Arachnoid Cyst
An arachnoid cyst is a fluid-filled sac that develops between the brain or spinal cord and the arachnoid membrane, one of the three protective layers (meninges) that cover the central nervous system. These benign, congenital cysts contain cerebrospinal fluid and are typically present from birth, though they may not be discovered until later in life.
What is it?
Arachnoid cysts are cerebrospinal fluid-filled sacs that form within the arachnoid membrane, which is the middle layer of tissue covering the brain and spinal cord. Most arachnoid cysts are primary (congenital), meaning they develop during fetal development when the arachnoid membrane splits or duplicates abnormally. Secondary arachnoid cysts can develop after head injury, infection, or brain surgery, though these are less common.
These cysts occur in approximately 1% of the population and are found three times more often in males than females. The most common location is the middle cranial fossa (near the temporal lobe), accounting for about 50% of cases, though they can occur anywhere along the brain or spinal cord. Most arachnoid cysts remain stable in size throughout life and never cause symptoms, often discovered incidentally during imaging for unrelated reasons. When symptoms do occur, they typically result from the cyst’s mass effect—pressing on adjacent brain tissue—or rarely from cyst rupture or hemorrhage.
Important to Know
The majority of arachnoid cysts are asymptomatic and require no treatment beyond periodic monitoring with imaging to ensure stability. Treatment is only considered if the cyst causes symptoms, grows significantly, or results in complications. Symptoms vary depending on the cyst’s location and size, and may include headaches, seizures, developmental delays in children, balance or coordination problems, or vision changes. When intervention is needed, surgical options include cyst fenestration (creating openings to allow fluid drainage) or cyst-peritoneal shunting. The decision to treat is individualized based on symptoms, cyst characteristics, and patient factors. Most people with arachnoid cysts live normal, healthy lives without any intervention.