According to the Columbia University Medical Center, type 3 CM occurs when part of the brain protrudes out of an opening in the back of the skull area. This is usually seen in infants with spina bifida. Spina bifida is a neurological condition that causes a portion of the spinal cord to develop outside the body. Type 4 CM occurs when the back of the brain fails to develop normally, according to John Hopkins Medicine. Most babies born with this condition do not survive infancy.
