Diagnosis

Infratentorial Hydatid Cyst

The CT showed a well-defined, rounded, hypodense mass in the right cerebellum. No enhancement was seen. These findings were suggestive of a unilocular cyst.

Exploratory craniotomy and fine needle aspiration revealed clear fluid. Enucleation of the cyst was performed. Its wall was whitish in colour and the material obtained was subjected to histopathological examination, which confirmed a hydatid cyst. No scolex was seen in the fluid.

Hydatid disease though prevalent throughout the world, is mostly found in cattle-raising and sheep-raising countries. The liver and the lungs are the most frequently involved organs [1]. Intracranial hydatidosis is a rare entity with an incidence of 2-5% [2]. Although the cyst can occur anywhere within the brain, most of the intracerebral cysts are supratentorial, in a cortical location in the distribution of the middle cerebral artery [3]. Extradural location is a very rare event [3]. Cerebral hydatid cysts usually occur in children and young adults. Primary hydatid cysts of the brain are usually large and solitary, lying just a few millimeters below the cortex whereas the infrequent secondary intracerebral cysts are usually multiple and often occur after embolisation from cysts ruptured elsewhere [4].

CT findings of a hydatid cyst include a large well-defined round cystic lesion, with a density similar to water or CSF, which usually shows absence of surrounding edema or contrast enhancement. Significant distortion of the brain parenchyma, ventricular displacement and / or hydrocephalus are common. Rarely the cyst may show rim enhancement, an enhancing nodule simulating a cystic astrocytoma or calcification in the wall of a degenerated cyst [2], [5].

Differential diagnosis include arachnoid cyst, cystic astrocytoma and cystic malformations. Arachnoid cysts are usually posterolaterally located. Though other features are similar, arachnoid cysts show irregularity of their inner border [6]. The spherical shape of a cystic lesion favours the diagnosis of a hydatid rather than an arachnoid cyst. Cerebellar cystic astrocytomas show higher attenuation values than hydatid or arachnoid cysts, due to higher protein content. The presence of surrounding edema and the homogeneous, enhancing solid component (mural nodule) of the tumor distinguishes it from a hydatid cyst [5]. Abscesses show perifocal edema and peripheral enhancement [3], [7]. A porencephalic cyst often communicates with the ventricle. Dandy-Walker malformations can be easily differentiated on the basis of characteristic findings and associated features