Spina bifida occulta normally rarely causes any problems. People may have hair growth or a dimple in the bottom of their back. Spina bifida cystica has different levels of severity. A cyst like sac is evident on the spine. In the mildest form, the sac contains spinal fluid and the tissues that cover the spinal cord. The nerves can be intact or mildly damaged, normally by tethering. In the most severe type, the sac also contains nerves and part of the spinal cord. This can lead to paralysis, muscle wastage and loss of sensation. Hydrocephalus (an accumulation of fluid which causes a significant enlargement of the head) is almost always present in the most severe type, approximally 90% of cases.