There has been active debate about the treatment of arachnoid cysts. The treatment depends mostly upon the location and the size of the cyst. If the cyst is small, does not disturb the surrounding tissue and cause any symptoms, some doctors will refrain from treatment. In the past, doctors placed shunts in the cyst to drain its fluid. Now, with micro neurosurgical techniques and endoscopic tools available for minimally invasive surgery, more doctors are opting for the surgical removal of the membranes of the cyst. Alternatively, some of them open the cyst, so that its fluid can drain into the cerebrospinal fluid and be absorbed.
This answer is based on source information from the National Institute of Neurological Disorders and Stroke.