Holoprosencephaly: single-lobed brain, one undivided ventricle. Failure of cleavage of telencephalon and diencephalon. Incomplete variants: semilobar, lobar holoprosencephaly. Associated with defects of skull base, dura, face, eyes, olfactory apparatus.

Polymicrogyria: excessive number of gyri, all smaller than normal. Some familial.

Lissencephaly: (“smooth brain”); absence or primitive appearance of gyri; may occur with chromosome abnormalities or intrauterine insults. Associated with muscle and eye malformations in Walker-Warburg syndrome, Fukuyama muscular dystrophy.

Agenesis of corpus callosum (complete or partial): may occur in isolation or in conjunction with many conditions: holoprosencephaly, septo-optic dysplasia, many syndrome complexes with multiple dysmorphisms, some metabolic diseases (e.g., pyruvate dehydrogenase deficiency).

Macrocephaly: head size >2 standard deviations above mean for age. Causes: hydrocephalus, mass lesions (infant, young child); inherited.

Megaloencephaly: brain weight ≥2.5 standard deviations above mean for sex and age or brain weight >1,600 g. May occur alone. Progressive brain enlargement with neurologic deterioration seen with neurocutaneous, neuronal storage, degenerative disorders.