Hydatid disease (echinococciasis) is a tissue infection of humans caused by the larval stage of a small tapeworm, most commonly Echinococcus granulosus. Dogs, sheep, cattle, and camels are the major intermediate hosts. The ova that are ingested by humans penetrate the intestinal mucosa and enter the portal circulation. Most are filtered out by the liver or lui.g, but some escape into the general circulation to involve kidney, bones, brain, and other tissues. The larvae that are not destroyed develop into hydatid cysts, which have a double wall composed of a thick outer membrane (exocyst) and a thin inner wall of germinal cells (endocyst). The cysts may grow slowly over a period of years (mimicking a tumor), rupture into adjacent tissues, or become calcified.
Hydatid cysts are the most frequent cause of hepatic calcification in endemic areas. Patients infected with the common Echinococcus granulosus typically have complete oval or circular calcification at the periphery of the mother cyst. Within the mother cyst, there may be multiple daughter cysts with arclike calcifications. Hydatid cyst calcification generally develops 5 to 10 years after the liver has been infected and can be present in either active or inactive cysts. Extensive dense calcification suggests quiescence of the parasitic process; segmental calcification (nonhomogeneous, striped, trabeculated) suggests cystic activity and may be an indication for surgery.
Hydatid disease frequently causes diffuse hepatomegaly that may be associated with an enlarging liver mass, portal hypertension, splenomegaly, jaundice, and ascites, producing a pattern often indistinguishable from that of carcinoma of the liver. Computed tomography and ultrasound are of value in demonstrating uncalcified or partially calcified hydatid cysts. On arteriograms, vessels are stretched around the avascular cysts, and there is usually a virtually diagnostic halo of contrast material (representing the exocyst) surrounding the lesion.
Large parent cysts in the liver can communicate with the biliary tree. The periodic shedding of daughter cysts into the bile duct causes recurrent episodes of biliary colic and can produce round or irregular filling defects in the bile duct or cyst cavity. Daughter cysts can be trapped in the region of the ampulla and cause obstruction of the common bile duct.
A rarer and more malignant form of hydatid disease is the alveolar type, due to Echinococcus multilocularis. The natural intermediate hosts of this organism are small rodents, and, unlike the generally indolent Echinococcus granulosus disease, alveolar hydatid disease can be a fulminant, even fatal, condition. The striking radiographic feature of alveolar hydatid disease is liver calcification, which typically appears as multiple small ra-diolucencies. measuring 2 to 4 mm in diameter, surroui jed by rings of calcification that, in turn, lie within larger reas of amorphous calcifications up to 10 to 12 cm in diameter. The combination of hepatomegaly along with portal hypertension, splenomegaly, jaundice, and ascites may produce a pattern indistinguishable from that of carcinoma of the liver.
Nonparasitic Liver Cysts
Nonparasitic hepatic cysts vary in size from a few millimeters to several centimeters in diameter and appear on CT as sharply delineated, round or oval low-density lesions. Although most commonly single, hepatic cysts may be multiple; innumerable multifocal cysts occur in polycystic liver disease. Hepatic cysts have very thin walls and no internal septations and do not enhance following the intravenous administration of contrast material. At times, cystic neoplasms and old hematomas may have attenuation values identical to simple hepatic cysts. In such cases, ultrasound may permit differentiation by demonstrating internal septations and irregularities of the inner margin of the wall of cystic tumors, an appearance that is not seen in non-neoplastic hepatic cysts. On rare occasions, percutaneous fine-needle biopsy may be required to differentiate a simple hepatic cyst from a necrotic cystic metastasis.
On ultrasound, hepatic cysts appear as echo-free lesions with smooth walls and good posterior enhancement. Radionuclide scans show hepatic cysts as photopenic areas devoid of isotope activity.
Nonparasitic liver cysts may occasionally become large enough to be symptomatic. Because simple aspiration alone has not been effective in preventing recurrence, and because surgical correction is associated with considerable morbidity, a number of sclerosing agents have been instilled into hepatic cysts to attempt to prevent their reformation. In initial studies, the injection of alcohol following cyst aspiration has proved to be an effective technique without major complications.photopenic cyst in the liver · calcified liver cyst · photopenia liver cyst ·