What Causes a Brain Cyst?

A brain cyst is a small, noncancerous sac filled with air, liquid or another type of tissue. How it can affect you depends on where in the brain it's located and what's causing it.

The good news on brain cysts is that these cysts "are usually benign," says David Weintraub, MD, director of functional neurosurgery at North Shore University Hospital in Manhasset, New York. They don't usually cause symptoms, and a vast majority of them will stay the same size.

"Certain types of cysts, however, can draw more fluid into them and grow over time," Dr. Weintraub adds. "Cysts that grow over time often do cause symptoms and need to be treated with surgery."

Brains cysts can occur at any time in a person's life, even before birth. And the reasons behind cyst development are many.

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An arachnoid cyst is one type of cyst that can develop in the fetal brain. The cyst may be caused by an abnormal splitting of the arachnoid membrane, one of the three layers of tissue that protect the brain and spinal cord, says the National Organization for Rare Disorders (NORD). While usually present from birth, these cysts can also arise as a result of a brain tumor, injury, infection or brain surgery, explains NORD.

Oftentimes arachnoid cysts cause no symptoms at all. But when they do, it's usually due to the size of the cyst or its location in the brain, adds NORD.

Other types of cysts, including those called dermoid and epidermoid cysts, also can be caused by abnormalities that occur in the developing brain of a fetus. These cysts form when skin cells or tissue become trapped on the inside of the fetal brain, according to St. Luke's Health System in Kansas City, Missouri.

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Colloid cysts usually form in the central part of the brain and can lead to a blockage of cerebrospinal fluid. If you have a colloid cyst, it can cause headaches when you're in a certain position, says St. Luke's. These headaches usually strike in adulthood.

Pineal cysts are named for the pineal gland on which they grow. The causes of these cysts aren't known, says the Mischer Neuroscience Institute at Memorial Hermann-Texas Medical Center and McGovern Medical School at UTHealth. They're typically found in young adults, and women more so than men.

Some brain cysts, such as arachnoid cysts, can form later in life from causes other than developmental abnormalities. When this happens, they're called secondary arachnoid cysts. They may be caused by an injury to the brain or as a complication of brain surgery, according to the National Institute of Neurological Disorders and Stroke. Meningitis, a swelling of the membranes covering the brain, may also cause secondary arachnoid cysts to develop.

Small hard cysts in the brain can be the result of parasites, particularly a tapeworm called Taenia solium that humans may ingest by eating undercooked pork, explains the World Health Organization (WHO). If the tapeworm larvae invade the brain and forms cysts, the condition is called neurocysticercosis.

WHO says this condition is the most frequent cause of epilepsy in the developing world. While it's possible for Americans who never travel outside of the U.S. to become infected, the Centers for Disease Control and Prevention says it's rare.

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