Birth defects of the central nervous system (brain, spine, or spinal cord), believed to occur in the first month of pregnancy. The most common type related to Chiari being Spina Bifida.
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Spina Bifida
A congenital a neural tube defect in which the spinal canal does not close before birth and part of the meninges (Meningocele) and sometimes part of the spinal cord itself (Myelomeningocele) creates a sac-like herniation in the back. Additionally, a hidden form (Spina Bifida Occulta) is now known to exist, where little/none of the meninges or spinal cord protrude through the back.