Teaching Case

A patient presented to the Emergency Department with fever (39 degrees Celsius), acute onset right-sided flank pain, nausea and vomiting. Nephrolithiasis was initially suspected (positive Giordano sign). The patient’s blood test revealed eosinophilia and gradually deteriorating transaminasemia.

The ultrasonography [US] did not confirm the initially suspected nephrolithiasis, but revealed a large well-defined cystic mass in the right liver lobe. Interestingly, the mass contained hyperechoic membranes "floating" in the fluid, thus raising suspicion of hydatid cyst with detachment of the endocyst. The mass had no vascularity. The computed tomography [CT] examination which followed depicted a non-calcified cystic mass with septations. The cyst wall was thin, but ill-defined and an adjacent dilated biliary radicle was demonstrated. A small ipsilateral pleural effusion was also seen. Magnetic resonance imaging [MRI] confirmed the diagnosis by demonstrating a cyst with undulating linear structures inside, representing the detached endocyst. The low signal intensity rim on T2-weighted images was interrupted by a small wall defect. Magnetic Resonance Cholangiopancreatography [MRCP] showed communication with the biliary tree. After treatment with albendazole, the 6-month follow-up with CT scan demonstrated decrease of the size of the cyst and no signs of inflammation.

The most common cause of hydatid disease in humans is infestation by the parasite Echinococcus granulosus. The hydatid cysts are most frequently located in the right liver lobe. [1]

Four groups for hydatid cysts have been proposed: [1, 2, 3]
- Simple cysts containing no internal architecture except sand
- Cysts with daughter cysts and a central matrix
- Cysts with a totally calcified wall (inactive)
- Complicated cysts with rupture and superinfection.

Complications of hydatid cysts include rupture, infection, exophytic growth and portal vein involvement. Lewall and Mc Corkell have classified rupture into three types. [4]
- Contained rupture: the endocyst ruptures but the pericyst remains intact. The "floating membranes" inside the cavity of the cyst is the most characteristic sign. Complete detachment of the membrane inside the cyst has been referred to as the US "water lily sign". Collapsed parasitic membranes appear at MR imaging as twisted linear structures within the cyst ("snake sign"). Usually, asymptomatic.
- Communicating rupture: the cyst contents pass into the biliary radicals that have been incorporated into the pericyst.
- Direct rupture: both the endocyst and the pericyst ruptures, allowing spillage of hydatid material into the peritoneal cavity, pleural cavity, abdominal wall and hollow viscera.

Both US and CT may demonstrate a cyst wall defect, particularly in direct communication. [3, 5] Indirect cysto-biliary fistula MR-signs are considered to be: deformation of the cyst or decrease in the size of the cyst in the follow-up of untreated patients, beak-like projection extending from the cystic wall, dilated ducts distal to the cyst containing hydatid debris (which may show intraductal loss of signal density). [6, 7] Biliary communication of the cyst may cause biliary obstruction and subsequently cholangitis.

Hydatid cysts may have a low-signal-intensity rim on T2-weighted MR images, which probably represents the pericyst (rich in collagen). [8] MRI may demonstrate interruption of the low-signal-intensity rim of the cyst wall as well as extrusion of contents through the defect. [8]

Contrast enhanced CT [CECT] is recommended when infection is suspected. [9] Infection occurs only after rupture of both the pericyst and the endocyst, which allows bacteria to pass easily into the cyst. [3, 4, 5]

Early surgical intervention is mandatory in identified intrabiliary rupture. During resection of the cyst, the opening in the bile duct is detected and properly sutured. [10] Small defects may be seen only after the evacuation of the cyst content. [11] These communications elevate the recurrence rate of complex hydatid cysts in the postoperative period.