Cystic liver lesions represent a diverse group of conditions that can pose diagnostic challenges. Understanding the differential diagnosis is crucial for accurate identification and appropriate management. This article provides an in-depth exploration of congenital cystic liver lesions, focusing on their distinguishing features and imaging characteristics to aid in differential diagnosis.
Understanding Ductal Plate Malformations
Ductal plate malformations (DPMs) are at the heart of many congenital cystic liver lesions. These malformations arise from disruptions in the normal development of bile ducts during embryogenesis. Specifically, they result from the incomplete remodeling and resorption of the cylindrical ductal plates, a process essential for forming the mature biliary system. These developmental anomalies can affect both the intrahepatic and extrahepatic bile ducts, leading to a spectrum of cystic conditions.
Figure 1: Diagram illustrating the progression of ductal plate malformation in relation to bile duct development, from the liver’s center to its periphery.
The size of the affected ducts in DPMs is directly related to the specific stage of bile duct development that was disrupted. This developmental context is key to understanding the diverse presentations of cystic liver diseases.
Simple Hepatic Cysts: The Basics
Simple hepatic cysts (SHCs), also known as biliary cysts or bile duct cysts, are common, benign congenital lesions. These cysts are essentially detached segments of the ductal tree that have become dilated. While the exact origin of SHCs is debated, a prevalent theory suggests they arise from the dilatation of bile duct hamartomas. Importantly, SHCs do not communicate with the main biliary tract.
Prevalence studies indicate that SHCs are found in 2.5% to 18% of the population, with a higher incidence in women and increasing prevalence with age. These lesions are typically asymptomatic and benign. The cysts are lined with cuboidal biliary epithelium that continuously produces serous fluid. However, large cysts can cause symptoms like abdominal pain or early satiety due to compression. Complications such as hemorrhage or infection are rare, but can transform an SHC into a “complex cyst.”
Imaging Characteristics of Simple Hepatic Cysts
Imaging plays a pivotal role in diagnosing SHCs. On ultrasonography (US), SHCs appear as well-defined, unilocular lesions with thin, often imperceptible walls. They are anechoic, exhibiting increased through-transmission, a hallmark of fluid-filled cysts.
Computed tomography (CT) scans reveal SHCs as hypoattenuating lesions, typically in the 0–20 Hounsfield Units (HU) range, consistent with fluid density. On magnetic resonance imaging (MRI), SHCs are hypointense on T1-weighted images and intensely hyperintense on T2-weighted images, further confirming their fluid content.
A key diagnostic feature of SHCs is the absence of internal nodules and lack of enhancement after intravenous contrast administration in US, CT, or MRI. SHCs can be single or multiple and may appear bilobular or multilocular due to the fusion of adjacent cysts. The characteristic imaging findings across different modalities usually allow for a confident diagnosis of SHCs.
Figure 2: Imaging of a simple hepatic cyst found incidentally in a 64-year-old male. (a) Ultrasonography showing a round, anechoic lesion with through transmission (arrows). (b) T2-weighted MRI showing a hyperintense lesion. (c, d) CT scans without and with contrast showing a non-enhancing, hypoattenuating lesion.
Figure 3: Large simple hepatic cyst in a 64-year-old female with abdominal pain. (a) Axial and (b) coronal CT scans showing the extensive size of the cyst.
Complex Simple Hepatic Cysts: Recognizing Complications
While SHCs are typically straightforward, complications can arise, transforming them into “complex cysts” and potentially leading to diagnostic errors, especially when considering tumorous cysts.
- Hemorrhage: Hemorrhagic cysts can increase in size and exhibit heterogeneous content with internal septations seen as echogenic material on US. Pain may occur during the ultrasound examination. CT scans show higher attenuation than simple fluid, and MRI signals vary based on the blood’s stage. Fluid-fluid levels may be observed. Importantly, the internal septa do not enhance, although a thin peripheral rim enhancement (pseudo-capsule) can be present and should not be mistaken for tumor.
Figure 4: Subacute hemorrhage within a simple hepatic cyst in a 70-year-old male. Ultrasonography reveals a mobile, hyperechoic area resembling a “fern leaf”.
Figure 5: Bilobulated hemorrhagic hepatic cyst in a 73-year-old female. (a) Non-enhanced CT showing hyperattenuation in the inferior part. (b) T2-weighted MRI showing heterogeneous hypointensity in the inferior part. (c) Contrast-enhanced ultrasonography confirming non-enhancement of the cystic portion.
Figure 6: Hemorrhagic cyst in a 55-year-old male. (a) T1 fat-sat MRI showing hyperintense lesion. (b) T2-weighted MRI showing heterogeneous hyperintensity. (c, d) T1-fat-sat MRI without and with subtraction, showing no enhancement after contrast.
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Infection: Infected SHCs are rare, except in polycystic liver disease. Imaging may reveal a thickened wall with heterogeneous enhancement, fluid-fluid levels, and gas bubbles within the cyst. Differentiating infected cysts from abscesses can be challenging unless prior imaging documented the presence of SHCs.
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Rupture: Rupture of an SHC is also rare. A “floating wall” sign within the cyst is suggestive of rupture. Subcapsular cyst rupture may lead to perihepatic fluid collections.
Figure 7: Ruptured hepatic cyst in an 89-year-old male with severe abdominal pain. CT scan showing a floating wall (arrow) and perihepatic fluid.
- Large Symptomatic SHCs: Very large SHCs, though often asymptomatic, can cause pain or duodenogastric compression. Treatment options include surgical cyst fenestration or radiological sclerotherapy. Diagnostic aspiration can be used to assess if symptoms are related to the cyst.
Bile Duct Hamartomas: Von Meyenburg Complexes
Bile duct hamartomas (BHs), also known as von Meyenburg complexes, are benign lesions resulting from the failure of small interlobular bile ducts to involute during late embryogenesis. These asymptomatic lesions are composed of disconnected, minimally dilated bile ducts lined by biliary epithelium. Autopsy studies place their prevalence around 5.6% in adults. BHs can occur in otherwise healthy livers or in conjunction with other ductal plate abnormalities. While rare, there have been reported cases of cholangiocarcinoma associated with BHs in the absence of other DPMs.
Imaging Features of Bile Duct Hamartomas
BHs are typically located subcapsularly or peripherally within portal tracts. On US, they appear as multiple small hyperechoic nodules, often exhibiting comet-tail artifacts, creating a characteristic “snowstorm” pattern. BHs are poorly visualized on CT, appearing as small, round or irregular lesions with fluid attenuation and no enhancement.
MRI offers superior detection, particularly with strongly T2-weighted sequences, revealing a “starry sky” appearance. Magnetic resonance cholangiopancreatography (MRCP) confirms the lack of communication with the biliary tree. In some instances, a thin, persistent enhancing rim may be seen due to compressed surrounding liver parenchyma. The combination of these imaging features usually allows for definitive diagnosis. Differentiating BHs from SHCs can be challenging when BHs are few, but clinically, this distinction is less critical as both are benign. Key differentiating factors include the smaller size and peripheral distribution of BHs.
Figure 8: Bile duct hamartomas in a 52-year-old female. (a) Ultrasonography showing multiple hyperechoic nodules (“snowstorm”). (b) Coronal MRCP showing small hyperintense nodules without biliary communication. (c) CT scan showing hypoattenuating nodules. (d) Axial MRCP for better detection.
Caroli Syndrome and Disease: Rare Biliary Dilatations
Caroli disease (CD) and Caroli syndrome (CS) are exceptionally rare congenital ductal plate abnormalities, with a prevalence of less than one in a million. CS is more common than CD and it’s crucial to differentiate between them due to different pathophysiologies. CD involves abnormal development of large bile ducts, while CS affects both central and peripheral ducts. CS is characterized by cystic biliary dilatation and congenital hepatic fibrosis, whereas CD solely involves biliary dilatation.
The origin of CD is largely unknown and typically nonhereditary, with rare autosomal dominant inheritance. CS, in contrast, is autosomal recessive. In CD, the abnormality is thought to be an early interruption of ductal plate remodeling. In CS, abnormalities occur both early and late in development and involve peripheral bile ducts. Both CD and CS require careful follow-up due to an increased risk of cholangiocarcinoma. Bile stasis in cysts and bile ducts can also lead to stone formation.
Caroli Disease: Central Bile Duct Dilatation
Pathophysiology and Clinical Presentation
Caroli disease arises from a failure of periportal biliary epithelium regression during embryogenesis, leading to aneurysmal dilatation of large intrahepatic bile ducts. The liver parenchyma itself remains normal. Common symptoms include abdominal pain, jaundice, and/or cholangitis.
Imaging of Caroli Disease
Imaging reveals an abnormal biliary tract with focal, saccular dilatations of intrahepatic bile ducts without stenosis. A characteristic “central dot sign” is often present within the dilated ducts, representing a residual portal vein and arterial branch. Doppler or contrast-enhanced CT/MRI can confirm the vascular nature of this central dot.
MRCP is the preferred imaging modality for Caroli disease. It clearly demonstrates the communication of cystic hepatic lesions with the biliary tract. Hepatobiliary MRI contrast agents can further confirm this communication. Liver atrophy is typically absent in CD.
Figure 9: Caroli disease in a 24-year-old male. (a) T2-weighted MRI and (b) T1 portal venous phase MRI showing fusiform and round dilatations (arrows and arrowheads). (c) MRCP showing the dilatations and no fibrosis.
Caroli Syndrome: Dilatation and Hepatic Fibrosis
Pathophysiology and Clinical Presentation
Caroli syndrome is distinguished by the combination of bile duct dilatations and congenital hepatic fibrosis. CS can be associated with autosomal recessive polycystic kidney disease (PKHD1 mutation). Symptoms are often related to the consequences of liver fibrosis. Long-term follow-up is essential for CS, not only for managing fibrosis complications but also due to a significant risk of cholangiocarcinoma (around 7%).
Imaging of Caroli Syndrome
Imaging features of CS are similar to CD, with cysts communicating with dilated bile ducts and the presence of the “central dot sign”. However, cysts in CS are often smaller than in CD.
Figure 10: Caroli syndrome in a 7-year-old female. Ultrasonography showing cysts (stars) connected to dilated biliary tracts (arrows), with central dot sign (arrowhead).
The associated hepatic fibrosis in CS can lead to characteristic liver dysmorphia, including hypertrophy of the left and caudate lobes, normal size or hypertrophy of segment IV, and hypotrophy of the right lobe. Signs of portal hypertension, such as portosystemic collaterals, splenomegaly, and ascites, may also be present.
Figure 11: Caroli syndrome in a 53-year-old female. (a) Portal venous phase MRI showing peripheral dilated bile ducts. (b) T2-weighted MRI showing hyperintense ducts with central dot sign (arrows). Note spleno-renal shunts due to portal hypertension.
Polycystic Liver Disease: Extensive Cystic Involvement
Polycystic liver disease (PLD) encompasses conditions with numerous cysts in the liver. PLD can manifest as autosomal dominant polycystic liver disease (ADPLD), primarily affecting the liver, or as part of autosomal dominant or recessive polycystic kidney disease (ADPKD or ARPKD), involving both liver and kidneys. ADPLD is rarer (≈ 1:100,000) than ADPKD (≈ 1:600–1,000) and ARPKD (≈ 1:20,000). While the exact number of cysts for diagnosis is not definitive, the international PLD registry suggests at least 10 hepatic cysts for a PLD diagnosis.
Complications of PLD are similar to SHCs and include organ compression, hemorrhage, infection, and rupture. Detecting infection in PLD can be challenging, but 18-Fluorine-fluorodeoxyglucose positron emission tomography (18-F-FDG PET) imaging can be helpful.
Imaging of Polycystic Liver Disease
In PLD, cysts, resembling SHCs, are distributed diffusely throughout the liver. They may coalesce and show linear calcifications between them. Hepatomegaly and vascular compression with venous collaterals are common findings.
Figure 12: Infected cysts in a 77-year-old male with ADPLD. (a) Non-enhanced CT showing pressurized cysts (arrow) and non-infected cysts (arrowhead). (b) Portal venous phase showing enhanced wall of infected cyst (arrowhead).
Figure 13: Infected cyst in a 65-year-old male with ADPLD. (a) Non-enhanced CT showing a more attenuating cyst (cross). (b) FDG-PET showing increased metabolism in the infected cyst.
Figure 14: Autosomal dominant polycystic kidney disease in a 38-year-old male. (a) Portal venous phase CT and (b) T2-weighted MRI showing hepatic and kidney cysts (arrows).
Conclusion: Navigating the Differential Diagnosis
Differential diagnosis of cystic liver lesions requires a comprehensive approach, integrating clinical context, imaging findings, and knowledge of the distinct characteristics of each condition. Understanding the spectrum of ductal plate malformations and their manifestations as simple hepatic cysts, bile duct hamartomas, Caroli disease/syndrome, and polycystic liver disease is essential for accurate diagnosis and appropriate patient care. This guide serves as a valuable resource in navigating the complexities of cystic liver lesions and refining diagnostic strategies.
