were receiving antithrombotic treatment (97%, mostly aspirin), and almost half were taking an angiotensin-converting enzyme inhibitor (45.2%).

Medical comorbidities, present in 57.5% of the cohort, centered mainly on asthma (12%) and sleep apnea (9%); in the neurodevelopmental domain, one-third of patients (32.6%) had at least one diagnosis, led by ADHD (18.9%). Overall, 6 in 10 patients carried a psychiatric diagnosis, with anxiety present in 34.1%, and antidepressants prescribed in 11.8% of the entire cohort, a figure that rose to 23.6% among adults.

The authors conclude that FON already represents one of the largest prospective datasets on Fontan circulation, and that this initial analysis demonstrates the potential of a collaborative learning network to advance research and quality of care in this rare disease population. Its future integration into the Single Ventricle Outcomes Network (SV-ONE) will extend this follow-up across the entire lifespan and support increasingly personalized care.

COMMENTARY:

This is an interesting study, well conceived, addressing one of the areas of greatest current interest in congenital heart disease: the Fontan circulation. Beyond simply describing a contemporary cohort, these data help clarify how the clinical profile of the Fontan patient is evolving and, above all, which aspects of long-term follow-up remain far from standardized.

Before drawing clinical lessons, it is worth considering who makes up this cohort. Women account for 42% of the total but are, on average, two years older than men (17.6 versus 15.4 years; p<.001) — a difference that may reflect early survival patterns rather than a true sex effect, and one that deserves dedicated analysis in future series. The limited ethnic representation is also striking: only 7.3% of participants are Black and 14% are of Hispanic or Latino origin, below what would be expected given the demographics of the United States and Canada. The authors attribute this, cautiously, to the population served by the earliest recruiting centers, or to the underrepresentation already documented in clinical trials.

Splitting the cohort into three age brackets makes sense for a registry designed to follow patients across the lifespan, and it reveals some notable patterns, such as shifts in the surgical technique used depending on the era in which each group was operated on. Something similar applies to imaging: echocardiography was performed in nearly every patient, yet cardiac MRI was available in only just over half of the cohort (57%) and was considerably more common in adults than in children. Cardiac catheterization use also varied across age groups, reflecting differing strategies for invasive assessment over the course of follow-up. This variability raises a question that goes beyond cardiac anatomy itself: how to standardize surveillance across the patient’s lifetime, particularly because detecting complications requires early, integrated, multidisciplinary management.

One notable gap is the absence of data on Fontan-associated liver disease, one of the complications of greatest concern as these patients approach adulthood. The authors explain that liver assessment is not captured at registry entry and will instead be gathered through long-term follow-up — a reasonable decision, but one that leaves unanswered a question that comes up routinely in practice: when, and with which test, the liver should be monitored (ultrasound elastography, MRI, transjugular biopsy) before the transition to adult congenital heart disease care. It is also worth noting that asthma emerges as the most frequent medical comorbidity, which raises the question of how much some of these diagnoses may overlap with the restrictive respiratory dysfunction or pulmonary lymphatic involvement characteristic of Fontan physiology.

Regarding treatment, one notable finding is the still limited — and in children nearly anecdotal — use of SGLT2 inhibitors, a drug class now widely incorporated into heart failure management, whose role in univentricular physiology remains to be established. But the most concerning finding in this study has nothing to do with the heart: six in ten patients carry a mental health diagnosis, and nearly one in four adults is taking antidepressants, a rate several times higher than in the general population. Psychological screening should be just as routine as an annual echocardiogram.

In conclusion, the Fontan circulation does not end in the operating room: it is a lifelong chronic condition whose psychological burden is as significant as its cardiac one. FON does not yet have answers to questions such as longitudinal cardiac surveillance, liver disease, or the role of newer medications, but this first snapshot sets an important precedent — a shared framework capable of turning between-center variability into collaborative research that identifies which patients are at risk and when intervention is needed.

REFERENCE:

Glenn TM, Kasparian NA, Amdani S, Renaud D, DiMaria MV, Teh J, et al; SV-ONE Investigators. The Fontan Outcomes Network: Findings After 2 Years and 1121 Participants. Circulation. 2026 Apr 21;153(16):1182-1195. doi: 10.1161/CIRCULATIONAHA.125.078152. Epub 2026 Mar 25. PMID: 41878814; PMCID: PMC13157274.