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Arachnoid cysts are sacs of spinal fluid that grow on the arachnoid membrane, one of the three layers of tissue protecting the brain. They can also grow at the base of the brain. Arachnoid cysts are something you are born with, but they may not grow large enough to cause problems, or even to be noticed, until later in life. Sometimes arachnoid cysts are asymptomatic. They are only discovered by accident when the brain is scanned for some other reason. Sometimes arachnoid cysts cause awful problems; this usually happens when the cysts are unusually large.

What are the symptoms of an enlarged, problematic arachnoid cyst?

  • Macrocephaly, or enlargement of the head. This is a cranial deformity that would be noticed in a growing child.
  • Bobble-head doll syndrome. Cysts in the suprasellar cistern, behind the cortex at the front of the brain, may cause brain changes that cause bobbing and nodding of the head.
  • Attention deficit hyperactivity disorder may have a relationship to cysts in a region of the brain called the left middle cranial fossa. It's more or less back from the eyes inside the brain.
  • Seizures.
  • Non-specific headaches that don't occur in the same part of the head every time.
  • Behavioral changes.
  • Nausea.
  • Ataxia (loss of muscle control).
  • Hemiparesis (paralysis or muscle weakness, but on just one side of the body).
  • Hydrocephalus (buildup of spinal fluid).
  • Musical hallucinations.
  • In the elderly, a combination of Alzheimer's-like symptoms including hemiparesis, loss of bladder control, and dementia.

Cysts have been associated with depression, Ménière's disease, hallucinations, paranoia, and manic-depression. Sometimes, however, even large cysts cause no symptoms at all.All of the symptoms can be caused by other conditions, and the only practical way you can know for sure that you have an arachnoid cyst is for your doctor to order an MRI.

Most arachnoid cysts don't cause symptoms and don't require treatment. Treating a cyst requires neurosurgery, but modern methods aren't as scary as they used to be.

  • Some brain cysts can be treated with endoscopic surgery. The surgeon makes a 3-millimeter cut to the femoral artery in the groin, and inserts a tiny catheter into it and threads it up to the brain. This very, very, thin catheter can be equipped with tools for visually inspecting and surgically removing the cyst, without the need ever to open the skull. This procedure does not even require general anesthesia, and it may be possible to go home the same day as you have the operation.
  • Needle aspiration through a burr hole is another way of removing the fluid from a cyst. The surgeon has to make a small incision in the scalp and drill a hole through the skull to insert the needle, but the trauma to the brain is still relatively minimal. Doctors can also use this procedure to biopsy the brain. It's not a procedure you want to have unless absolutely necessary, but complications are relative rare.
  • Any kind of procedure involving "fenestration" requires opening your skull. These procedures are inherently risky and have a relatively long recovery time, in hospital. Skilled neurosurgeons with dedicated teams in well-equipped hospitals are a must, but there will be risks in procedure of this kind no matter how skilled your surgeon may be.

If you have an arachnoid cyst, you aren't alone. About 1 out of every 100 people in the general population has this brain abnormality. Only 1 in 500 out of the general population needs medical treatment of an arachnoid cyst. About 80 percent of the time, they are benign. There's no reason to panic just because you have been told you have the cyst. Treatment can be scary, but modern methods have made it a lot safer than it used to be, and the overwhelming odds are that you will feel better, sometimes a lot better, after you have it.

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