Atelencephalic microcephaly in a 21-week human fetus.

Atelencephalic microcephaly, a rare and extreme disorder, is known morphologically by only six cases. Derivatives of the telencephalon are absent or dysplastic, while more caudal structures are normal or mildly deformed. A more extensive form, aprosencephaly, involves structures of the diencephalon and may be associated with holoprosencephalic facies. Extracranial anomalies may be present in both atelencephaly and aprosencephaly. We describe the seventh and youngest specimen, a 21-week female with atelencephaly. Maternal and gestational histories were unremarkable; the fetus was obtained by therapeutic abortion following diagnosis of a severe cranial malformation by ultrasound. A small and depressed, but intact, calvarium covered the brain. The forebrain was rounded and showed fused hemispheres, absent gyri, olfactory bulbs, and tracts. Caudal structures were mildly deformed. An oval mass of bone filled much of the middle cranial fossa. By light microscopy, several dysplastic changes were apparent in the forebrain. Ventricles were not present; small round cells resembling those of the germinal matrix were prominent in the forebrain. Pyramidal tracts were absent at all levels. The process responsible for these changes cannot be established with certainty; the changes are, however, in keeping with previous damage, such as that accepted for other encephaloclastic disorders. The insult in atelencephaly presumably occurs after closure of the rostral neuropore; earlier damage, with more widespread consequences, is possible for aprosencephaly. As with other destructive processes, etiology in atelencephaly and aprosencephaly is most likely heterogeneous.