American Journal of Neuroradiology

Case of the Week

Section Editors: Matylda Machnowska¹and Anvita Pauranik²
¹University of Toronto, Toronto, Ontario, Canada
²BC Children's Hospital, University of British Columbia, Vancouver, British Columbia, Canada

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September 1, 2022

Disseminated Systemic Hydatidosis

Background:
- Hydatidosis is a zoonosis caused by Echinococcus granulosus when contaminated water or food containing parasitic larvae is ingested.
- It is endemic in many countries, especially in rural regions where agricultural and livestock activities facilitate its transmission. There is a high incidence in Latin American, African, and Asian countries.
- The liver and lungs are the most common locations.
- Atypical locations include renal parenchyma, skeletal muscle, vertebrae, pelvis, femur, tibia, ribs, skull, humerus, and fibula.
Clinical Presentation:
- It can be asymptomatic for many years and its clinical manifestations depend on the affected anatomic site, the size of the cysts (mass effect), and/or any other complication, such as bacterial infection or rupture of the cyst.
- Anaphylactic shock, usually following cyst rupture, is the most serious complication.
Key Diagnostic Features:
- On MRI, cysts are hypointense on T1WI and hyperintense on T2WI. Daughter cysts can be clearly differentiated from the internal cyst matrix or hydatid sand, which demonstrates slightly higher signal intensity on T1-weighted MR images and intermediate signal intensity on T2-weighted MR images.
- Cysts show a hypointense wall on T2WI that is called the "border sign." It is a fibrous capsule that represents the pericyst rich in collagen and calcium. It is a nonspecific finding and may have postcontrast enhancement in late acquisitions. As the amount of hydatid sand increases, the internal signal on T2WI decreases.
- Classification of hydatid cysts on MRI:
  - Type 1: Cysts are hypointense on T1WI and hyperintense on T2WI; border sign; active cysts
  - Type 2: Multiseptated and multivesicular hyperintense on T2WI daughter cysts; active cysts
  - Type 3: Heterogeneous cysts with hypointense internal detached membranas; transitional cysts
  - Type 4: Cysts are iso- to hypointense on T2WI; no daughter cysts; mainly inactive cysts
  - Type 5: Very hypointense wall and low/intermediate signal on T2WI; inactive cysts
  - In our case, the patient showed type 1 and type 2 cysts.
Differential Diagnoses:
- Epidermoid cyst: Cystic lesions iso- or slightly hyperintense to CSF on T1WI and T2WI (hyperintense on T1WI if high protein content); hyperintense on DWI; no surrounding edema
- Neurenteric cyst: Isointense to CSF on T1WI and T2WI; no enhancement; very rare
- Arachnoid cyst: Isointense to CSF on T1WI and T2WI; no restriction on DWI
- Cysticercosis: Cystic lesions with peripheral enhancement and no bone invasion
- Aneurysmal bone cyst: Septated cyst with fluid-fluid levels; may show septal and peripheral enhancement
- Synovial cyst: Extradural cyst adjacent to a facet joint
Treatment:
- Albendazole 400 mg PO twice a day, with meals, for 3 cycles of 30 days with no interruptions