Hydranencephaly is a condition in which the brain's cerebral hemispheres are absent and replaced by sacs filled with cerebrospinal fluid.
@"Unlike in Anencephaly where the damage to the brain happens at conception, in Hydranencephaly the baby's brain develops normally until something happens to cut off the flow of blood to the baby's brain. The affected part of the brain then starts to die and the tissue is reabsorbed by the body and replaced with cerebral spinal fluid (CSF). The "something" that cuts off the flow of blood to the baby's brain can be quite brief. Some of the most common causes are a stroke in the baby, prenatal drug exposure, and the death of a twin in utero. In many of the children the cause is unknown. The damage to the brain usually occurs in the 2nd or 3rd trimester of pregnancy and can occur up to a year after birth as well. While the damage to the hemispheres is typically extensive, the child's brainstem is usually (but not always) intact." - Taken from http://hydranencephaly.com/@