Hydranencephaly, Alobar Holoprosencephaly, and Massive Hydrocephalus

The main differential considerations for very large ventricles (> 20 mm) on prenatal ultrasound are hydranencephaly, alobar holoprosencephaly, and massive (maximal) hydrocephalus.

Hydranencephaly is thought to arise from bilateral internal carotid artery occlusions with intact posterior circulation. This results in a large cerebrospinal fluid space anteriorly where the frontal and temporal lobes would have formed and intact areas in the posterior circulation, namely preserved thalami. The third ventricle will be normal. You will also see a falx.

Holoprosencephaly is caused by the failure of division of the forebrain (specifically the prosencephalon). There are varying degrees of frontal lobe fusion. There may or may not be associated facial anomalies. Alobar holoprosencephaly is the most severe form and is characterized by a monoventricle communicating with a dorsal cyst; fused thalami, basal ganglia, and choroid plexus; and absent third ventricle, interhemispheric fissure, and corpus callosum. A ridge of tissue arising from the posterior aspect of the boomerang-shaped frontal lobes separates the monoventricle from the dorsal cyst. This ridge corresponds to the hippocampal fornix and is sometimes referred to as the hippocampal ridge.

Finally, in massive (maximal) hydrocephalus, there is a falx and separate ventricles with a septum pellucidum. The cortex, although squeezed extremely thin due to the large ventricles (circumferential mantle), can be seen (best with MRI). The third ventricle is dilated. Look for aqueductal stenosis.

References

• Filly RA, Chinn DH, Callen PW.. Alobar holoprosencephaly: ultrasonographic prenatal diagnosis. Radiology. 1984 May;151(2):455-9.
• Neuroradiology: The Requisites
• Yassin OM, El-Tal YM. Solitary maxillary central incisor in the midline associated with systemic disorders. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 1998 May;85(5):548-51.