These cysts result either from a congenital splitting of the arachnoid membrane or from an irritation of the leptomeninges by inflammation or hemorrhage. They occur in both children and adults. Common sites are along the sylvian fissure, in the cerebellopontine angle, and over the cerebellar hemispheres. Clinically, they may present with seizures, focal deficits and, in infants, with hydrocephalus. They may also remain asymptom atic, to be diagnosed incidentally on neuroimaging or at autopsy (Fig. 11.41). The cyst wall consists of collagenous tissue lined with arachnoid cells. The underlying brain may be indented and gliotic.
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