Primary cerebral hydatid cyst: An unusual cause of very slowly progressive hemiparesis in a child.

Sir,

Hydatid cyst, the cystic larval stage of Echinococcus granulosus (dog tapeworm), occurs in humans following accidental ingestion of eggs released from the intestine of the definitive hosts (canines).[1] These eggs then hatch in the intestine into hexacanth embryos that penetrate the intestinal wall and enter the portal circulation.[1]

A 5-year-old girl presented with gradually progressive right hemiparesis over 1½ years. Magnetic resonance imaging (MRI) of the brain showed a single large (8 cm × 7 cm × 7 cm) lesion hypointense on T1 and fluid attenuation inversion recovery sequences and hyperintense on T2 sequences. It was well marginated with no perilesional edema, not enhancing on contrast and was causing mass effect on the ventricle with midline shift. Subfalcine herniation was noted. The lesion had a T2 hypointense wall at its periphery. Hydatid disease was suspected [Figure 1a and b]. Ultrasonography of the abdomen and chest X-ray were normal. The patient underwent left frontoparietal craniotomy, transsulcal approach and en masse removal of the cystic lesion and its coverings using the Dowling technique (instilling saline between the cyst and brain parenchyma and delivery). There was no intraoperative rupture. The lesion was lined by a milky-white thicker outer layer and an extremely thin transparent inner layer containing fluid and some particulate matte [Figure 1c]. Histopathology showed an acellular outer lamellated layer and an inner germinal layer containing multiple papillary projections with brood capsules containing multiple scolices with distinct hooklets [Figure 1d]. The postoperative period was uneventful with resolution of hemiparesis and computed tomography scan showed good brain expansion with no residual cyst. The patient was put on antihelminthics to treat any occult cysts that may have been undetected. Cardiac evaluation done postoperatively showed no evidence of patent ductus arteriosus or patent foramen ovale.

Figure 1

(a) Axial fluid attenuation inversion recovery sequence showing a left hemispheric cystic hypointense lesion with no perilesional edema; (b) coronal T2 sequence showing the lesion is hyperintense with a well-defined margin and is causing mass effect and subfalcine herniation; (c) specimen removed intact with part of the thicker outer membrane peeled off showing a transparent thinner inner membrane with particulate matter and (d) microscopic picture showing a lamellated ectocyst (green arrows) and a thinner germinal layer (blue arrows) with brood capsules containing multiple scolices with hooklets (red stars) indicating the cyst is a primary one

Cerebral hydatid cysts are exceedingly uncommon and account for only 1–4% of all patients infected with Echinococcus.[2] Although they can occur anywhere in the brain, they are most commonly found supratentorially in the middle cerebral artery territory.[1] Seeding of the brain is usually secondary to a ruptured cyst elsewhere in the body that releases scolices in the blood stream. Such secondary cysts are sterile and have no brood capsules or daughter cysts inside.[13] Surprisingly, though, in our case, though the cyst was a primary one, i.e. fertile with secondary cysts inside. This is uncommon[3] as the liver and lung usually filter the oncospheres that enter the portal circulation[1] and further this patient had no evidence of a right to left intra- or extra-cardiac shunt.

As in this case, intracerebral hydatid cysts occur more commonly in children than adults (50–93% are seen in children).[1] They can attain a considerable size by the time they are diagnosed.[2] Their rate of expansion may vary between 1.5 and 10 cm per year and hence a significant time lag may occur between the time of infection and onset of clinical symptoms.[13] Children tend to present with features of raised intracranial pressure.[1] Our case, though, had focal neurological deficits - A finding more common in adults.[1] The differential diagnosis on imaging in such cases would include porencephalic cysts, cystic tumors, and intracerebral arachnoid cysts.[2] However, lack of reactive changes in adjacent parenchyma and hypointense rim on T2 imaging are clues to the diagnosis.[2]

Surgical removal is the preferred treatment of hydatid cysts[4] and these should ideally be removed into with all layers intact. Contamination of the operative field by cyst fluid predisposes to anaphylaxis and recurrence and must be avoided.[345] However, in large cysts where cortical damage would be likely if en masse removal is attempted - cyst puncture, aspiration of fluid, instillation of hypertonic saline (a scolicidal agent), reaspiration and finally removal is an acceptable alternative.

In conclusion, though cerebral hydatid cysts are uncommon, even in endemic zones, they must be regarded as a differential diagnosis in children with a cystic intracranial space occupying lesion. Absence of perilesional edema, lack of mural nodule or wall enhancement on contrast, hypointense margin on T2 imaging (halo sign)[5] and fluid contents being isointense to cerebrospinal fluid in all sequences on MRI must alert the clinician to this possibility as also a history of very slow progression which is unlikely in a neoplastic lesion. Despite the presence of barriers to prevent the entry of larvae into the systemic circulation, primary cerebral hydatid cysts may occur and since they cannot be differentiated from secondary sterile cysts on imaging, in all cases undergoing surgery attempt must be made to remove them without rupturing the wall and spilling the contents.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.