| 2009, Volume 41, Number 1, Page(s) 049-055 | |||||||||||||||||||||||||||||||||
| [ Summary ] [ PDF ] [ Similar Articles ] [ Mail to Author ] [ Mail to Editor ] | |||||||||||||||||||||||||||||||||
| Hepatic Hydatid Disease: Radiographics Findings | |||||||||||||||||||||||||||||||||
| Pınar Polat1, S. Selçuk Atamanalp2 | |||||||||||||||||||||||||||||||||
| 1Atatürk University, Faculty of Medicine, Department of Radiology, Erzurum, Turkey 2Atatürk University, Faculty of Medicine, Department of General Surgery, Erzurum, Turkey |
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| Keywords: Hydatid disease imaging, Hydatid cyst, Liver, Imaging modalities | |||||||||||||||||||||||||||||||||
| Abstract | |||||||||||||||||||||||||||||||||
Hydatid disease (HD) is a unique parasitic disease that primarily
affects the liver and is endemic in many parts of the
world. There are four types of hydatid cysts (HCs) with various
levels of organ involvement. All four HC types can be
seen in the liver, with the right lobe being the most common
site of involvement. There are many potential intrahepatic
and extrahepatic complications that affect the liver and other
organs. Intrahepatic complications include vascular and biliary
complications and infection. With rare portal vein compression
and decreased vascular supply, the involved lobe may
show atrophic changes while the other lobe becomes hypertrophic.
Intrahepatic biliary dilatation results from large cysts
compressing intrahepatic ducts or due to cyst rupture into
the biliary ducts. Cyst infection may also occur via a similar
mechanism. Extrahepatic complications result from exophytic
growth and hematogeneous dissemination. Exophytic growth
causes migration of cysts into the lungs, mediastinum, heart
and peritoneum. Hematogeneous dissemination involves all
other organs. |
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| Introduction | |||||||||||||||||||||||||||||||||
Hydatid disease (HD) is a common parasitic disease produced
by the larval stage of the Echinococcus tapeworm.
There are two types of echinococcus infection.
E. granulosus is common in regions where animal husbandry
is common (the Mediterranean, Africa, South America, the
Middle East, Australia, and New Zealand). The eastern part of
Turkey is an endemic region. E. granulosus is a more frequent
cause of hydatid disease in humans than E. multilocularis 1-5. In this paper, we discuss a variety of radiological and pathological findings in surgically- and laboratory-proven cases of hepatic hydatid disease at our hospital over the past 10 years. In addition to radiographic findings, we present the local complications of hydatid disease.
Life Cycle of E. granulosus
Cyst Structure and Radiographic Findings The middle laminated membrane and the germinal layer form the true cyst wall, usually referred to as endocyst; the acellular laminated membrane is occasionally referred to as the ectocyst. The thicknesses of these layers depend on the tissue in which the cyst is located. The layers tend to be thick in the liver, less well developed in muscle, absent in bone, and sometimes visible in the brain 1-3.
Types of hydatid disease and radiographic findings Type I cysts constitute the initial and active phase of hydatid disease. The three layers are intact. The external rupture of these cysts, due to either iatrogenic or traumatic causes, can cause disseminated disease. Type I cysts are important in disease spread to other anatomic sites in addition to the well-described hematogeneous and lymphatic route. The walls of ruptured type I cysts are generally imperceptible since they become infected and rupture. Ultrasound (US) reveals an anechoic, well-defined cystic lesion with small echogenic foci or “falling snowflakes” consistent with hydatid sands changing with patient position (Figure 1). Computed tomography (CT) detects a water-attenuating lesion with well-defined borders. Pseudo contrast enhancement at the cyst wall can be seen due to compressed host tissue (Figure 2).
True wall enhancement is seen in infected Type I cysts. Magnetic resonance imaging (MRI) reveals a homogeneous hypointense lesion on T1-weighted images and a homogeneous hyperintense lesion on T2-weighted images. The presence of a hypointense rim at the cyst periphery has been described as a characteristic of hydatid cysts (as opposed to non-parasitic cysts), but it is non-specific. This hypointense rim may be seen in long-standing Type I cysts due to a fibrotic response of neighboring host tissue or to slight calcifications within the cyst wall. Type II hydatid cysts Type II HCs represent the active phase of hydatid diseases in the parasite life cycle and in the dissemination of the hydatid disease. Although Type I HCs cannot spread to host parenchyma other than by external rupture, type II HCs can be spread to nearby tissue by outpouching a new cyst from main cyst cavity. An hourglass appearance and additional type II HCs can be seen. This type of HCs can be classified into three stages according to the arrangement of daughter vesicles within the cyst cavity. In type IIA HCs, daughter cysts arrange at the cyst periphery. The CT density of the mother cyst is higher than the daughter cysts (Figure 3). MR imaging shows the daughter cysts as hypointense or isointense relative to the maternal matrix on T1- and T2–weighted images. Type IIB HCs contain larger, irregularly shaped daughter cysts that occupy almost the entire volume of mother cyst, creating a “rosette” appearance (Figure 4). Type IIC HCs are type IIA and type IIB cysts that contain scattered calcifications within the cyst wall and daughter cysts within the cavity. Scattered calcification at the cyst wall does not imply a dead cyst in the presence of daughter cysts, but simply degeneration at the cyst wall (Figure 5).
Type III: Calcified HCs Type III HCs constitute the inactive or dead phase of HD. In this phase, HD cannot spread, and in the absence of mass effect or other complications, there is no need for surgery. These HCs are seen in three types: (1) Total and thick continuous calcification (ring-like) of the cyst wall (Figure 5), (2) total calcification within the cyst matrix and a decrease in cyst size (Figure 6), and (3) curvilinear calcification within the ruptured internal membranes (Figure 7). CT is the preferred imaging method to evaluate these types of HCs due to calcification.
Type IV: Complicated HCs The complications of HCs can be seen in all types of HCs except completely calcified Type III cysts. The complications, explained below, can be due to profound cyst size and subsequent mass effect on neighboring organs. Other complications include internal and external rupture of HCs, secondary site involvement due to invasion of various anatomic barriers (e.g., diaphragm) and superinfection. Ruptures can be seen in 50-90% of cases. Internal cyst rupture is detected by the detachment of the endocyst from the pericyst and is probably related to decreasing intracystic pressure, degeneration, host response, trauma or response to medical therapy and percutaneous drainage 1-3. Internal rupture causes death of the parasite. In the acute phase of rupture, internal membranes can be seen as floating structures inside the cavity; this is called the “water lily sign” (Figure 8). With time, cystic fluid decreases, and the HC mimics a solid mass. Collapsed membranes within the cavity are detected as serpentine structures (Figure 9).
The involvement of various organs
Hepatic Hydatid Disease
The complications of hydatid disease in liver One of the most important complications occurs when HCs (type I and II) reach large sizes due to an active growth phase. Large type I and II HCs can compress neighboring tissue and cause mass effect. If there is compression of vascular structures, especially portal vein, the involved lobe becomes atrophic due to decreased portal venous supply, and compensatory hypertrophy of the other lobes results. This is especially true for E. alveolaris due to microinvasion of portal veins and small biliary ducts. However, we also detected compensatory hypertrophy of uninvolved lobes and atrophy in the involved lobe without complete biliary obstruction (Figure 10).
Large cysts in the hepatic parenchyma can cause biliary duct dilatation by either compression of a nearby duct by mass effect or by perforation into biliary ducts. The compression of nearby biliary ducts can cause microerosion within the bile duct wall and fistula formation. There can be wide perforation that allows cystic contents to spill within the biliary ducts. Hydatid sands in Type I HCs, daughter cysts in type II HCs and ruptured germinal membranes in type IV HCs (Figure 11) may pass through the fistula or perforation, obstruct bile flow, dilate biliary ducts and cause jaundice.
In the presence of a small fistula, it is difficult to show direct communication between cyst and biliary tracts by imaging methods. MR cholangiography may show small biliary ducts entering in the HCs. The detection of dilated biliary ducts and echogenic material within the lumen may be suspicious in the presence of HCs by ultrasound. A ruptured internal membrane floating freely within the cystic cavity may pass through the communication between HC and biliary tract. This may be seen as tubular structures within the biliary lumen. When daughter cysts pass through a direct communication, a cystic lesion may also be seen within the biliary lumen. Rupture of HCs The rupture of HCs can be contained (internal), communicating, and direct. Contained or internal rupture occurs when the detachment of the endocyst from the pericyst takes place, but the pericyst remains intact. Internal rupture may be related to degeneration, trauma or response to therapy, and it represents the inactive phase of the HCs. Communicating rupture implies passage of cystic contents into biliary tracts. Communicating rupture may be seen in type I, II and IV HCs that internally rupture. When both the pericyst and endocyst rupture, direct rupture occurs. Internal rupture of cyst implies the death of the parasite, but external rupture causes dissemination to other organs. Type II HCs can only rupture externally. Trauma, degeneration and iatrogenic rupture due to surgical and percutaneous treatment cause external rupture of type I and II HCs (Figure 12). Sudden death, anaphylactic shock and dissemination of disease can be seen with cystic content spillage into the peritoneal cavity.
When type I and II HCs are found near the hepatic capsule, extension other neighboring organs is easier. Rupture can also be seen in these locations due to a lack of protective tissue surrounding the cyst and deficient pericyct. Spillage of cystic content within peritoneal cavity causes disseminated disease (Figure 13).
Infection of the HCs The presence of air within the cystic cavity is also an important clue. Cyst infection is generally seen in HCs that rupture. Rupture may be internal (communicating) or direct. Rupture of HCs permits bacteria to pass easily into cyst. The wall becomes thicker and exhibits contrast enhancement on CT and MR imaging. Patchy contrast enhancement of neighboring liver parenchyma represents inflammatory changes. CT is superior in detecting gas or air-fluid levels within the cyst (Figure 14).
Exophytic growth The two most common routes of exophytic growth are via the bare area of the liver and gastrohepatic ligament. HCs extend to lung and mediastinum when located at the bare area of liver. The cyst in located near the gastrohepatic ligament can extend into peritoneal cavity (Figure 13). Transdiaphragmatic lung, mediastium and cardiac involvement Transdiaphragmatic involvement of lungs and mediastinum occurs in 0.6- 16% of hepatic hydatid cysts. Transdiaphragmatic migration is most common in HCs located in the 7th and 8th segments of right lobe due to their close proximity to the diaphragm. All HC types can be spread in this way including completely calcified cysts that migrate into the thoracic cavity by adherence to the diaphragmatic surface. When a HC is located in close proximity to the diaphragm and grows large, inital indirect or reactional findings may be seen on imaging methods, including pleural effusion, atelectasis or consolidation due to decreased diaphragmatic motion and incomplete lung expansion. Elevation of the diaphragm due to mass effect may also be seen with large HCs (Figure 15).
A direct sign of transdiaphragmatic migration is the presence of cystic lesion growing through diaphragm into lung, mediastinum and heart (Figure 16). The cyst typically has a characteristic hourglass shape. Sagittal and coronal MR images are helpful. Migration to the lung is frequent in HCs located in the 7th to 8th segments of the liver while mediastinal and cardiac involvement is associated with the 4th and 2nd liver segments.
Migration of HCs into lung from liver is seen as a continuation of the main cyst in the liver. Erosion or adhesion to the diaphragm is the cause of cyst migration. If cyst erodes the bronchial wall, as seen in primary lung HCs, internal perforation is considered. Expectoration of cystic membranes can occur with a bronchial fistula. Direct perforation of HCs into lung parenchyma causes prominent parenchymal consolidation and widespread hydatid disease. Perforation of HCs into the pleural cavity causes pleural empyema or multiple pleural cysts. Peritoneal seeding Peritoneal HD is almost always secondary to hepatic disease. Peritoneal echinococcosis in our series was generally due to previous hepatic surgery or spontaneous or traumatic rupture. Spontaneous rupture is a possibility with HCs located near the hepatic capsule or gastrohepatic ligament that grow exophytic. Peritoneal hydatid disease is generally multiple and remains undetected until cysts are large enough to produce symptoms (Figure 13). The overall incidence of peritoneal disease in cases of abdominal HD is approximately 13%. CT and MR imaging is valuable by imaging the entire peritoneal cavity. All kinds of HCs can be seen in the abdominal cavity. Conflict interest statement The authors declare that they have no conflict of interest to the publication of this article. |
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