Abdominal imaging findings in adult patients with Fontan circulation
The Fontan procedures, designed to treat paediatric patients with functional single ventricles, have markedly improved the patient’s survival into adulthood. The physiology of the Fontan circuit inevitably increases systemic venous pressure, which may lead to multi-system organ failure in the long-term follow-up. Fontan-associated liver disease (FALD) can progress to liver cirrhosis with signs of portal hypertension. Focal nodular hyperplasia-like nodules commonly develop in FALD. Imaging surveillance is often performed to monitor the progression of FALD and to detect hepatocellular carcinoma, which infrequently develops in FALD. Other abdominal abnormalities in post-Fontan patients include protein losing enteropathy and pheochromocytoma/paraganglioma. Given that these abdominal abnormalities are critical for patient management, it is important for radiologists to become familiar with the abdominal abnormalities that are common in post-Fontan patients on cross-sectional imaging.
• Fontan procedure for functional single ventricle has improved patient survival into adulthood.
• Radiologists should be familiar with unique imaging findings of Fontan-associated liver disease.
• Focal nodular hyperplasia-like nodules commonly develop in Fontan-associated liver disease.
• Hepatocellular carcinoma, protein-losing enteropathy, pheochromocytoma/paraganglioma may develop.
KeywordsLiver cirrhosis Focal nodular hyperplasia Heart defects, congenital Diagnostic imaging Digestive system neoplasms
The Fontan procedure was first introduced for patients with tricuspid atresia to allow systemic blood return to enter the pulmonary arteries without passage through a ventricle in 1968 by Fontan and Baudet [1, 2]. Subsequently, the procedure has been applied for most forms of functional single ventricles [3, 4, 5, 6]. Surgical modifications of the procedure along with advances in postoperative management led to prolonged patient survival, frequently into adulthood [7, 8, 9].
The Fontan circuit allows the entire systemic venous return to enter the pulmonary vascular bed directly, bypassing the right ventricle. There are two ways to connect the systemic venous return to pulmonary arteries: either through the right atrium (namely atriopulmonary circulation) or by direct connection of superior vena cava (SVC) and inferior vena cava (IVC) to pulmonary arteries (namely cavopulmonary circulation). Cavopulmonary circulation is subdivided according to the type of IVC connection to the pulmonary artery either by intra-atrial lateral tunnel conduit or external cardiac conduit . Due to the absence of a pumping ventricle, the transmission of pulmonary vascular resistance to the systemic venous circulation inevitably leads to chronically elevated systemic venous pressure. Thus, all organ systems of the body are exposed to chronic venous congestion, potentially leading to multi-system organ failure [11, 12, 13].
Fontan-associated liver disease (FALD) has been recognised as the disease process of liver structure and function resulting from the Fontan circulation, excluding other plausible causes such as viral hepatitis or alcohol toxicity. FALD is a broad term encompassing hepatic parenchymal change, hypervascular regenerative hepatic nodules and ultimately hepatic cirrhosis  . FALD is considered one of the major non-cardiac determinants of mortality in adult Fontan patients [15, 16, 17, 18]. Other abdominal abnormalities found in adult patients with Fontan circulation include signs of portal hypertension such as ascites or varices, protein losing enteropathy (PLE) and pheochromocytoma/paraganglioma.
Although the imaging features of abdominal abnormalities after Fontan operation have been described in several publications mostly for the paediatric population [19, 20, 21, 22], abdominal imaging findings in adult patients with Fontan circulation have not been well described in detail. In this review, we summarise the cross-sectional imaging findings of the hepatic and extrahepatic abnormalities in the abdomen in adult patients with Fontan circulation.
Fontan-associated liver disease
Among the gastrointestinal system, the liver is particularly vulnerable to the elevated systemic venous pressure in Fontan circulation compared to other organ systems for four principal reasons [13, 14]. Firstly, most venous return from the gastrointestinal system passes through the liver. Secondly, portal venous flow to the liver relies on the pressure gradient between the hepatic and portal veins that decreases as the systemic venous pressure increases, in contrast to other organs in which the blood flow relies on the pressure gradient between the arterial and venous systems. Third, the natural response of diversion of gastrointestinal blood to other organs such as brain, heart and muscles further compromises already precarious circulation of the liver. Lastly, lymphatic drainage disturbance due to increased central venous pressure causes lymphatic-mediated hepatic congestion and sinusoidal dilatation, which in turn attributes to the liver injury .
Adult patients with Fontan circulation are prone to developing FALD as they age. It frequently results in chronic hepatic congestion, liver cirrhosis, portal hypertension, focal nodular hyperplasia (FNH)-like nodules and even hepatocellular carcinoma (HCC) [15, 24, 25, 26]. The possible mechanisms of hepatic dysfunction in FALD include chronic passive venous congestion, reversed blood flow with deep intrahepatic reflux during atrial contraction , and reduced hepatic blood supply with subsequent hypoxic damage from relatively low cardiac output after Fontan operation [27, 28]. Histopathological changes begin with sinusoidal dilatation, parenchymal atrophy and progressive sinusoidal fibrosis in the perivenular distribution, similar to that of right heart failure, but more exaggerated over time in patients with Fontan circulation .
The severity of FALD is positively related to the duration of the Fontan circulation, elevated hepatic venous pressure, failing Fontan circulation and increased serological markers such as hyaluronic acid or gamma-glutamyltransferase [11, 29, 30]. Monitoring FALD with imaging surveillance is essential in guiding patient management such as medical therapy for liver cirrhosis, placement on the liver transplantation list or treating HCC when it is detected. In post-Fontan patients who are considered for heart transplantation, severe FALD is considered a contraindication for heart transplantation or combined heart-liver transplantation [16, 26, 31].
Given the clinical importance of FALD in adult patients with Fontan circulation, various non-invasive screening/surveillance protocols for FALD have been suggested regarding the timing, intervals, and modalities [16, 18, 26, 32, 33]. Liver biopsy still remains the “gold standard” for assessing the severity of liver fibrosis and cirrhosis. However, reported liver changes secondary to FALD are not uniformly distributed and therefore liver biopsy may overestimate or underestimate the presence or nature of FALD [19, 28, 34]. Further, the risk of biopsy-related bleeding complication is likely to be higher due to the frequent use of warfarin and elevated systemic venous pressures [32, 35]. Therefore, non-invasive imaging surveillance may play a major role for the surveillance of FALD and its related changes.
Imaging of hepatic parenchymal changes
Imaging of hepatic nodules
FNH-like nodules in FALD are often small and multiple, and show a predilection for the right lobe in the periphery (mostly within 2 cm of the liver surface) . On CEUS, FNH-like nodules in FALD show enhancement features similar to FNH; hyper-enhancement in the arterial phase, centrifugal enhancement, central stellate vasculature and sustained enhancement without washout (Supplementary material 1). MRI using liver-specific contrast agent is useful for characterising FNH-like nodules by demonstrating iso- or hyper-enhancement in the hepatobiliary phase. The FNH-like nodules may show central hypointensity in the hepatobiliary phase representing a central scar (Fig. 3c).
HCC typically shows [25, 32, 51, 52, 53, 54, 55, 56, 57] arterial-phase hyper-enhancement followed by late and mild washout on dynamic contrast-enhanced imaging. CEUS is potentially useful for indeterminate hepatic nodules in FALD on CT or MRI, especially for the patients who are contraindicated for contrast-enhanced CT or MRI. Suspicion of HCC is raised upon large size, interval change in size or echogenicity, mass-like appearance or nodules causing contour abnormality on the liver surface (Fig. 4a). However, it is important to note that the FNH-like nodules can infrequently demonstrate washout in the delayed phase, mimicking HCC [58, 59] (Supplementary material 1). The reported incidence of delayed-phase washout in FNH-like nodules is approximately 10%, in the setting of cardiac cirrhosis secondary to constrictive pericarditis and alcoholic cirrhosis. The pathophysiology for washout of FNH-like nodules is not clear. Wells et al.  speculated that the washout may not be related to an abnormality of the nodule itself, but a reflection of the background parenchymal contrast retention due to parenchymal congestion and fibrosis.
MRI using liver-specific contrast agent may be useful for the differentiation between FNH-like nodules and HCC, as HCC mostly show hypointensity in the hepatobiliary phase. DWI may be also useful in differentiating between HCC and FNH-like nodules. In our experience of a single case of HCC in FALD, HCC showed strong restricted diffusion, whereas FNH-like nodules did not show diffusion restriction, compatible with the study by Wells et al.  (Figs. 3d and 4d). The authors suggested other ancillary findings favouring benign FNH-like nodules, including hypointensity on T2-weighted images.
Imaging of vascular abnormalities
On Doppler US, normal hepatic vein shows a triphasic waveform, consisting of a hepatopetal (coming towards the liver) phase occurring during atrial systole and two hepatofugal phases related to atrial and ventricular diastole. Loss of the hepatopetal phase is typically considered to represent increased stiffness of the liver such as in hepatic fibrosis, steatohepatitis and cirrhosis [60, 61, 62].
Portal venous waveform on Doppler US normally shows slightly phasic hepatopetal flow with mildly decreased flow during inspiration due to transient compression of the compliant liver parenchyma and easily collapsible portal venules and hepatic sinusoids [61, 68]. In patients with Fontan circulation, the portal veins invariably show hepatopetal flow with variable phasicity, but the flow velocity is lower than in the normal population [69, 70]. While the dilatation of the IVC and hepatic veins is frequently seen in post-Fontan patients, the diameter of the main portal vein and intrahepatic portal veins is usually small, possibly due to reduction of portal perfusion secondary to increased sinusoidal pressure with venous stasis  (Fig. 5). Bland portal vein thrombosis seems to be rare in post-Fontan patients with no reported cases compared to Budd-Chiari syndrome, where portal vein thrombosis occurs up to 18% [71, 72]. Hepatofugal flow of the portal vein, which can be found in severe portal hypertension or Budd-Chiari syndrome with large extrahepatic portosystemic collaterals, has not been reported in post-Fontan patients [63, 66, 70, 73, 74, 75] . It can be explained by relatively high systemic venous pressure in Fontan circulation, preventing the development of large extrahepatic portosystemic collaterals.
On the arterial-phase CT with contrast injection through the upper extremity veins, dense opacification in the dependent portion of the IVC and hepatic veins is occasionally seen in post-Fontan patients. This is likely due to passive retrograde flow of the contrast material which is heavier than blood in the absence of effective right heart function (Supplementary material 2). This phenomenon might be different from the opacification of IVC and all hepatic veins in patients with right heart failure [38, 76, 77], which is the result of reflux of contrast media into the inferior vena cava during right atrial contraction or tricuspid regurgitation.
Signs of portal hypertension
Extrahepatic complications of portal hypertension secondary to liver cirrhosis such as splenomegaly and ascites are commonly seen in adult Fontan patients. Compared to liver cirrhosis secondary to hepatitis, extrahepatic portosystemic shunts are uncommon or small in size in FALD, even in a decompensated state, likely due to the high systemic venous pressure in Fontan circulation and relatively low pressure gradient between the portal and systemic veins to promote the formation of large portosystemic collaterals  (Supplementary material 3).
Protein losing enteropathy
Protein losing enteropathy (PLE) is characterised by enteric loss of proteins including albumin, immunoglobulins and clotting factors. Patients with PLE present with peripheral oedema, ascites, diarrhoea, weight loss and malabsorption. The reported incidence of PLE in Fontan patients ranges from 3 to 18%, with a reported mortality of 50% within 5 years after initial diagnosis [78, 79, 80]. Elevated systemic venous pressure and mesenteric vasoconstriction, which is the natural reaction to decreased cardiac output, might be the cause of PLE. Additionally, lymphatic dilatation, stasis and leakage, and inflammatory reaction secondary to elevated systemic venous pressure, further contribute to the development of PLE [19, 80, 81].
Early clinical diagnosis of PLE is often difficult because the patients may have a long asymptomatic period [80, 81, 82]. As multiple intensive therapeutic approaches enable an improved survival rate of up to 88% at 5 years , early recognition of excessive enteric loss of protein is important. Suggestive imaging findings including ascites, diffuse bowel wall thickening, and severe mesenteric oedema along the root of the mesentery on CT or MRI, should prompt a clinical evaluation to rule out PLE .
Pheochromocytoma and paraganglioma
Fontan procedures, designed to treat patients with functional single ventricles, have markedly improved the patient’s survival into adulthood. The physiology of the Fontan circuit inevitably increases systemic venous pressure, which may lead to multi-system organ failure in the long-term follow-up. FALD is most common among post-Fontan abdominal complications and can progress to liver cirrhosis with signs of portal hypertension. FNH-like nodules frequently develop in FALD as small multiple nodules with arterial-phase hyper-enhancement and show imaging findings similar to FNH. Imaging surveillance is often performed to monitor the progression of FALD to liver cirrhosis and to detect HCC, which infrequently develops in FALD. Extrahepatic abdominal abnormalities in post-Fontan patients include signs of portal hypertension such as ascites, splenomegaly and varices, protein losing enteropathy, and pheochromocytoma/paraganglioma. Given that these abdominal abnormalities are critical for patient management, it is important to be familiar with the typical imaging findings of the abnormalities on cross-sectional imaging.
Compliance with ethical standards
The scientific guarantor of this publication is Dr. Tae Kyoung Kim.
Conflict of interest
The authors of this manuscript declare no relationships with any companies whose products or services may be related to the subject matter of the article.
Statistics and biometry
No statistical experience was required (review article).
Institutional Review Board approval was not required (review article).
• Review article
• Performed at one institution
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