Isolated renal hydatid disease in a child: a case report

A 12-year-old patient presented to the emergency department for chronic left hypochondrium pain, evolving for 5 months without other associated signs. Anamnesis revealed that the patient had frequent contact with dogs without any other notable medical history. Physical examination was unremarkable. The routine biological workup did not reveal any abnormalities.

Abdominal ultrasound found a well-circumscribed cystic lesion of the left kidney, with peripheral wall calcifications, containing finely echogenic moving foci (falling snowflakes sign) together with hyperechoic floating structures (membranes). On abdominal MRI, the lesion showed a hypointense rim on T2-weighted images (rim sign), surrounding a content which is isointense on T1-weighted and hyperintense on T2-weighted images, with hypointense curvilinear floating structures corresponding to collapsed parasitic membranes (serpent sign/ water lily). No contrast enhancement was depicted. These findings were compatible with type II hydatid cyst according to the classification of Gharbi (CE3a according to WHO classification).

Background:

The hydatid cyst is a parasitic infection in which humans are accidental hosts. It is due to the development of the larval form of Echinococcus granulosus in a human tissue, mainly the liver and lung. Renal localization remains uncommon, occurring in 2-3% of cases [1,2]. It is even rarer in children.

Clinical Perspective:

RHD can remain asymptomatic for a long time. It manifests later by nonspecific symptoms, including back pain, a palpable mass, or dysuria. [1,3]. Cystic rupture may occur into the urinary tract and cause renal colic or even trigger an anaphylactic reaction [3].

The diagnosis of RHD is based on epidemiological, clinical, biological, and radiological features. Medical imaging plays a key role in diagnosing hydatid disease.

Imaging Perspective:

Ultrasound is the first-line key examination. It determines the number, the localization and the stage of hydatid lesions [4].

• Type I cysts represent the initial development of the parasite and resemble simple cysts. However, unilocular cystic lesion with double-contour thick wall strongly suggests a diagnosis of hydatid cyst [5].
• Type II cyst results from splitting of the endocyst and pericyst, which presents as water-lily sign when complete.
• Type III represents a multivesicular cyst with honeycomb appearance.
• Type IV cyst contains a heterogeneous matrix and can be confused with an infected cyst, abscess or renal neoplasm.
• Type V cyst presents with partial or complete wall calcification [1,2,5].

Contrast-enhanced CT is indicated whenever the diagnosis is uncertain, especially for type I and IV cysts. It helps to determine the exact topography of lesions, search for extrarenal locations, and assess possible communication with the urinary tract [6].

Cyst wall can be thickened or calcified. Internal septa and parietal post-contrast enhancement may be present [2].

MRI is often ordered when previous examinations prove inconclusive. The hydatid cyst shows a characteristic hypointense rim on T2-weighted images representing the pericyst. Its fluid appears hypointense on T1-weighted and hyperintense on T2-weighted images [2,5]. Collapsed parasitic membranes and internal septa are hypointense on all MRI sequences and may exhibit a post-contrast enhancement [6].

Outcome:

Treatment for RHD is mainly surgical, however, renal resection should be as limited as possible. Medical treatment is associated on a case-by-case basis [7]

Take-Home Message / Teaching Points

RHD is rare in children. Imaging plays a fundamental role in the diagnosis, characterization and assessment of the lesions. Atypical hydatid cyst forms often require the use of different imaging methods in order to confirm the diagnosis.