The exact cause of arachnoid cysts is not known. Researchers believe that most cases of arachnoid cysts are developmental malformations that arise from the unexplained splitting or tearing of the arachnoid membrane, being classified as primary. There are also arachnoid cysts that arise from secondary causes, such as trauma, surgery, intracranial hemorrhage and infection.^[1]

• Arachnoid cysts are not tumors. They are small sacs that develop between the brain and the arachnoid membrane;
• They are filled with intracranial cerebrospinal fluid and lined with arachnoid membranes;
• Arachnoid cysts can be primary, in which the cause is unknown and develops in a fetus during pregnancy;
• And it can also be secondary to CNS infections, trauma, brain tumors and may be a complication of brain surgeries.^[1]
• There have been cases of arachnoid cysts appearing in multiple members of a family (familial cases) which suggests that there might be a genetic component playing a role in the development of arachnoid cysts in some patients.^[2]
• They may also be related to Marfan’s syndrome, agenesis of the corpo callosum and other disorders.^[2]