Department of Pathology, University of Oklahoma Health Sciences Center

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Answer: (E) Alobar holoprosencephaly.

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Discussion:

• Pathology of the case: On the frontal view (Panel A), the two cerebral hemispheres appear to be fused in the midline. The occipital half of the cerebral hemispheres are reduced to a thin translucent seam (Þ in Panel B) as illustrated in the photo that is taken under water and out of water (Þ in Panel C). These features are typical for an alobar holoprosencepahly. As only the frontal half of the telencephalon exists, it would appear as like a shield on lateral view and this type of holoprosencephaly is also called the shield type holoprosencephaly. The thalamus and the basal ganglia appear as two fused hemispheres. There is a single, dilated  ventricle with no septum pellucidum found (Panel C).  On a coronal cut, the gray matter cross over the midline and no corpus callosum is present (Panel D). None of the other answer provided have these features.

• De Morsier Syndrome (Septo-optic dysplasia) is characterized by optic nerve hypoplasia, aplasia or defects of the septum pellucidum (inconstant finding), and pituitary-hypothalamic dysfunction (hypopituitarism). The floor of third ventricle, a structure embryonically related to the development of the optic placodes, is abnormal in some cases. Olfactory aplasia is frequent. Other features include elevation of the tentorium cerebelli and lateral and transverse sinuses and torcula, lack of patency of the foramina of Magendie and Luschka, and hydrocephalus. Not all these features are present in every case. 

• Chiari II malformation: Displacement of part of the inferior cerebellar vermis and an elongated and malformed medulla oblongata into the cervical spinal canal. Typically, it is associated with a neural tube defect.

• Encephalocele: Herniated protrusion of brain substance through a congenital or traumatic opening of the skull. Congenital encephalocele, like anencephaly, is usually associated with hypoplastic lungs and often with other internal anomalies. About 80-90% are occipital encephalocele. The next common is parietal encephalocele. Basal and frontoethmoidal encephaloceles are rare.

• Dandy-Walker syndrome as originally defined has 6 features:

• a cyst-like dilation of the fourth ventricle,

• an abnormal cerebellar vermis,

• elevation of the tentorium cerebelli and lateral and transverse sinuses and torcula (torcular Herophilli),

• lack of patency of the foramina of Magendie and Luschka,

• enlargement of the posterior fossa,

• hydrocephalus.

Not all six features are found in each case. The three essential features are: agenesis of the vermis, cystic dilatation of the fourth ventricle and enlargement of the posterior fossa. Hydrocephalus is a frequent but inconstant finding.

Comment: KarMing-Fung@ouhsc.edu