Specific Imaging Findings Hydranencephaly presents as a replacement of the cerebral hemispheres by large, fluid-filled cystic cavities that are widely communicating with rudiments of the lateral ventricles posteriorly. Remnants of the basal portions of the temporal lobes and of the posterior parts of the occipital and parietal lobes are usually present, and the cortex may be normal or dysplastic. The cerebral falx is intact and appears to float within the cystic cavities; typically, one or more parafalcine cerebral rudiments are found. The basal ganglia are absent, whereas the thalami are notably present and abut the overlying cavitations. The brainstem is normally developed, but is usually thinned as a result of Wallerian degeneration; the cerebellum is also normal. MR angiography can show abrupt cut-off or hypoplasia of the vessels distal to the supraclinoid internal carotid arteries, but can also display an entirely patent, albeit stretched, craniocervical arterial tree. The skull may be enlarged as a consequence of CSF pulsation. Pertinent Clinical Information During pregnancy there is usually no obvious maternal illness or distress. The diagnosis is postnatal in a majority of cases, and may be delayed due to a surprisingly silent clinical course despite the severe pathological findings. Patients may elicit medical attention because of progressive increase of head circumference during the first weeks or months of life. Seizures may also appear. The prognosis is usually dismal with death in infancy in most cases. However, prolonged survival has also been reported and very rare cases of hemi-hydranencephaly, where only one cerebral hemisphere is affected, have been incidentally discovered in adults.
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