This patient had an established diagnosis of substantial brain abnormality and was evaluated at an age of 11 months by a pediatric neurosurgeon who found the patient to have macrocephaly (head circumference 55 cm), severe developmental delays, open compressible anterior fontanelle, no signs of intracranial hypertension.

The MRI, in this case, is somewhat indeterminate with features both of hydranencephaly and alobar holoprosencephaly.

In favor of hydranencephaly is that the residual cortical tissue is scattered as islands, with most preserved tissue being in the posterior temporal lobes and occipital poles (posterior circulation) territory. The thalami are not completely fused (thalamic adhesion is present, but so is the third ventricle). There is no pancake cortex or horseshoe cortex as is typically seen in alobar holoprosencephaly. Nor is there evidence of facial abnormalities, commonly seen in holoprosencephaly. 

In favor of holoprosencephaly is the fact that the falx appears entirely absent. 

Overall, the absence of the falx is felt to be the strongest feature, and a diagnosis of alobar holoprosencephaly is somewhat favored over hydranencephaly. 

[Editors: this case has been reviewed by two pediatric neuroradiologists, both favoring alobar holoprosencephaly]

Case courtesy of Dr Guillermo Bolivar