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Parasitic Disease

4.1.7.4.1 Hydatid Cysts

The two most prevalent subtypes of hydatid disease are: a unilocular cyst caused by Echinococcus gran-ulosus (worldwide prevalence) and will have a right lobe localization; and a multilocular cyst caused by E. alveolaris (northern hemisphere only) (Czermak et al. 2001). Children usually become infected through exposure to canine feces or by eating food contaminated with tapeworm eggs, which hatch in human small intestine under the influence of gastric and intestinal secretions.

An abdominal mass with pain is a common manifestation. Loss of appetite is a frequent symptom, and weight loss and weakness may occur. Approximately 80% of hydatid cysts will be localized in the liver, and may compress the porta and biliary ducts producing jaundice or cholangitis.

Hepatic echinococcal cysts have been classified into four groups based on US findings: type I is a simple anechoic fluid-filled cyst; type IR is a cyst with undulating membrane secondary to rupture; type II is a cyst with a daughter cyst, and type III is a densely calcified, echogenic cyst with shadowing. It has been proposed that the natural progression is from type I to III.

Fig. 4.13a-c. CT of candidiasis. a Multiple rim-enhancing lesions in the liver, focal defects in the spleen: candida in an immunocompromised patient. b,c Ascites, enhancing pancreatic tissue

Fig. 4.13a-c. CT of candidiasis. a Multiple rim-enhancing lesions in the liver, focal defects in the spleen: candida in an immunocompromised patient. b,c Ascites, enhancing pancreatic tissue

On CT, E. granulosus cysts appear as well-demarcated thin or thick walled hypo-attenuating structures. CT can identify the small dissections of the parasitic fluid into the pericystic space with collapse of the parasitic membrane. This is pathognomonic of E. granulosus and has been called the "snake" sign because of the undulated membrane appearance. Peripheral calcifications may be seen in advanced stages.

Hydatid cyst can be identified on MRI as unilocular str uctures with low signal intensity inside the cyst on Tl-weighted images. At the bottom of the cyst a structure with intermediate signal intensity can be visualized and it represents the hydatid sand. On T2 sequences, the cyst will have high signal intensity with a low intensity rim surrounding the lesion. Typically, a mother cyst will have greater signal intensity than a daughter cyst. In some patients, the unilocular cyst will have an increased intensity inside the cyst on Tl-weighted images, which corresponds to proteins and lipids (Kuhn et al. 2004).

4.1.7.4.2 Amebiasis

Amebiasis is a parasitic infection caused by the pro-tozoon Entamoeba histolytica. It is the third leading parasitic cause of death worldwide, surpassed only by malaria and schistosomiasis. Amebiasis can localize in many different locations besides the bowel. One of the most common extraintestinal manifestations is the amebic liver abscess, which usually occurs in children less than 3 years of age, with a peak incidence in the first year of life (Elizondo et al. 1987; Giovagnoni et al. 1993).

The child will present with abdominal pain, fever, and hepatomegaly at the initial examination. An acute abdomen may be the initial manifestation of a ruptured amebic abscess in some patients; however, this is very uncommon.

US is preferred for the evaluation of amebic liver abscess because of its low cost, rapidity, and lack of adverse effects (Kuhn et al. 2004). Amebic abscess b a c can be identified on US as hypoechoic lesions with good sound transmission through the cyst. Sometimes it may be difficult to differentiate between amebic versus pyogenic abscesses by US. However, amebic abscesses are usually multiple and in very close proximity to one another. They have a central area of liquefaction that will be seen on US as a central hypoechoic area. Some are more likely to have a peripheral halo and better defined borders compared to pyogenic abscesses.

CT may demonstrate multiple cysts adjacent to the liver capsule, with ill-defined margins.

On MRI, a well-defined lesion with heterogeneous signal intensity on T1-weighted images and with high signal intensity on T2-weighted images can be identified. Some abscesses may show a peri-lesional rim with increased signal intensity that corresponds to edema. After treatment the abscesses are typically seen as homogeneous lesions with low signal intensity on T1-weighted images, and with concentric rims and decreased of edema on T2-weighted images.

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