The surgery also appears to reverse hindbrain herniation, in which the back of the brain slips down into the spinal canal.
More serious defects involving herniation of abdominal contents outside the infant's body are omphalocele and gastroschisis.
Type II malformation, sometimes called Arnold Chiari malformation, is more severe than Type I and involves herniation of a more significant part of the cerebellum, part of the fourth ventricle, and parts of the brain stem.
In these children, hydrocephalus is caused by obstruction of the fourth ventricle due to its herniation into the spinal column.
Type IV malformation consists of an underdevelopment of the cerebrum and involves no herniation of brain tissue into the spinal area.