What is the natural course/history of untreated spina bifida?

It Depends. It depends on the extent of the spina bifida. Some forms of spina bifida are mild and only involve more of the superficial bony elements of the lumbar spine. Other forms of spina bifida may be more extensive involving not only the bony elements of the spine but also the coverings of the neural elements and the neural elements themselves. Natural history is dependent on the form of spina bifida.