A brain cyst is a small sac that is filled with air, liquid or another type of material. Cysts are different than tumors, explains MayoClinic.com. A cyst is small, noncancerous sac. A tumor, in contrast, is caused by the abnormal growth of cancerous cells, forming a lump full of tissue. A brain cyst may not cause any symptoms or it may cause a variety of different symptoms, depending on where in the brain it is located. Brain cysts can have several different causes.
Abnormal Membrane Formation
Many types of brain cysts arise while a fetus is developing in the womb, caused by a developmental abnormality. For example, arachnoid cysts, also called leptomeningeal cysts, often develop in the subarachnoid space of the brain, an area between the layers of membranes that surround the brain. Doctors think these arachnoid cysts are caused by an abnormal split that forms while the membranes are growing in the fetal brain. The abnormal split results in a tiny sack in the membrane that fills with cerebrospinal fluid, the American Brain Tumor Association explains.
Abnormal Brain Development
Other types of cysts, including dermoid and epidermoid cysts, also occur because of an abnormality of brain development. These cysts occur when cells that are programmed to turn into skin tissue accidentally become trapped on the inside of the developing fetal brain. These cells continue to grow into skin tissue and often form a small cyst containing skin and hair follicles, in the case of dermoid cysts, or skin and cholesterol crystals, in the case of epidermoid cysts, reports the American Brain Tumor Association.
Pineal cysts are another type of brain cyst that may form in a developing fetus or in an young adult after puberty. These cysts are named for the pineal gland on which they grow. Doctors do not understand the exact cause of pineal cysts, but believe it may be related to the shrinking of the pineal gland that normally occurs after puberty, according to the American Brain Tumor Association.
Cysts Caused by Tumors
Although cysts and tumors are completely different types of structures, some brain cysts may be caused by the rapid growth of cells that cause tumors, explains the American Brain Tumor Association. As cancerous cells grow out of control and form tumors, they may create spaces between tissues in the brain that fill with cerebrospinal fluid or other material.
The pork tapeworm, called Taenia solium, burrows into human tissue and forms small, hard cysts. If the tapeworm burrows into the brain and forms cysts, the condition is called neurocysticercosis, the U.S. Centers for Disease Control and Prevention says.
Certain brain cysts, such as arachnoid cysts, can form later in life from causes other than developmental abnormalities. Secondary arachnoid cysts, which is the name used for cysts that develop later in life, may be caused by an injury to the brain or as a complication of brain surgery, explains the National Institute of Neurological Disorders and Stroke. Another condition known as meningitis, which is a swelling of the membranes covering the brain, may also result in the development of secondary arachnoid cysts.