The most serious type of Spina Bifida – a neural tube defect where the spine fails to close in utero, causing the spinal cord, membranes and nerves to protrude through a sac-like opening through the back. When a Myelomeningocele is accompanied by a Chiari Malformation, it is usually considered a Chiari 2 malformation. Approximately 90% of those with a myelomeningocele have Chiari 2. Most patients with Chiari 2 also have hydrocephalus and/or Tethered Cord Syndrome.