“By the Luck of Chance: When a 1-in-500 Condition Meets a 1-in-10,000 Disease”

Description of Clinical Presentation: A 23-year-old male patient with single ventricle (pulmonary atresia with ventricular septal defect) underwent staged surgical correction with initial palliation, Glenn procedure, and extracardiac Fontan completion with fenestration closure. He has preserved baseline functional status but demonstrates mild-to-moderate exercise limitation, with peak oxygen consumption (VO₂ max) of 16.4 ml/kg/min and a drop in oxygen saturation to 80% during exertion. In this context, cardiac MRI was requested for follow-up, which provided additional information beyond the limited assessment of his underlying congenital heart disease.

Diagnostic Techniques and Their Most Important Findings: Cardiac magnetic resonance imaging was performed using black-blood morphological sequences, cine imaging, phase-contrast flow assessment, 3D gadolinium-enhanced angiography, native and post-contrast T1 mapping, and late gadolinium enhancement imaging 10 minutes after contrast administration.

The patient has a morphologically left single ventricle receiving the mitral valve, with notable elongation of the anterior mitral leaflet, and a large interventricular communication connecting to a vestigial right ventricle. The vestigial right ventricle gives rise to the aorta, consistent with associated transposition of the great arteries. Systolic function is preserved (ejection fraction 55%), and the ventricle is not dilated. Marked hypertrophy is observed in the interventricular septum (20 mm) and vestigial right ventricular wall (18 mm). Native septal T1 mapping was 1027 ms. Late gadolinium enhancement shows heterogeneous fibrosis in the mid-apical septum and the free wall of the vestigial right ventricle. The extracardiac Fontan conduit is patent.

Learning Points from this Case:

1. 
   Cardiac MRI is a cornerstone for the follow-up of adults with surgically corrected complex congenital heart disease. It allows comprehensive assessment of ventricular morphology and function, evaluation of long-term surgical outcomes, and detection of potential complications over time.
   
   


2. 
   In this patient, MRI provided additional value by revealing a coexisting hypertrophic cardiomyopathy. Although rare, such associated pathologies are clinically significant, particularly because they may increase arrhythmic risk and affect overall management, highlighting the need for vigilance during imaging evaluation.
   
   


3. 
   Family screening and, when available, genetic evaluation are essential to identify at-risk relatives and guide preventive strategies.
   
   

Key CMR findings in a single ventricle with Fontan circulation

Parameter	Value
Patient	23 y, 80 kg, 185 cm, BSA 2.03 m²
Ventricle	Single ventricle with vestigial RV
SV EDV / ESV	220 / 100 ml (108 / 48 ml/m²)
SV EF	55%
LA / RA	34 ml / 7 cm²
Septum thickness	20 mm
RV vestigial free wall thickness	18 mm
Mitral valve	Elongated anterior leaflet
T1 native septal	1027 ms
Late gadolinium enhancement	Mid–apical septum & vestigial RV wall, heterogeneous
Fontan conduit	Extracardiac, permeable
Situs	Solitus, mesocardia

Top row: three long-axis cine-SSFP images showing inflow into the single ventricle through the AV valve, a vestigial RV, marked hypertrophy of the interventricular septum and RV free wall, and the aorta arising from the vestigial RV, with a wide outflow tract. Bottom row: corresponding late gadolinium enhancement images demonstrating heterogeneous myocardial fibrosis in the mid–apical septum and vestigial RV free wall.
Top row: four short-axis cine-SSFP slices. The first slice shows a wide interventricular communication. In the first and second slices, the aorta arises from the vestigial right ventricle. Hypertrophy of the interventricular septum and the free wall of the vestigial RV is evident in all slices. Bottom row: late gadolinium enhancement images. In the second and third slices, intramyocardial enhancement is visible at the RV insertion points in the interventricular septum. In the third and fourth slices, diffuse fibrosis of the vestigial RV free wall is observed.