Hydatid disease is a parasitic condition constituting a public health problem in many traditional cattle-breeding countries, including the Mediterranean basin, where it is endemic [1]. It is secondary to the infestation of the organism by the hexacanth embryo of Echinococcus granulosus [1]. The Mediterranean basin currently presents the world's largest focus of hydatidosis, its infestation index in developing countries varies between 5 and 18 cases/100,000 inhabitants per year [7]. The localizations frequently encountered are hepatic (48%) and pulmonary (36%) while cerebral localization is rare and does not exceed 2% of cases [7]. The rarity of cerebral localization may be explained by the passage of the parasite through two hepatic and pulmonary filters before reaching the large circulation [1]. It is more common in pediatric population and young adults (50 to 70% of cases) [7] and most often occurs before the age of 15. However, all age groups may be affected [7].
Concerning cerebral involvement, there are 3 forms of this parasitic disease, namely the intracerebral form, extradural form and combined or disseminated form. Extradural hydatid cyst, whether primary or secondary, remains very rare or even exceptional when it is associated with or followed by distinct solitary of multiple intracerebral disseminations [3]. Hilmani et al. [8] published in 2017 a case of a 6-year-old patient operated on, using the classic Dowling-Orlando technique, for a frontal inta-axial hydatid cyst without rupture and who came back with recurrence after 3 years in the same frontal area with a another small adjacent extra dural cyst. They explain that their current combined hydatid location is explained by the rupture of the first cyst. In 2010, Lakhdar et al. [9] published a case of a 37-year-old woman operated on for a combined extradural and extracranial hydatid cyst with multiple septae of the posterior cerebral fossa with occipital lysis and submuscular extension. They suggest that hydatid bone involvement in their patient could be intradiploic at first, then the extension took place in both directions, blowing the bone and breaking both inner and outer table of the skull, thus resulting in the cranial extradural form.
In this third form of combined or disseminated cysts, intra-axial localization may occur following a dural defect, or through a normal dura mater as indicated in our patient and published in our previous report [10], in whom the dura mater was perfectly intact during first surgery. The first mechanism may be an embolization of embryos or scolex through blood in the primary form with a tendency to develop around the midline as it was seen in our patient. The second mechanism may be due to the existence of a large venous network in this zone [5] plus a previous rupture of another hydatid cyst.
Clinical progression of the disease in adults is faster than in children. This is explained by the inextensibility of the cranial box in adults. The clinical picture is dominated by raised intracranial pressure as it was seen in our patient, visual disturbances and focal deficit syndromes [7, 11].
Cerebral CT scan remains the examination of choice and of first intention as it makes it possible to specify the seat, the number, the volume and the contents of the cyst as well as its relationship with the neighboring structures thus helping in the choice of the surgical approach [1, 7]. In the majority of cases, the CT appearance is typical showing a single cyst, spherical or oval, located in the middle of the cerebral parenchyma, with a thin and regular wall, without enhancement and having the same density as the CSF [1] as it was seen in our patient. The almost constant absence of peripheral enhancement is explained by the thinness of the pericyst and the hydatid membrane adhering to this pericyst [1]. It is not possible to distinguish on CT between pericyst and cystic membranes unless, exceptionally, fragments of proligorous membrane become floating when the cyst cracks [12]. On the other hand, the passage of hydatid fluid into the cerebral tissue will cause an inflammatory reaction and thickening of the pericyst. Thus edema and contrast enhancement, when they exist, sign the insufficiency of tightness linked to the fissuring of the cyst [1, 13]. In our patient, CT was not enhanced. However, it did not reveal any edematous reaction around both cysts.
Cerebral localizations are generally hemispherical supratentorial and subcortical, particularly in the territory of the middle cerebral artery at the level of the parietal lobe [13], with a laminated cortex and virtually no detectable gray matter between the vault and the cyst wall as it was seen in our case where both pericysts were very thin and fragile, which ruptured at corticotomy. MRI currently not only provides additional diagnostic information for cerebral hydatid disease but also allows more appropriate therapeutic planning. It highlights a spherical fluid lesion, with fine contours edges, containing a fluid with the same imaging characteristics as CSF [7] with a very thin wall (pericyst) in relative T1 hypersignal and characteristic T2 hyposignal [6] which is concordant with our case. The absence of a hypersignal on the FLAIR sequence as well as a significant hyposignal on the diffusion-weighted sequence, as observed in our patient, also characterize the hydatid cyst [1]. Thus MRI demonstrates the signal characteristics of the hydatid cyst and the possible adhesions that the pericyst may have with neighboring structures, a very important element in the planning of the surgical act preventing accidental rupture [1].
The differential diagnosis may arise with certain cystic lesions, in particular the arachnoid or epidermoid cyst, cystic astrocytoma, craniopharyngioma and even the brain abscess but in an endemic region as in our patient and in front of his history of hydatid disease, the diagnosis of hydatid cyst is evoked from the beginning [1].
The treatment of cerebral hydatid cyst is surgical [7]. Its aim is to excise the entire cyst without causing its rupture in order to avoid the dissemination of the scolices, the risk of recurrences and anaphylactic reactions which would be responsible for circulatory collapse and cardiac arrest, thus guaranteeing a definitive cure [1, 7]. The most widely used surgical technique consists of delivery of the cyst by hydro-dissection using hypertonic saline. In our patient, the resection of both cysts was not done by the classic technique of hydrodissection according to Dowling-Orlando, given the covering cerebral cortex was quite thin and Both cysts‘walls were very thin and fragile, which ruptured at corticotomy.
While cerebral hydatidosis is still considered by most authors to be a benign disease, functional motor and/or visual sequelae and postoperative complications such as subdural hematoma or bacterial meningitis may occur, especially if the hydatid cysts are multiple and/or recurrent [7]. In our patient no complications were encountered after surgery and the postoperative course were fortunately uneventful.
Our case is therefore among the exceptional cases reported in the literature taking into consideration the rarity of cerebral pediatric hydatid pathology in its extradural form with concomitant or subsequent intra-axial dissemination.