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Neuroradiology Case of the Week

Case 125

Humera Ahsan, MBBS, MCPS, Mashhood Ul Haque Qazi
and Per-Lennart Westesson, MD, PhD, DDS

Clinical Presentation: A 21-year-old female presented with a history of headaches. She also gave history of previous brain surgery.

Radiological Findings: Axial T2 weighted image shows a well encapsulated round cystic mass in the left parietal region. The signal intensity of the cyst is similar to CSF both on T1 and T2-weighed images. The capsule of the mass is slightly hyperintense on T1 and hypointense on T2-weighted images. No surrounding mass effect or edema is seem. There is also evidence of craniotomy.

Discussion: Hydatid disease is caused by the infestation of the larvae of taenia echinococcus. The definite hosts of echinococcus are various carnivores, the common being the dog. All mammals (more often being sheep and cattle) are intermittent hosts. Humans get infected through the faeco-oral route by ingestion of food or milk contaminated by dog faeces containing ova of the parasite or by direct contact with dogs. The eggs loose their enveloping layer in the stomach, releasing the embryos. The embryos pass through the wall of the gut into the portal system and are carried to the liver where most larvae get entrapped and encysted. Some may reach the lungs and occasionally, some may pass through the capillary filter of the liver and lungs and get entry into the systemic circulation. Cerebral hydatid cysts are usually supratentorial and often involve the middle cerebral artery territory because of the embolic nature of the infestation [1,12].
     Intracranial hydatid disease is rare, with reported incidence of 1-2% of all cases with hydatid disease [2]. Hydatid cyst is endemic in the Middle East, Mediterranean countries, South America, North Africa and Australia [3]. Cerebral hydatid cyst is more common in the pediatric population [1,4]. This high incidence in children is probably related to patent ductus arteriosus [5].
     Intracranial hydatid cysts are more frequently located in the supratentorial compartment. The other less common sites reported are skull [6], cavernous sinus [7], ventricles [4], etc. Cerebral hydatid cysts are slow growing and present late when they increase in size and become large. There is no consensus on the growth rate of the hydatid cysts of the brain and has been variably reported between 1.5-10 cm/year [1,8]. Intracranial hydatid cysts are commonly solitary. Multiple intracranial cysts are rare. Intracranial hydatid cyst may also be classified as primary or secondary. The primary cysts are formed as a result of direct infestation of the larvae in the brain without demonstrable involvement of other organs. In primary multiple cysts, each cyst has a separate pericyst with brood capsule scolices and these originate from multiple larvae affecting brain after crossing the gastrointestinal tract, liver, lungs and right side of heart without affecting them. The primary cysts are fertile as they contain scolices and brood capsules, hence rupture of primary cyst can result in recurrence. The secondary multiple cysts results from spontaneous, traumatic or surgical rupture of the primary intracranial hydatid cyst and they lack brood capsule and scolices. Thus, these are infertile, and the resultant risk of recurrence after their rupture is negligible.
     The patients with intracranial hydatid cysts usually presents with focal neurological deficits and features of raised intracranial pressures; the later may be due to the large size or due to interference with pathway of CSF flow. Erashin et al. [2] observed that 18 out of 19 cases presented with raised intracranial pressure. MR and CT scans characteristically show hydatid cyst as a spherical, well-defined, non-enhancing cystic lesion without peripheral edema [9,10]. The fluid density is generally equal to that of CSF on both CT and MR scan. A fine rim of peripheral enhancement with perilesional edema may be seen in the presence of active inflammation [10]. The presence of a hypointense rim, especially on T2-weighted MR images, is characteristic of HC of the brain [11]. The lack of surrounding edema and the marked mass effectmake it easy to distinguish cerebral HC from abscess and cystictumor.
     The treatment of hydatid cyst is surgical and the aim of surgery is to excise the cyst without rupture to prevent recurrence and anaphylactic reaction. Various surgical options include, puncture and aspiration of the cyst fluid through a small hole in the cyst wall, cortical incision over cyst and expulsion of hydatid cyst by insuffalation of air in the contralateral ventricle and the most commonly done procedure designed to give birth to the intact cyst by irrigating saline between cyst wall-brain interface.

References:

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