In clinical practice, an arachnoid cyst is actually a manifestation of the enlarged nature of the arachnoid granules, which are essentially a membranous structure encasing cerebrospinal fluid. Arachnoid cysts may be present in many people, but some people do not have any clinical symptoms and grow more slowly, so they do not require special treatment. If the arachnoid cyst grows rapidly and compresses the brain tissue within a short period of time during observation, then aggressive treatment, such as surgery, is indicated. Some of the causes of arachnoid cysts may be congenital, some may be due to encephalitis as a child or in early childhood, resulting in blockage of arachnoid granules that eventually expand to form arachnoid cysts, and some may be due to genetic mutations.