In this extreme form of cerebral wall defect, both cerebral hemispheres are transformed into two thin-walled membranous cysts lacking any recognizable gyral pattern (Figs. 13.12 and 13.13). The temporo-basal, occipital, and the orbital regions may be partially present. Histologically, the fibrotic wall of the cyst is lined with glioneuronal tissue: Some areas contain cortical remnants with neurons, glial cells and calcifications. The ependymal layer usually is disrupted, and the deep gray structures may show minor anomalies. Hydranen-cephaly most likely results from a hypoxic-ischemic insult, but an association with fetal infection, twinning, and maternal gas poisoning have also been suggested.

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