Someone was born with a sack of fluid on the back of the head. Parents were told, that if it burst, it would result in the death of baby. It was removed by surgery within the first week of life. Question: What is it? What causes it? What are the long-term effects of it, if any, even if it is removed and the patient lives? What is the medical term for such a sac, if there is one? What is the name of the surgery to remove such a sac?