Liver

Hydatids in the liver have a structure similar to lesions in other sites, but the host reaction (pericyst), which plays a role in the development of the cyst is particularly intense in the liver. About 85% of liver hydatids occur within the right lobe, probably because this lobe is much larger in size than the left and has a greater portal venous circulation. Cysts in the liver may be superficial, near the capsule (Fig. 3.56A), or lie deeply in the liver substance. Superficial cysts may enlarge in any direction, elevating the diaphragm or bulging into the peritoneal cavity. More deeply situated cysts may cause compression and atrophy of the surrounding hepatic tissues. They may cause biliary stasis and, if ruptured, reactive hepatitis. Secondarily infected ruptured cysts may cause cholangitis. Communicating or direct rupture may produce severe portal fibrosis and portal hypertension in some patients. If the cyst is located near the porta hepatis, there may be obstructive jaundice from compression of the main biliary ducts.

Central or deep hydatids may undergo direct intrahepatic rupture of the pericyst, resulting in the cyst contents no longer being contained and spilling into the liver parenchyma (Fig. 3.50). Sectional imaging of liver cysts which have undergone communicating rupture demonstrates detached endocyst floating in the remaining cyst fluid - the "snake or serpent sign" (Figs. 3.27, 3.44D, 3.45D, 3.49) which is the cross-sectional imaging counterpart of the "floating water lily sign" in the lung. The cyst may become appreciably smaller and less spherical (Fig. 3.28), and there may be evidence of downstream biliary obstruction. Communicating rupture may be diagnosed directly, by the demonstration of hydatid debris in the biliary tree, or when contrast enters the cyst cavity during endoscopic retrograde cholangiopancreatography (ERCP) (Fig. 3.29). Communicating rupture occasionally shows a fat-fluid level in the cyst cavity.

Dilated pericystic ducts, which are apparent in 25% of liver hydatids, are usually much more tortuous than blood vessels (Fig. 3.22, 3.34B). A clear distinction between the two is possible only if CT scans are performed with intravenous contrast enhancement - a distinction that is not as easy on ultrasound examinations. Dilated bile ducts in the unruptured hydatid are always upstream (in terms of bile flow) of the cyst. The reasons for biliary dilatation and biliary perforation of the pericyst are probably related. Dilated bile ducts which perforate the pericyst and allow herniation of a small knuckle of delicate endocyst may predispose to rupture. Dilated ducts should be considered an indication that unseen communicating ducts exist downstream, threatening biliary obstruction after rupture. At the time of rupture, small daughter cysts and fragments of endocyst may be forced into the upstream ducts, but they do not cause obstruction as the pressure within the cyst cavity soon drops as hydatid fluid leaves via the downstream ducts. Bile flow then returns the debris to the pericyst cavity. The bile that floods the pericyst cavity probably always kills the parasite. Biliary obstruction, which occurs if formed elements enter downstream ducts, characteristically produces fluctuating jaundice which may disappear for short periods, presumably because of the compliance of the obstructing fragments.

Cysts located beneath Glisson's capsule protrude from the external surface of the liver, are dome-shaped, and may become pedunculated. Growing cysts at the dome of the liver or spleen exert pressure, leading to elevation or a local bulge of the hemidiaphragm and often restricted movement (Figs. 3.41 and 3.42). This should not be confused with eventration or other abnormality of the diaphragm or prolapse of intra-abdominal organs into the chest. Other entities such as amebic abscess, subphrenic abscess, and primary or metastatic malignant disease must be distinguished. Unlike amebic or other subphrenic abscesses, unruptured liver hydatids confined to the hepatic parenchyma usually do not stimulate a pleural effusion or lung reaction. Rupture of an infected hydatid cyst located in the upper right lobe of the liver or in the spleen may, however, lead to a subdiaphragmatic abscess (Fig. 3.59) . Rupture of a hydatid cyst, especially if direct, may be accompanied not only by abdominal pain but also by allergic phenomena, such as urticaria or asthma, and less often by collapse, and rarely by anaphylactic shock.

Cysts growing towards the diaphragm may penetrate it and perforate into the pleura, pulmonary parenchyma, bronchi or pericardium (this is one reason why cysts in the left lobe of the liver have a worse prognosis) (Fig. 3.60). Even if not infected, a slowly growing cyst may cause pressure atrophy of diaphragmatic muscle fibers and prolapse into the pleural cavity, accompanied by a pleural effusion. If there are extensive pleural adhesions, prolapse may even occur into the lung parenchyma. If the cyst ruptures, in addition to a marked pleural effusion, an allergic pneumonitis or even a tension pneumothorax may develop. The long-term damage, if no acute allergic reaction has occurred, may be secondary dissemination of echinococcosis in the pleura and lung parenchyma. Prophylactic treatment with effective chemotherapy is now available and should be considered in such cases.

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Fig. 3.60 A 24-year-old Saudi Arabian man with multiple abdominal cysts; one in the left lobe of the liver prolapsed into his chest. (A) Noncontrast CT scan of the lower chest reveals the prolapsed hydatid (arrow). (B) MRI (T2-weighted; SE 2000/100) shows the prolapsed hydatid with a low-intensity rim (small black arrows), high- intensity contents with a daughter cyst (large black arrow), and halo caused by reactive or compressed lung parenchyma (white arrows). (C) Microscopic pathology shows parasitic membranes with bile pigment from an old biliary-cystic fistula, confirming that this cyst had originated in the liver and prolapsed into the chest. (Courtesy of Dr. von Sinner).