(Radiographics. 2000;20:795-817.)
© RSNA, 2000
Hydatid Disease: Radiologic and Pathologic Features and Complications1
(CME Available in print version and on RSNA Link)
Iván Pedrosa, MD,
Antonio Saíz, MD,
Juan Arrazola, MD,
Joaquín Ferreirós, MD and
César S. Pedrosa, MD
1 From the Department of Diagnostic Imaging, Hospital Clínico San Carlos, Universidad Complutense, C/ Martín Lagos s/n, 28040 Madrid, Spain. Presented as a scientific exhibit at the 1998 RSNA scientific assembly. Received April 5, 1999; revision requested May 10 and received July 7; accepted July 7. Address reprint requests to I.P. (e-mail: ipedrosa@hotmail.com).
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Abstract
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Hydatid disease primarily affects the liver and typically demonstrates characteristic imaging findings. However, there are many potential local complications (eg, intrahepatic complications, exophytic growth, transdiaphragmatic thoracic involvement, perforation into hollow viscera, peritoneal seeding, biliary communication, portal vein involvement, abdominal wall invasion). Furthermore, secondary involvement due to hematogenous dissemination may be seen in almost any anatomic location (eg, lung, kidney, spleen, bone, brain). Ultrasonography (US) is particularly useful for the detection of cystic membranes, septa, and hydatid sand. Computed tomography (CT) best demonstrates cyst wall calcification and cyst infection. CT and magnetic resonance (MR) imaging may demonstrate cyst wall defects as well as the passage of contents through a defect. Chest radiography, US, CT, and MR imaging are all useful in depicting transdiaphragmatic migration of hydatid disease. CT is the modality of choice in peritoneal seeding. US and CT demonstrate rupture in most cases that involve wide communication. Indirect signs of biliary communication include increased echogenicity at US and fluid levels and signal intensity changes at MR imaging. CT allows precise assessment of osseous lesions, whereas MR imaging is superior in demonstrating neural involvement. Familiarity with atypical manifestations of hydatid disease may be helpful in making a prompt, accurate diagnosis.
Index Terms: Echinococcosis, **.20832 • Liver, calcification, 761.81 • Liver, cysts, 761.311 • Liver, echinococcosis, 761.2083 • Parasites, **.20832
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LEARNING OBJECTIVES FOR TEST 5
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After reading this article and taking the test, the reader will be able to:
• Discuss the prevalence of hydatid disease in various anatomic locations.
• Describe the pathophysiologic features of hydatid disease and their imaging appearances.
• Recognize the most frequent complications associated with hepatic hydatid disease.
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Introduction
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Hydatid disease is a worldwide zoonosis produced by the larval stage of the Echinococcus tapeworm (Fig 1). The two main types of hydatid disease are caused by E granulosus and E multilocularis. The former is commonly seen in the great grazing regions of the world—particularly the Mediterranean region, Africa, South America, the Middle East, Australia, and New Zealand—and is the most frequently encountered type of hydatid disease in humans (1–3). The classical findings in hydatid disease are well known; however, findings related to disease complications and unusual anatomic locations are less frequently described in the literature.
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Figure 1. Geographic distribution of hydatid disease. Map shows areas in which hydatid disease is endemic due to the transmission of E granulosus by means of the dog-sheep cycle (solid red areas). Red stripes indicate areas where transmission occurs by means of alternative life cycles in which carnivores such as wolves and foxes serve as definitive hosts and goats, camels, and horses serve as intermediate hosts. Transmission by means of alternative life cycles is common in North Africa, the Middle and Far East, the United States, Canada, and Iceland.
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In this article, we present the life cycle of E granulosus and its importance in the development of hepatic echinococcosis. We also discuss and illustrate a variety of radiologic and pathologic findings in over 500 surgically proved cases of hydatid disease seen at our institution over the past 16 years. Many of these cases involved local complications (eg, intrahepatic complications, exophytic growth, transdiaphragmatic thoracic involvement, perforation into hollow viscera, peritoneal seeding, biliary communication, portal vein involvement, abdominal wall invasion) or involvement of more distant anatomic sites due to hematogenous dissemination (eg, lung, kidney, spleen, bone, brain).
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Life Cycle of E granulosus
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The life cycle of E granulosus involves two hosts (Fig 2). The definitive host is usually a dog but may be some other carnivore. The adult worm of the parasite lives in the proximal small bowel of the definitive host, attached by hooklets to the mucosa. Eggs are released into the host's intestine and excreted in the feces (1–4). Sheep are the most common intermediate hosts. They ingest the ovum while grazing on contaminated ground. The ovum loses its protective chitinous layer as it is digested in the duodenum. The released hexacanth embryo, or oncosphere, passes through the intestinal wall into the portal circulation and develops into a cyst within the liver (Fig 3). When the definitive host eats the viscera of the intermediate host, the cycle is completed (1,3,4). Humans may become intermediate hosts through contact with a definitive host (usually a domesticated dog) or ingestion of contaminated water or vegetables (1,3,4). Once in the human liver, cysts grow to 1 cm during the first 6 months and 2–3 cm annually thereafter, depending on host tissue resistance (3–5).
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Figure 2. Life cycle (dog-sheep cycle) of E granulosus. Diagram shows the most prevalent life cycle of E granulosus, in which a dog and sheep serve as the definitive and intermediate hosts, respectively.
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Figure 3a. Calcified cysts in an intermediate host. (a) Photograph of a resected specimen from a sheep liver shows two calcified cysts (arrows). (b) Computed tomographic (CT) scan of the specimen shows the larger cyst (arrow). Dense calcification of the pericyst and cyst contents is common in end-stage hydatid disease and implies the death of the parasite.
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Figure 3b. Calcified cysts in an intermediate host. (a) Photograph of a resected specimen from a sheep liver shows two calcified cysts (arrows). (b) Computed tomographic (CT) scan of the specimen shows the larger cyst (arrow). Dense calcification of the pericyst and cyst contents is common in end-stage hydatid disease and implies the death of the parasite.
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Hydatid Cyst Structure
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The hydatid cyst has three layers: (a) the outer pericyst, composed of modified host cells that form a dense and fibrous protective zone; (b) the middle laminated membrane, which is acellular and allows the passage of nutrients; and (c) the inner germinal layer, where the scolices (the larval stage of the parasite) and the laminated membrane are produced. The middle laminated membrane and the germinal layer form the true wall of the cyst, usually referred to as the endocyst, although the acellular laminated membrane is occasionally referred to as the ectocyst (1,2,4).
Daughter vesicles (brood capsules) are small spheres that contain the protoscolices and are formed from rests of the germinal layer. Before becoming daughter cysts, these daughter vesicles are attached by a pedicle to the germinal layer of the mother cyst. At gross examination, the vesicles resemble a bunch of grapes (Fig 4). Daughter cysts may grow through the wall of the mother cyst, particularly in bone disease (1).
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Figure 4. Multivesicular cyst. Photograph of a human kidney that has been sectioned along the midcoronal plane demonstrates a large cyst with the typical "bunch of grapes" appearance (black arrows) due to the presence of daughter cysts (arrowheads). White arrows indicate the ureter. (Courtesy of Mónica García-Cosío, MD, Department of Pathology, Hospital Ramón y Cajal, Madrid, Spain.)
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Cyst fluid is clear or pale yellow, has a neutral pH, and contains sodium chloride, proteins, glucose, ions, lipids, and polysaccharides. The fluid is antigenic and may also contain scolices and hooklets. When vesicles rupture within the cyst, scolices pass into the cyst fluid and form a white sediment known as hydatid sand (Fig 5) (1,2).
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Figure 5. Scolex in a middle-aged man who presented with jaundice, fever, and right upper quadrant pain. Photomicrograph (original magnification, x 40; Papanicolaou stain) clearly depicts a scolex obtained from pleural fluid. E granulosus uses hooklets (arrows) to attach itself to the duodenal mucosa of the host.
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Hydatid Disease in Humans
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Once the parasite passes through the intestinal wall to reach the portal venous system or lymphatic system, the liver acts as the first line of defense and is therefore the most frequently involved organ. In humans, hydatid disease involves the liver in approximately 75% of cases, the lung in 15%, and other anatomic locations in 10% (1–3).
Hepatic Disease
The right lobe is the most frequently involved portion of the liver. Imaging findings in hepatic hydatid disease depend on the stage of cyst growth (ie, whether the cyst is unilocular, contains daughter vesicles, contains daughter cysts, is partially calcified, or is completely calcified [dead]).
Calcification is seen at radiography in 20%–30% of hydatid cysts and usually manifests with a curvilinear or ringlike pattern representing calcification of the pericyst (3). During the natural evolution toward healing, dense calcification of all components of the cyst occurs. Although the death of the parasite is not necessarily indicated by calcification of the pericyst, it is implied by complete calcification (Fig 6) (3,4,6).
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Figure 6. Completely calcified hydatid cysts in a 62-year-old patient. Digital scout image from an abdominal CT examination shows three round, densely calcified lesions (arrows). The lesions were discovered incidentally. The patient confirmed that they had been present for many years.
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The ultrasonographic (US) appearance of hydatid cysts may vary. Several classification schemes based on cyst appearance have been proposed (6–8). The cyst wall usually manifests as double echogenic lines separated by a hypoechogenic layer (6). Simple cysts do not demonstrate internal structures, although multiple echogenic foci due to hydatid sand may be seen within the lesion by repositioning the patient. The echogenic foci quickly fall to the most dependent portion of the cavity without forming visible strata (8). This finding has been referred to as the snowstorm sign (6,9).
Detachment of the endocyst from the pericyst is probably related to decreasing intracystic pressure, degeneration, host response, trauma, or response to therapy (3,6,7,10). The cyst may appear as a well-defined fluid collection with a localized split in the wall and "floating membranes" inside the cavity (3,4,7,9,11). Complete detachment of the membranes inside the cyst has been referred to as the US water lily sign because of its resemblance to the radiographic water lily sign in pulmonary cysts (3,6). US is the most sensitive modality for the detection of membranes, septa, and hydatid sand within the cyst (6).
Multivesicular cysts manifest as well-defined fluid collections in a honeycomb pattern with multiple septa representing the walls of the daughter cysts (7). Daughter cysts appear as cysts within a cyst. When daughter cysts are separated by the hydatid matrix (a material with mixed echogenicity), they demonstrate a "wheel spoke" pattern (5). The matrix represents hydatid fluid containing membranes of broken daughter vesicles, scolices, and hydatid sand. Membranes may appear within the matrix as serpentine linear structures, a finding that is highly specific for hydatid disease (5,6).
When the matrix fills the cyst completely, a mixed echogenic pattern is created that mimics a solid mass (Fig 7) (3,7,10). Because differentiation of this lesion from other hepatic masses or abscesses is usually difficult, it is important to look for daughter vesicles or membranes within the lesion that may help in making a correct diagnosis (12).
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Figure 7a. Hydatid cyst mimicking a solid mass. (a) US image shows a round lesion with a mixed echogenic pattern in segment IV of the liver (L). Note the serpentine structures within the matrix representing collapsed membranes (arrowheads). PV = left portal vein. (b) Unenhanced CT scan obtained in a different patient shows a hydatid cyst with the characteristic solid appearance growing exophytically from the right hepatic lobe. There are several minimally calcified foci within the cyst (arrows).
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Figure 7b. Hydatid cyst mimicking a solid mass. (a) US image shows a round lesion with a mixed echogenic pattern in segment IV of the liver (L). Note the serpentine structures within the matrix representing collapsed membranes (arrowheads). PV = left portal vein. (b) Unenhanced CT scan obtained in a different patient shows a hydatid cyst with the characteristic solid appearance growing exophytically from the right hepatic lobe. There are several minimally calcified foci within the cyst (arrows).
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Cyst calcification usually occurs in the cyst wall, although internal calcification in the matrix may also be seen. US demonstrates a hyperechoic contour with a cone-shaped acoustic shadow (7). When the cyst wall is heavily calcified, only the anterior portion of the wall is visualized and appears as a thick arch with a posterior concavity (7). As stated earlier, partial calcification of the cyst does not always indicate the death of the parasite; nevertheless, densely calcified cysts may be assumed to be inactive (4,8).
CT is indicated in cases in which US fails due to patient-related difficulties (eg, obesity, excessive intestinal gas, abdominal wall deformities, previous surgery) or disease complications. CT has a high sensitivity and specificity for hepatic hydatid disease (13). Intravenous administration of contrast material is not necessary unless complications are suspected, especially infection and communication with the biliary tree (13).
CT may display the same findings as US. Cyst fluid usually demonstrates water attenuation (3–30 HU) (14). Calcification of the cyst wall or internal septa is easily detected at CT (Fig 8). A hydatid cyst typically demonstrates a high-attenuation wall at unenhanced CT even without calcification (Fig 9). There is no clear explanation for this finding, which could be missed in patients with increased liver attenuation due to hemochromatosis, drugs (eg, amiodarone), and so on, or because of hepatic parenchymal enhancement following contrast material administration (6). Detachment of the laminated membrane from the pericyst can be visualized as linear areas of increased attenuation within the cyst (Fig 10) (1,5,6). Daughter vesicles manifest as round structures located peripherally within the mother cyst. In our experience, they usually contain fluid with a lower attenuation than that of the fluid in the mother cyst (Fig 11).
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Figure 8. Calcified unilocular hydatid cyst. Contrast material-enhanced CT scan shows a round lesion with water attenuation and a ringlike pattern of calcification (arrows). This pattern represents calcification of the pericyst and strongly suggests a diagnosis of hydatid cyst.
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Figure 9. Typical unilocular hydatid cyst. Unenhanced CT scan shows a large hydatid cyst with a noncalcified, high-attenuation wall in the right hepatic lobe (arrows). This finding can be missed if only contrast-enhanced CT is performed. Because of its elasticity, the cyst accommodates itself to neighboring structures.
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Figure 10. Hydatid cyst with collapsed parasitic membranes. Unenhanced CT scan shows a dense circular area of increased attenuation within the cyst representing detached membranes (arrows).
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Figure 11a. Hydatid cyst with multiple daughter vesicles. (a) US image shows three distinct daughter vesicles (arrows) with the typical peripheral location within the mother cyst. A hydatid matrix with a solid appearance is seen filling the rest of the cavity. (b) Unenhanced CT scan shows the typical peripheral location of the daughter vesicles within the mother cyst (arrows). There is partial calcification of the pericyst.
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Figure 11b. Hydatid cyst with multiple daughter vesicles. (a) US image shows three distinct daughter vesicles (arrows) with the typical peripheral location within the mother cyst. A hydatid matrix with a solid appearance is seen filling the rest of the cavity. (b) Unenhanced CT scan shows the typical peripheral location of the daughter vesicles within the mother cyst (arrows). There is partial calcification of the pericyst.
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Hepatic hydatid cysts may have a low-signal-intensity rim on T2-weighted magnetic resonance (MR) images. This finding has been proposed as a characteristic sign of hydatid disease. It probably represents the outer layer of the hydatid cyst (pericyst), which is rich in collagen and is generated by the host (15). When present, daughter cysts are seen as cystic structures attached to the germinal layer that are hypointense relative to the intracystic fluid on T1-weighted images and hyperintense on T2-weighted images (15). Collapsed parasitic membranes appear at MR imaging as twisted linear structures within the cyst (5). Although cyst wall calcification is more clearly depicted at CT, MR imaging is superior in demonstrating irregularities of the rim. These irregularities probably represent incipient detachment of the membranes (Fig 12) (15).
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Figure 12. Noncalcified hydatid cysts. Axial spin-echo T1-weighted MR image demonstrates two large cystic lesions in the right hepatic lobe. The more anterior cyst has high signal intensity, probably due to a reduction in the water content of the fluid. The cyst also contains several round, nodular, low-signal-intensity lesions representing daughter cysts (arrowheads). The mother cyst has a characteristic low-signal-intensity rim (straight solid arrows). The more posterior cyst has homogeneous low signal intensity with a double ring: The hypointense outer ring represents the pericyst (open arrows), and the partially "wrinkled," intermediate-signal-intensity inner ring represents incompletely detached membranes (curved arrow).
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Local Complications
Intrahepatic Complications.—Intrahepatic complications of hydatid cysts include cyst rupture and infection. Although rupture may be related to minor trauma, the natural history of hepatic hydatid cysts implies rupture as a complication in 50%–90% of cases (9). Passage of the cyst contents into the host's blood circulation can produce anaphylaxia due to the antigenic nature of the cyst fluid (3,5), although cyst rupture may be clinically silent (3).
Three different types of cyst rupture have been described in the literature: contained, communicating, and direct (1,4,9,13). Contained ruptures occur when the endocyst ruptures but the pericyst remains intact. Endocyst detachment is seen at cross-sectional imaging as floating membranes within the cyst. Contained rupture may be related to degeneration, trauma, or response to therapy. Communicating rupture implies passage of the cyst contents into the biliary radicles that have been incorporated into the pericyst (9,11). Direct rupture occurs when both the pericyst and endocyst rupture, allowing free spillage of hydatid material into the peritoneal cavity, pleural cavity, hollow viscera, abdominal wall, and so on (16). Direct rupture is more frequent in lesions located near the edge of the liver, where there may be less protection for the cyst due to a deficient pericyst and little host tissue to offer support (4). In communicating and direct ruptures, the cyst empties and may became smaller and less spherical (4,9,11).
Both US and CT may demonstrate a cyst wall defect and passage of the cyst contents through the defect, particularly in direct communication (Fig 13) (4,9). MR imaging may demonstrate interruption in the low-signal-intensity rim of the cyst wall as well as extrusion of contents through the defect (16).
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Figure 13. Intrahepatic rupture of a hydatid cyst in a patient with acute abdominal pain in the right upper quadrant that was not related to trauma. Unenhanced CT scan shows a partially collapsed cyst that has lost its normal spherical shape. The lesion is partially calcified, and a daughter cyst is identified within the lesion (arrowhead). An incomplete wall is seen along the lateral aspect of the cyst in connection with a hypoattenuating area caused by intrahepatic extravasation of the cyst contents (arrows).
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Infection occurs only after rupture of both the pericyst and endocyst (communicating and direct rupture), which allows bacteria to pass easily into the cyst (5%–8% of cases) (4,9,11,13). At clinical examination, infection usually manifests as a hepatic abscess. US and CT findings are similar to those in other hepatic abscesses. US findings are nonspecific. Although the lesion usually demonstrates poorly defined margins, it may remain well defined (3). Findings that suggest infection include a solid appearance, a mixed pattern produced by solid and fluid elements, internal echogenic foci, and air or air-fluid levels within the cyst (3,5,8).
CT is the modality of choice for demonstrating cyst infection. Infected cysts may manifest at CT as poorly defined masses, in contrast to the more clearly defined masses seen in uncomplicated cases (13). Contrast-enhanced CT may reveal the typical high-attenuation rim representing abscesses surrounding the lesion (17). Occasionally, patchy areas of contrast-enhanced liver parenchyma are seen in the vicinity of the lesion representing inflammatory changes (Fig 14). CT also most clearly depicts gas or air-fluid levels within the cyst (17).
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Figure 14a. Infected hydatid cyst in a patient with a history of hepatic hydatid disease. The patient presented with sudden onset of pain in the right upper quadrant, fever, and leukocytosis. (a) Unenhanced CT scan reveals an irregular, bilobulated cystic lesion in the right lobe of the liver. The thin, high-attenuation wall of the lesion (solid arrows) is surrounded by an area of low-attenuation liver parenchyma (open arrows). (b) On a dynamic CT scan obtained with bolus injection of contrast material, the cyst wall demonstrates enhancement as well as multiple layers ("split wall") (straight arrows), findings that indicate early separation of the laminated membrane from the pericyst. There is also marked enhancement of the surrounding liver parenchyma (curved arrows), probably due to perilesional inflammatory changes.
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Figure 14b. Infected hydatid cyst in a patient with a history of hepatic hydatid disease. The patient presented with sudden onset of pain in the right upper quadrant, fever, and leukocytosis. (a) Unenhanced CT scan reveals an irregular, bilobulated cystic lesion in the right lobe of the liver. The thin, high-attenuation wall of the lesion (solid arrows) is surrounded by an area of low-attenuation liver parenchyma (open arrows). (b) On a dynamic CT scan obtained with bolus injection of contrast material, the cyst wall demonstrates enhancement as well as multiple layers ("split wall") (straight arrows), findings that indicate early separation of the laminated membrane from the pericyst. There is also marked enhancement of the surrounding liver parenchyma (curved arrows), probably due to perilesional inflammatory changes.
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Exophytic Growth.—Hydatid cysts may use the natural routes provided by the liver capsule, ligaments, and peritoneum to progress beyond the boundaries of the liver. The two most common routes of exophytic growth are the bare area of the liver and the gastrohepatic ligament (Fig 15).
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Figure 15a. Exophytic growth. (a) Drawing illustrates the two most common routes of exophytic growth: the bare area of the liver (A) and the gastrohepatic ligament (B). Growth through the bare area leads to involvement of the diaphragm (arrows) and extension into the thorax. From our experience, the gastrohepatic ligament appears to be the path by which the cyst reaches the stomach (S). (b) CT scan demonstrates a partially calcified multivesicular cyst in the right lobe of the liver. The cyst has grown posteriorly through the bare area of the liver and has a typical hourglass shape. Note the proximity of the cyst to the diaphragm (arrows), which facilitates transdiaphragmatic thoracic involvement. Arrowheads indicate daughter vesicles.
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Figure 15b. Exophytic growth. (a) Drawing illustrates the two most common routes of exophytic growth: the bare area of the liver (A) and the gastrohepatic ligament (B). Growth through the bare area leads to involvement of the diaphragm (arrows) and extension into the thorax. From our experience, the gastrohepatic ligament appears to be the path by which the cyst reaches the stomach (S). (b) CT scan demonstrates a partially calcified multivesicular cyst in the right lobe of the liver. The cyst has grown posteriorly through the bare area of the liver and has a typical hourglass shape. Note the proximity of the cyst to the diaphragm (arrows), which facilitates transdiaphragmatic thoracic involvement. Arrowheads indicate daughter vesicles.
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Transdiaphragmatic Thoracic Involvement.—Involvement of the diaphragm and thoracic cavity occurs in 0.6%–16% of cases of hepatic hydatid disease (18). Previously reported mortality rates in these cases vary from 5.6% to 43.7% (6). Transdiaphragmatic migration of hydatid disease from the posterior segments of the right hepatic lobe has been reported to be a common complication and is probably related to their proximity to the diaphragm (6). In our experience, the bare area of the liver is by far the most common route of transdiaphragmatic migration. This area has been described as a potential pathway for the migration of hepatic abscesses from the liver to the mediastinum (19). This may be due to the lack of peritoneal covering in this particular area, resulting in decreased resistance to cyst growth. Transdiaphragmatic migration from the right hepatic lobe through the diaphragm via other routes is less common.
Transdiaphragmatic migration varies from simple adherence to the diaphragm to rupture into the pleural cavity, seeding in the pulmonary parenchyma, and chronic bronchial fistula (Fig 16). Surgical classification into five progressive stages has been proposed (18).
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Figure 16. Diaphragmatic involvement. Unenhanced CT scan demonstrates a cystic lesion with a partially calcified septum (straight arrow). There is no evidence of perforation of the diaphragm (curved arrows), although at surgery, the cyst was seen to adhere to the diaphragmatic surface.
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Chest radiography may show pleural effusion, elevation of the diaphragm, lung consolidation, or laminated atelectasis at the lung base. Occasionally, an hourglass-shaped lesion or a loculated pleural effusion similar to an empyema can be seen in the posterior thorax on the lateral projection (6).
US can help confirm the presence of hepatic hydatid disease and demonstrate pleural effusion, although the diaphragmatic defect is rarely seen.
CT is valuable for demonstrating transdiaphragmatic migration of hydatid disease and evaluating the thoracic component (Figs 17, 18) (18).
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Figure 17. Transdiaphragmatic migration in a patient with hydatid disease at a lower hepatic level. Unenhanced CT scan reveals multiple cysts. The most external cyst is seen splitting the leaves of the diaphragm (straight arrow), the middle cyst has an incomplete posterior wall and appears to be growing into the chest cavity (arrowheads), and the medial cyst is clearly located in the thoracic cavity (curved arrow). These findings were confirmed at surgery.
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Figure 18. Transdiaphragmatic migration in a 56-year-old man with a history of surgery for hepatic hydatid disease. Lateral chest radiography demonstrated a lesion that appeared to be a lung nodule. Contrast-enhanced CT scan obtained at the level of the dome of the diaphragm shows a partially calcified cyst originating in the posterior segment of the right hepatic lobe and growing through the diaphragm into the lung (arrows). The cyst has the characteristic hourglass shape. There is a triangular, low-attenuation area in the lateral aspect of the right lobe (arrowhead) that is related to the prior surgery.
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Sagittal and coronal MR imaging is also very useful in demonstrating the migration of the cyst through the diaphragm (Fig 19) (17). MR imaging allows accurate presurgical diagnosis based on various grades of transdiaphragmatic migration and so proves helpful in surgical planning (Fig 20) (18).
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Figure 19a. Pleural empyema secondary to a perforated hepatic hydatid cyst in a patient with a history of hepatic hydatid disease. The patient presented with acute chest pain, fever, and leukocytosis. Chest radiography demonstrated findings typical of empyema. (a) Sagittal T1-weighted MR image obtained after intravenous administration of contrast material shows a posteriorly located hepatic cyst (C) in communication with a loculated pleural collection (P). There is marked enhancement of the thickened pleura (arrows). (b) Sagittal T2-weighted MR image obtained at the same level shows multiple vesicles within the mother cyst (C), the fistula (arrows), and the pleural cavity (P). Note the exophytic growth of the cyst through the bare area of the liver (L). K = right kidney.
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Figure 19b. Pleural empyema secondary to a perforated hepatic hydatid cyst in a patient with a history of hepatic hydatid disease. The patient presented with acute chest pain, fever, and leukocytosis. Chest radiography demonstrated findings typical of empyema. (a) Sagittal T1-weighted MR image obtained after intravenous administration of contrast material shows a posteriorly located hepatic cyst (C) in communication with a loculated pleural collection (P). There is marked enhancement of the thickened pleura (arrows). (b) Sagittal T2-weighted MR image obtained at the same level shows multiple vesicles within the mother cyst (C), the fistula (arrows), and the pleural cavity (P). Note the exophytic growth of the cyst through the bare area of the liver (L). K = right kidney.
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Figure 20a. Pulmonary involvement with a bronchial fistula in a patient with cough, fever, malaise, and a high white blood cell count. (a) Coronal fat-suppressed fast spin-echo T2-weighted MR image obtained at the hepatic level shows a large pulmonary cyst with partial rupture of the wall (black arrows). Another cyst is seen superiorly in the lung parenchyma (C) with a high-signal-intensity band along its lateral aspect due to pleural effusion (arrowheads). White arrow indicates linear structures within the cyst representing detached membranes. (b) On a sagittal fat-suppressed fast spin-echo T2-weighted MR image, the central portion of the diaphragm is clearly absent (solid arrows). The cyst contains an air-fluid level (arrowheads) due to the presence of a bronchial fistula, a finding that was confirmed at surgery. Note the high signal intensity of the accompanying pleural empyema (E). Open arrow indicates linear structures within the cyst (cf a).
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Figure 20b. Pulmonary involvement with a bronchial fistula in a patient with cough, fever, malaise, and a high white blood cell count. (a) Coronal fat-suppressed fast spin-echo T2-weighted MR image obtained at the hepatic level shows a large pulmonary cyst with partial rupture of the wall (black arrows). Another cyst is seen superiorly in the lung parenchyma (C) with a high-signal-intensity band along its lateral aspect due to pleural effusion (arrowheads). White arrow indicates linear structures within the cyst representing detached membranes. (b) On a sagittal fat-suppressed fast spin-echo T2-weighted MR image, the central portion of the diaphragm is clearly absent (solid arrows). The cyst contains an air-fluid level (arrowheads) due to the presence of a bronchial fistula, a finding that was confirmed at surgery. Note the high signal intensity of the accompanying pleural empyema (E). Open arrow indicates linear structures within the cyst (cf a).
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Perforation into Hollow Viscera.—Spontaneous rupture of the cyst into hollow viscera is an extremely rare complication with an estimated frequency of 0.5% (6). This complication may be accompanied by clinical findings of hydatidemesis or hydatidorrhea (20). Typically, the communication is not discovered until surgery, although in some cases it is found at radiology (20). CT may demonstrate a cyst with an air-fluid level or orally administered contrast material inside the cavity. In some cases, CT performed with the patient in the left or right lateral decubitus position may help demonstrate filling or emptying of the cyst cavity. Barium-enhanced CT can be used to demonstrate the fistula between the cyst and the hollow viscus (Fig 21).
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Figure 21a. Hydatid cyst perforation into the gastric antrum. (a) Contrast-enhanced CT scan obtained at the level of the kidneys shows two congruous air-fluid levels. One air-fluid level is inside a calcified cyst (solid arrow), and the other is in the gastric antrum (open arrow). (b) Contrast-enhanced CT scan obtained with the patient in the left lateral decubitus position shows emptying of the cyst contents (C) into the antrum. The interruption of the cyst wall and the fistula are well depicted (arrow). Surgery revealed that the cyst was growing from the left lobe of the liver through the gastrohepatic ligament with a direct rupture into the gastric antrum.
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Figure 21b. Hydatid cyst perforation into the gastric antrum. (a) Contrast-enhanced CT scan obtained at the level of the kidneys shows two congruous air-fluid levels. One air-fluid level is inside a calcified cyst (solid arrow), and the other is in the gastric antrum (open arrow). (b) Contrast-enhanced CT scan obtained with the patient in the left lateral decubitus position shows emptying of the cyst contents (C) into the antrum. The interruption of the cyst wall and the fistula are well depicted (arrow). Surgery revealed that the cyst was growing from the left lobe of the liver through the gastrohepatic ligament with a direct rupture into the gastric antrum.
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Peritoneal Seeding.—Peritoneal echinococcosis is almost always secondary to hepatic disease, although some unusual cases of primary peritoneal involvement have been described (21). The overall frequency of peritoneal disease in cases of echinococcosis involving the abdomen is approximately 13% (21). Most of these cases are related to previous surgery for hepatic disease, although spontaneous, asymptomatic microruptures of hepatic cysts into the peritoneal cavity are not uncommon (12% of cases) (21). Peritoneal echinococcosis usually goes undetected until cysts are large enough to produce symptoms.
CT is the modality of choice in affected patients because it allows imaging of the entire abdomen and pelvis. Cysts may be multiple and located anywhere in the peritoneal cavity (Fig 22). Imaging findings are similar to those in hepatic disease. Peritoneal hydatid disease may grow and occupy the entire peritoneal cavity, simulating a multiloculated mass. This pathologic condition has been referred to as encysted peritoneal hydatidosis (6).
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Figure 22. Peritoneal seeding in the region of the transverse mesocolon in a patient with a history of surgery for hepatic hydatid disease. Unenhanced CT scan shows two cystic masses in the right lobe of the liver (arrows). There are multiple cysts (C) lying between the transverse colon (arrowheads) and the collapsed, contrast material-filled stomach (S), which is seen anterior to the pancreas (P).
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Biliary Communication.—Communication of hydatid disease with the biliary tree has been described in up to 90% of hepatic cysts (6,16). This can be explained by the fact that during cyst growth, small biliary radicles are incorporated into the pericyst (9,11). However, frank rupture into the biliary tree occurs in only 5%–15% of cases (6). Jaundice, fever, and chills are the most frequent symptoms related to biliary obstruction and cholangitis (22). Communicating rupture of a cyst into the biliary system may occur through small fissures or bile-cyst fistulas (55% of cases) or through a wide perforation that allows access to a main biliary branch (16). It is essential that imaging findings that are suspicious for biliary communication be reported to ensure adequate surgical management. However, dilatation of the biliary tree does not always indicate cyst rupture; it may result from direct compression of the biliary branches by the cyst or an associated common bile duct stone (6).
The only direct sign of rupture into the biliary tree is the visualization of the cyst wall defect or of a communication between the cyst and a biliary radicle (16). In cases of wide communication, US and CT demonstrate rupture in 46%–75% and 77% of cases, respectively (6,16). In some instances, the passage of hydatid material through the defect and subsequent filling of the biliary radicles or common bile duct may be seen. In such cases, US demonstrates anechoic, rounded or echogenic linear structures without posterior acoustic shadowing in the biliary tract (16). CT can demonstrate high-attenuation material passing through the cyst wall defect and filling the biliary radicles or common bile duct. CT is superior to US in depicting hydatid cyst contents in the distal segment of the common bile duct (Fig 23). Endoscopic retrograde cholangiopancreatography and percutaneous transhepatic cholangiography can demonstrate the communication in more detail (22).
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Figure 23a. Daughter cyst in the distal common bile duct in the same patient as in (a) Contrast-enhanced upper abdominal CT scan demonstrates a "wrinkled" cyst in the right hepatic lobe with peripheral enhancement due to associated infection (C). Note the biliary tree dilatation (arrows) and the marked dilatation of the common bile duct at the porta hepatis (arrowhead). There is also a right adrenal mass with low attenuation, a finding that is consistent with adenoma. (b) On a CT scan obtained at the lowest level of the head of the pancreas, a round, cystic structure is barely seen within the dilated common bile duct (arrow). Surgery revealed multiple daughter vesicles filling the distal common bile duct.
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Figure 23b. Daughter cyst in the distal common bile duct in the same patient as in (a) Contrast-enhanced upper abdominal CT scan demonstrates a "wrinkled" cyst in the right hepatic lobe with peripheral enhancement due to associated infection (C). Note the biliary tree dilatation (arrows) and the marked dilatation of the common bile duct at the porta hepatis (arrowhead). There is also a right adrenal mass with low attenuation, a finding that is consistent with adenoma. (b) On a CT scan obtained at the lowest level of the head of the pancreas, a round, cystic structure is barely seen within the dilated common bile duct (arrow). Surgery revealed multiple daughter vesicles filling the distal common bile duct.
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Indirect signs of biliary communication include increased echogenicity at US and fluid levels and signal intensity changes at MR imaging. An air-fluid level within the cyst, previously described as a sign of infection, is considered to be a sign either of rupture into the biliary tree or a hollow viscus or of a bronchopleural fistula (16). Lipid material that forms a fat-fluid level within the cyst has also been described as an indirect sign of biliary communication (Fig 24) (16).
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Figure 24. Intracystic fat-fluid level. Axial spin-echo T1-weighted (left) and fat-suppressed fast spin-echo T2-weighted (right) MR images demonstrate a fat-fluid level within a cyst. In our experience, this finding suggests that intracystic fat derives from the lipid elements in bile, implying that there is a communicating rupture. Communication with the biliary tree was proved at surgery. (Reprinted, with permission, from reference 16.)
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Portal Vein Involvement.—Compression of the portal vein and thrombosis with secondary cavernomatosis are rare and are caused by cysts located in the caudate lobe and hepatic bifurcation (23,24). Direct portal invasion by hydatid cyst contents is a very unusual complication that, to our knowledge, has not been previously reported in the literature. We have seen a case in which Doppler US demonstrated lack of flow in the portal vein and the presence of multiple small vessels with venous flow at the hepatic hilum in association with portal cavernomatosis. CT and MR imaging helped confirm cystic filling of the portal lumen and helped assess the lack of flow within the portal venous system (Fig 25).
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Figure 25a. Portal vein involvement in a patient with a history of surgery for hydatid disease of the right hepatic lobe. A Doppler US image obtained at the level of the porta hepatis (not shown) revealed the absence of flow in the main portal vein and helped confirm the presence of flow in collateral vessels (cavernous transformation of the portal vein). (a) Contrast-enhanced CT scan obtained at the level of the liver shows multiple cysts occupying the region of the main portal vein and its bifurcation (arrows). Multiple contrast material-filled vessels are seen surrounding the cysts (arrowheads), findings that are suggestive of portal cavernomatosis. A residual cyst is seen in the surgically reduced right lobe (C). (b, c) Axial (b) and coronal (c) maximum-intensity-projection images from a fat-suppressed T2-weighted MR imaging study show that the residual cyst in the right lobe (C) is connected by a cyst-filled track (arrowheads in b) to the right branch of the portal vein (RV in b). Multiple daughter vesicles are seen replacing the lumen of the main portal vein (straight arrows). In c, the gallbladder (G) and the common bile duct (curved arrow) are identified.
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Figure 25b. Portal vein involvement in a patient with a history of surgery for hydatid disease of the right hepatic lobe. A Doppler US image obtained at the level of the porta hepatis (not shown) revealed the absence of flow in the main portal vein and helped confirm the presence of flow in collateral vessels (cavernous transformation of the portal vein). (a) Contrast-enhanced CT scan obtained at the level of the liver shows multiple cysts occupying the region of the main portal vein and its bifurcation (arrows). Multiple contrast material-filled vessels are seen surrounding the cysts (arrowheads), findings that are suggestive of portal cavernomatosis. A residual cyst is seen in the surgically reduced right lobe (C). (b, c) Axial (b) and coronal (c) maximum-intensity-projection images from a fat-suppressed T2-weighted MR imaging study show that the residual cyst in the right lobe (C) is connected by a cyst-filled track (arrowheads in b) to the right branch of the portal vein (RV in b). Multiple daughter vesicles are seen replacing the lumen of the main portal vein (straight arrows). In c, the gallbladder (G) and the common bile duct (curved arrow) are identified.
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Figure 25c. Portal vein involvement in a patient with a history of surgery for hydatid disease of the right hepatic lobe. A Doppler US image obtained at the level of the porta hepatis (not shown) revealed the absence of flow in the main portal vein and helped confirm the presence of flow in collateral vessels (cavernous transformation of the portal vein). (a) Contrast-enhanced CT scan obtained at the level of the liver shows multiple cysts occupying the region of the main portal vein and its bifurcation (arrows). Multiple contrast material-filled vessels are seen surrounding the cysts (arrowheads), findings that are suggestive of portal cavernomatosis. A residual cyst is seen in the surgically reduced right lobe (C). (b, c) Axial (b) and coronal (c) maximum-intensity-projection images from a fat-suppressed T2-weighted MR imaging study show that the residual cyst in the right lobe (C) is connected by a cyst-filled track (arrowheads in b) to the right branch of the portal vein (RV in b). Multiple daughter vesicles are seen replacing the lumen of the main portal vein (straight arrows). In c, the gallbladder (G) and the common bile duct (curved arrow) are identified.
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Abdominal Wall Invasion.—Cysts may invade the right anterolateral abdominal wall from the right hepatic lobe and the anterior abdominal wall from the left hepatic lobe. Cysts usually pass through a small orifice, adopting an hourglass configuration (Fig 26). Imaging reveals a cystic mass within the abdominal wall that is similar to and in communication with the hepatic component of the hydatid cyst.
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Figure 26. Abdominal wall invasion. Unenhanced CT scan reveals a hepatic cyst with a partially calcified wall in the left lobe (C). The lesion is seen herniating through the anterior abdominal wall into the subcutaneous fat and has the classic hourglass shape.
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Hematogenous Dissemination
Lungs.—The lungs are the second most frequent site of hematogenous spread in adults and probably the most common site in children (15%–25% of cases) (3,25). Compressible organs such as the lung or brain facilitate the growth of the cyst, and this has been proposed as a reason for the high prevalence of the disease in childhood. Most cysts are acquired in childhood, remain asymptomatic for a long period of time, and are later diagnosed incidentally at chest radiography (Fig 27). Cysts are multiple in 30% of cases, bilateral in 20%, and located in the lower lobes in 60% (3,26). Calcification in pulmonary cysts is very rare (0.7% of cases) (26), although it may be seen in pericardial, pleural, and mediastinal cysts (4,27).
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Figure 27. Pulmonary hydatid cyst in a 3-year-old boy. Posteroanterior chest radiograph shows a well-circumscribed, masslike lesion with a polycyclic configuration in the left lower lobe (arrows). There is obliteration of the left costophrenic angle (arrowheads).
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Sudden coughing attacks, hemoptysis, and chest pain are the most common clinical symptoms (26,28). After cyst rupture, expectoration of cyst fluid, membranes, and scolices may occur. Rupture into the pleural cavity may also occur (28). Although allergic episodes may develop after cyst rupture, fatal anaphylaxis is uncommon (26,28). Bacterial infection of the cyst is the most serious complication commonly seen after rupture (26).
Uncomplicated cysts appear as well-defined masses. Centrally located cysts are usually round, although more peripheral cysts may be oval or polycyclic (29). Pulmonary hydatid cysts may vary from 1 to 20 cm (29). Cyst growth produces erosions in the bronchioles that are included in the pericyst, and as a result, air is introduced between the pericyst and the laminated membrane (3). This air collection appears as a thin, radiolucent crescent in the upper part of the cyst and is known as the crescent sign or meniscus sign (3,26,29). Some authors consider this to be a sign of impending rupture and an indication for emergency thoracotomy (3). As air continues to enter this space, the two layers separate completely and the cyst shrinks and ruptures, allowing the passage of air into the endocyst (3,29). An air-fluid level inside the endocyst and air between the pericyst and the endocyst with an "onion peel" appearance constitute the Cumbo sign (Fig 28) (29). After partial expectoration of the cyst fluid and scolices, the cyst empties and the collapsed membranes can be seen inside the cyst (serpent sign) (Fig 29) (5,29). When it has completely collapsed, the crumpled endocyst floats freely in the cyst fluid (water lily sign) (5,26,29). If the fluid is entirely evacuated by expectoration, the remaining solid components will fall to the most dependent part of the cavity ("mass within a cavity") (Fig 30).
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Figure 28. Open lung cysts in a child with symptoms of pulmonary infection including fever, cough, and expectoration. Chest radiograph obtained with the patient in the left lateral decubitus position demonstrates a large cavitary lesion with an air-fluid level in the inferior left lung (black arrow). Air is seen between the pericyst (white arrow) and the laminated membrane of the cyst (arrowhead). Taken together, these findings are known as the Cumbo sign. There is also a pulmonary infiltrate adjacent to the cyst as well as pleural effusion due to superimposed bacterial infection.
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Figure 29. Lung involvement in a child with previous episodes of cough and expectoration. Collimated lateral chest radiograph shows an intracystic serpentine structure representing collapsed membranes (serpent sign) (arrows).
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Figure 30. Open lung cyst in a 20-year-old man who had experienced a sudden coughing attack followed by expectoration of clear fluid. Chest radiograph obtained with the patient in the left lateral decubitus position reveals a cavitary lesion in the right upper lobe with solid contents that have settled in the most dependent part of the cavity ("mass within a cavity") (arrows). The solid component represents the detached, crumpled endocyst.
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Hydatid disease is a rare cause of recurrent acute pulmonary embolism. This complication may develop after invasion of the cardiovascular system or direct invasion of the inferior vena cava (30).
Kidneys.—Renal involvement occurs in 3% of cases (1,25,31,32). It usually remains asymptomatic for many years. The most common signs and symptoms are flank mass, pain, and dysuria (3,25). Cysts are frequently solitary and located in the cortex, and they may reach 10 cm before any clinical symptoms are noted (25). At excretory urography, uncomplicated cysts may create a bulge in the outline of the kidney and appear as a rounded mass that elongates the infundibula and calices. In up to 18% of cases, the cyst may rupture into the collecting system, leading to acute renal colic and hydatiduria. Several round filling defects may be seen in the excretory system due to daughter cysts (6). Ringlike calcification of the cyst wall may suggest a diagnosis of hydatid cyst (25). The US and CT features of renal hydatid cysts are similar to those of cysts in other locations (Fig 31).
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Figure 31. Renal hydatid cyst in a 45-year-old man with right flank pain. Unenhanced CT scan shows a round, cystic lesion in the right kidney with irregular, ringlike calcification of the pericyst (arrowheads). Small daughter cysts are faintly visualized within the cavity (straight arrows). The cyst is seen displacing the fat-filled renal sinus medially (curved arrow).
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Spleen.—The reported prevalence of splenic involvement in hydatid disease varies from 0.9% to 8% (6,33). In some series, the spleen is the third most common location of hydatid disease involvement after the liver and lungs (6). Splenic hydatid disease is mainly produced by systemic dissemination or intraperitoneal spread from a ruptured liver cyst (33,34). The most frequent clinical signs and symptoms are abdominal pain, splenomegaly, and fever (33).
Splenic hydatid cysts are usually solitary, and their imaging characteristics are similar to those of hepatic hydatid cysts. Cyst wall calcification may occur and is better depicted at CT than at radiography or US (33). CT may demonstrate the typical high-attenuation linear wall without calcification as well as daughter vesicles within the cyst.
Bone.—The frequency of osseous involvement in hydatid disease is 0.5%–4% (32). It is most commonly seen in the spine and pelvis, followed by the femur, tibia, humerus, skull, and ribs (3,32,35).
In bone involvement, pericyst formation does not occur, thereby allowing aggressive proliferation in an irregular branching fashion along the line of least resistance, especially the bone canals (3,32,36). The parasite replaces the osseous tissue between trabeculae due to the slow growth of multiple vesicles. With time, the parasite reaches and destroys the cortex, with subsequent spread of the disease to surrounding tissues (Fig 32) (3, 32,36,37). Extraosseous cysts may calcify, whereas intraosseous disease rarely demonstrates calcification (36).
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Figure 32. Hydatid disease involvement of the sacrum in a 50-year-old woman who presented with severe lower back pain and radicular pain. Contrast-enhanced CT scan obtained at the level of the iliac crest demonstrates sacral destruction and replacement by a multiloculated cystic mass, which also occupies the spinal canal and extends anteriorly into both psoas muscles and posteriorly into the soft tissues. Note the presence of multiple "septa" within the lesion, which actually represent the opposing walls of the daughter vesicles.
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Spinal hydatid disease, the most common form of bone involvement, is usually difficult to distinguish from tuberculous spondylitis or chronic osteomyelitis. Some imaging characteristics have been described as typical of spinal hydatid disease. These include lack of osteoporosis and sclerosis in host bone; absence of damage to the disk spaces and vertebral bodies and spread of the disease via a subperiosteal and subligamentous path (32,36,37); paraspinal extension; and, particularly, involvement of a contiguous rib (Fig 33) (37). Although CT allows precise assessment of osseous lesions and clearly depicts hydatid cyst calcification, MR imaging is superior in demonstrating involvement of neural structures (36).
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Figure 33a. Costal hydatid disease with spinal involvement in a middle-aged woman with clinical findings of spinal cord compression. (a) Axial contrast-enhanced T1-weighted MR image shows rib destruction by anterior and posterior low-signal-intensity masses (solid arrows). Another mass is seen within the spinal canal displacing the spinal cord anteriorly (open arrow). Note also the meningeal enhancement in the spinal canal (arrowheads). (b) Axial T2-weighted MR image shows the three lesions with high signal intensity (arrows), a finding that is consistent with their cystic nature. Note the different signal intensities of the cysts due to variations in distance from the surface coil.
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Figure 33b. Costal hydatid disease with spinal involvement in a middle-aged woman with clinical findings of spinal cord compression. (a) Axial contrast-enhanced T1-weighted MR image shows rib destruction by anterior and posterior low-signal-intensity masses (solid arrows). Another mass is seen within the spinal canal displacing the spinal cord anteriorly (open arrow). Note also the meningeal enhancement in the spinal canal (arrowheads). (b) Axial T2-weighted MR image shows the three lesions with high signal intensity (arrows), a finding that is consistent with their cystic nature. Note the different signal intensities of the cysts due to variations in distance from the surface coil.
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Hydatid disease involving other osseous locations typically manifests as different-sized areas of pure osteolysis that may become confluent and cause thinning of the cortex. Lesion growth causes breaking of the bone cortex and allows extension to the surrounding soft tissues (Fig 34) (32,37,38).
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Figure 34a. Iliac crest hydatid disease with extension into the soft tissues in a 68-year-old man with excruciating pain in the left gluteal region. (a) Contrast-enhanced CT scan shows an expanding lesion with water attenuation in the left iliac crest (solid arrow). Multivesicular cysts are seen within the soft tissues posterior to the iliac crest (open arrows). (b) Axial fat-suppressed fast spin-echo T2-weighted MR image demonstrates complete replacement of the iliac wing by a high-signal-intensity lesion (arrows). The masses within the posterior soft tissues also have high signal intensity due to their cystic nature. (c) Photograph of a resected specimen from the surgically removed iliac crest demonstrates multiple collapsed membranes within the medullary cavity (arrows). Note the perfect correlation between the surgical specimen and the MR image (cf b).
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Figure 34b. Iliac crest hydatid disease with extension into the soft tissues in a 68-year-old man with excruciating pain in the left gluteal region. (a) Contrast-enhanced CT scan shows an expanding lesion with water attenuation in the left iliac crest (solid arrow). Multivesicular cysts are seen within the soft tissues posterior to the iliac crest (open arrows). (b) Axial fat-suppressed fast spin-echo T2-weighted MR image demonstrates complete replacement of the iliac wing by a high-signal-intensity lesion (arrows). The masses within the posterior soft tissues also have high signal intensity due to their cystic nature. (c) Photograph of a resected specimen from the surgically removed iliac crest demonstrates multiple collapsed membranes within the medullary cavity (arrows). Note the perfect correlation between the surgical specimen and the MR image (cf b).
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Figure 34c. Iliac crest hydatid disease with extension into the soft tissues in a 68-year-old man with excruciating pain in the left gluteal region. (a) Contrast-enhanced CT scan shows an expanding lesion with water attenuation in the left iliac crest (solid arrow). Multivesicular cysts are seen within the soft tissues posterior to the iliac crest (open arrows). (b) Axial fat-suppressed fast spin-echo T2-weighted MR image demonstrates complete replacement of the iliac wing by a high-signal-intensity lesion (arrows). The masses within the posterior soft tissues also have high signal intensity due to their cystic nature. (c) Photograph of a resected specimen from the surgically removed iliac crest demonstrates multiple collapsed membranes within the medullary cavity (arrows). Note the perfect correlation between the surgical specimen and the MR image (cf b).
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Brain.—Hydatid disease affects the central nervous system in 1% of cases (1,25) and is usually diagnosed during childhood. Although hydatid disease may be located anywhere in the brain, it is most frequently located in both hemispheres, particularly in the territory of the middle cerebral artery. In young patients, radiography of the skull may demonstrate suture separation, unilateral enlargement of the vault, or erosion of the posterior clinoid processes (3). CT and MR imaging demonstrate a well-defined oval or cystic mass with an attenuation or signal intensity similar to that of cerebrospinal fluid. Although the lesion may cause extrinsic compression of the ventricular system with subsequent hydrocephalus, there is no associated edema as is typically seen in abscesses and cystic tumors (Fig 35) (3). The lesion does not enhance after intravenous administration of contrast material, and calcification is extremely rare (3,39).
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Figure 35a. Cerebral hydatid cyst in a patient with intracranial hypertension and papilledema. (a) Sagittal T1-weighted MR image of the brain reveals a bilobed, hypointense mass (arrows). Other MR images (not shown) demonstrated mass effect on the right lateral ventricle. (b) On an axial T2-weighted MR image, the lesion has high signal intensity due to its cystic nature. Note the lack of edema in the surrounding parenchyma. The lesion was confirmed to be a hydatid cyst at surgery. (Fig 35a and 35b courtesy of Ricardo Gomez Pereda, MD, Department of Diagnostic Radiology, Hospital Miguel Servet, Zaragoza, Spain.)
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Figure 35b. Cerebral hydatid cyst in a patient with intracranial hypertension and papilledema. (a) Sagittal T1-weighted MR image of the brain reveals a bilobed, hypointense mass (arrows). Other MR images (not shown) demonstrated mass effect on the right lateral ventricle. (b) On an axial T2-weighted MR image, the lesion has high signal intensity due to its cystic nature. Note the lack of edema in the surrounding parenchyma. The lesion was confirmed to be a hydatid cyst at surgery. (Fig 35a and 35b courtesy of Ricardo Gomez Pereda, MD, Department of Diagnostic Radiology, Hospital Miguel Servet, Zaragoza, Spain.)
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Other Locations.—Hydatid disease may involve almost any anatomic site due to hematogenous dissemination. Unusual locations include the heart, pericardium, orbit, gastric wall, retrocrural space, mediastinum, subcutaneous space, muscle, and adrenal glands (3,40–47). Hydatid disease should be included in the differential diagnosis when a cystic lesion is identified in a patient who lives in or has come from an area in which the disease is endemic, especially if any of the previously described imaging characteristics (eg, multivesicular cysts, calcification, intracystic membranes) are seen.
Orbital involvement occurs in less than 1% of cases (40). In areas in which it is endemic, hydatid disease is the second most common cystic lesion of the orbit (25% of cases) after dermoid cyst (41). Serologic findings and pertinent imaging findings in other locations help confirm the diagnosis.
Although hydatid disease may be present in the anterior mediastinum, when it is located posteriorly, the differential diagnosis should include bronchogenic, enteric, and intramural esophageal cysts; lymphangioma; and anterior meningocele (43).
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Conclusions
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Hydatid disease primarily affects the liver and typically demonstrates well-known, characteristic imaging findings. However, there are many potential local complications, and secondary involvement due to hematogenous dissemination may be seen in almost any anatomic location. Familiarity with atypical manifestations of hydatid disease may be helpful in making a prompt, accurate diagnosis.
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Acknowledgments
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We thank Simón Sánchez Moral and Juan Córdoba Ordóñez (Department of Regional Geographic Analysis, Universidad Complutense de Madrid, Spain) for the map shown in Figure 1.
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Footnotes
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** indicates multiple body systems. 
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References
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