Epidermoid cysts of the posterior fossa

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✓ Epidermoid cysts originating in the paramedian basal cisterns of the posterior fossa are congenital lesions that grow to a large size through slow accumulation of desquamated epithelium. These lesions grow between and ultimately displace cranial nerves, vascular structures, and the brain stem, causing a long course of progressive neurological deficits. The onset of symptoms usually occurs during the fourth decade of life. Epidermoid cysts are easily diagnosed with computerized tomography scans, which characteristically show a low-density extra-axial pattern. The primary surgical objective is to decompress the mass by evacuating the cyst contents and removing nonadherent portions of the tumor capsule; portions of the capsule adherent to vital structures should be left undisturbed. Aseptic meningitis is the most common cause of postoperative morbidity, and its incidence may be minimized by intraoperative irrigation with steroids followed by systemic therapy with dexamethasone. Symptomatic recurrences that occur many years after surgery should be managed with conservative reoperation.

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Figures

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    Axial computerized tomography scan obtained after intravenous infusion of contrast material. The parapontine cisterns are asymmetrical as a result of a low-density nonenhancing mass on the left (large arrows). The epidermoid cyst has eroded the petrous apex on that side and extends into the medial aspect of the left middle fossa (small arrows).

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    Axial computerized tomography scan through the tentorial incisura showing contiguous spread of the epidermoid cyst from the posterior fossa to the hippocampal region (arrows).

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    Left: Axial computerized tomography scan obtained after intrathecal instillation of metrizamide. The basilar artery (large arrow) is displaced away from the lesion (small arrows), which is outlined by the metrizamide. Right: Coronal reformation through the lesion shown left. Metrizamide fills the interstices of the epidermoid cyst (crosses) and demarcates its superior (large arrow) and lateral (small arrow) borders.

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    Axial computerized tomography scan showing a small low-density deposit of material (arrow) from the ruptured cyst over the right frontal convexity.

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