Hydranencephaly is a rare congenital deformity (a deformity that occurs during fetal development) that is characterized by the absence of the cerebral hemispheres of the brain.

Hydranencephaly may be caused by a genetic defect, infection associated with vessels, or a trauma that occurs after the twelfth week of pregnancy. Maternal exposure to carbon monoxide early in pregnancy.

Diagnosis may not be made for weeks or months following birth, because of the initial normal appearance and behavior of the newborn.

Most children die in their first year of life, although survival past the age of 10 can rarely occur. Currently, the oldest known survivor was 20 years, 6 months old.