Holoprosencephaly and septo-optic dysplasia.

Holoprosencephaly is a rare brain anomaly, which may be seen in liveborn neonates, although it is more common in stillbirths and abortions. It may occur alone or as part of a large number of genetic syndromes and associations. It is thought to arise early in embryogenesis (6 weeks), as the prosencephalon is separating into the two hemispheres. It is a complex anomaly with markedly varying degrees of severity. The most severe alobar form affects the entire prosencephalon. The brain shows no evidence of division into two hemispheres. The central thalami and basal ganglia remain undivided and confluent, and there is a large, dorsally located cyst connected to the large single ventricle. These infants usually die shortly after birth. The semilobar form has variable amounts of hemispheric formation with normalization occurring from dorsal to ventral. Some posterior falx, occipital, and temporal horn formation and evidence of a third ventricle are present. It is compatible with life. The lobar form has the most nearly normal appearance. It often exhibits a well-formed ventricular system, absence of the septum pellucidum, a shallow falx, abnormal connection of gray and white matter across the midline, and partially formed frontal lobes. Facial anomalies, especially hypotelorism and midline clefts, generally parallel the brain malformation in severity. Visual and olfactory symptoms plus pituitary abnormalities are often present. Septo-optic dysplasia is a related entity. It has a constellation of symptoms of hypotelorism, severe visual problems, and short stature or other pituitary symptoms. Primary imaging findings include optic nerve hypoplasia and absent septum pellucidum. Septo-optic dysplasia somewhat resembles lobar holoprosencephaly. Septo-optic dysplasia is more common than holoprosencephaly. Isolated absence of the septum pellucidum is also thought to be a mild brain anomaly.