An arachnoid cyst is a fluid filled sac that develops primarily in the uterus during the development of the brain and skull. The arachnoid membrane of the brain splits, and the split space fills with fluid known as cerebral spinal fluid. The fluid will enter the split area, but it cannot escape at the same rate as it enters. Because of this, the cyst will grow over time. The growth rate depends on many factors, but even as it grows, more times than not, the cyst will remain asymptomatic. This means that generally, an arachnoid cyst will not cause any symptoms.
Arachnoid cysts can grow anywhere on the brain with the primary areas being the side of the brain, and the top of the brain. When a cyst forms on the back of the brain, this is called a posterior fossa arachnoid cyst. A posterior fossa arachnoid cyst can create pressure on the brain stem and the cerebellum. These are extremely vital organs of the brain and they both produce tremendous functions that are required to live.
The brain stem controls a lot of our everyday functions that we do not even think about. One thing it controls is our body temperature. It controls our diaphragm for breathing, and it is where our hunger and thirst are controlled. It is also the path in which all our nerves leave the brain and travel throughout the body. The brain stem controls our pain, it is also where the nausea center is based, and it plays a role in the regulation of our hearts.
The cerebellum; on the other hand, helps control our balance. It plays a role in the reflex of our muscles, and plays a major role in our motor skills. Both parts of the brain have their very specific tasks, and when either one is compromised, it can reek havoc on the entire body.
If treatment is needed for an arachnoid cyst, usually the first choice is a fenestration of the cyst. This means going into the brain with a wand and opening up any walls that may have formed inside the cyst. These walls trap the fluid inside the cyst. The idea of this procedure is to get all the cerebral spinal fluid flowing in and out of the cyst efficiently so that the cyst does not continue to grow. If this procedure does not work, a neurosurgeon may decide to put a shunt in. A shunt helps to keep the cyst draining and not allow fluid to build up inside. If the cyst is small enough and in a space on the brain that is not too dangerous to remove, the neurosurgeon may be able to remove it altogether.
Symptoms can show themselves in many ways; from balance problems, nausea, vomiting, headaches, even seizures. Many cysts are discovered on accident when having a scan for another reason. They can be seen on Cat Scan, but the preferred diagnostic test for a brain cyst, is an MRI.