The tumors may be malignant or benign.[27] Unknown Post-haemorrhagic hydrocephalus Bleeding into the ventricles of the brain, especially in infancy, can lead to blockage of CSF drainage and cause hydrocephalus. Spina bifida Specifically spina bifida myelomeningocele can cause the development of hydrocephalus because the cerebellum will block the flow of CSF in a development of Chiari Malformation II. .125%[28] Congenital aqueductal stenosis A genetic disorder which can cause deformations of the nervous system. The defect is commonly associated with mental retardation, abducted thumbs and spastic paraplegia.[26] .003%[26] Craniosynostosis Craniosynostosis occurs when the sutures of the skull close too early.