Forebrain developmental abnormalities

Megalencephaly

• Abnormally large brain
• Less common than microencephaly 

Microencephaly

• Abnormally small brain
• Lots of causes (chromosomal abnormalities, fetal alcohol syndrome, HIV acquired in utero)

Lissencephaly

• Decrease in number of gyri
• Sometimes gyri are totally absent (agyria)

Polymicrogyria

• Small, numerous, irregularly formed gyri
• Can be caused by injury or genetic abnormality

Neuronal heterotopias

• Neurons in inappropriate locations along migrational pathways
• Associated with epilepsy

Holoprosencephaly

• Incomplete separation of cerebral hemispheres across the midline
• May have midline facial abnormalities (e.g., cyclopia, absence of olfactory cranial nerves)

Agenesis of the corpus callosum

• Absence of white matter bundles connecting cerebral hemispheres
• Patients may have other defects, or may be asymptomatic