What Is a Fossa Cyst?

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  • Written By: Brenda Scott
  • Edited By: Bronwyn Harris
  • Last Modified Date: 27 September 2019
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In anatomy, a fossa is an indentation along a bone, such as behind the knee, in the armpit, or in various places along the skull. A cyst is sac-like growth which forms in the body and is filled with fluid, gas or a solid matter. A fossa cyst is a liquid-filled cyst which forms in a fossa area in the body.

The most troublesome type of cyst is located in the brain, also known as an arachnoid fossa cyst, or cranial cyst. These usually begin while the brain and skull are forming in the womb. As the brain is developing, the arachnoid membrane splits and fills with spinal fluid and a cyst forms around the area enclosing the liquid. A very small number of arachnoid cysts have been connected to brain trauma instead of a congenital condition.

An arachnoid fossa cyst can form anywhere in the brain, though over half of those diagnosed are found in the fossa in the middle of the cranium, generally on the left side. These appear to occur more frequently in males than in females. Fossa cysts may never be detected unless they grow and begin to cause symptoms. A cranial fossa cyst is usually detected with a computed tomography (CT) scan or magnetic resonance imaging (MRI).


The most dangerous fossa cyst is a posterior arachnoid cyst, which occurs in the back of the brain. If a cyst grows in that area of the brain, it puts pressure on the cerebellum and brain stem and can cause nausea, vomiting, headaches, seizures, imbalance and blackouts. The posterior fossa on the skull is located near the top of the spinal cord and any cyst in this region should be treated to prevent a blockage of the spinal fluid and damage to the nerves which travel through this region into the brain.

Treatment of a fossa cyst depends upon where the cyst is located, and can include opening the cyst with a needle in order to allow the fluid to drain. If this procedure is ineffective, a shunt may be installed. Surgery may be possible if the cyst is easily accessible and can be accomplished without serious complications. Arachnoid fossa cysts are generally removed because of the significant damage they can cause.

A popliteal fossa cyst, also known as a baker’s cyst, occurs at the back of the knee and can result from an injury, gout, arthritis or some other condition which puts stress upon the joint. These cysts appear as a lump behind the knee and can shrink or grow, depending upon how much fluid is currently in the sac. The cyst itself does not generally cause significant pain unless it bursts, at which time the symptoms are similar to a blot clot. If the cyst has become painful or is causing knee instability it can be drained and treated with cortisone injections.


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Post 2

My son is 15 and has a subarachnoid cyst in the posterior fossa. I want to know if it's safe for him to fly with this? or if it is likely to cause him pain or worse?

Post 1

I have a Fossa cyst behind my left knee. My GP thought I had arthritis and was treating me for about 18 months for this. But the pain had gotten so bad.

It's been nearly two months now and I'm still waiting for a scan, but the pain is unbearable, and the heat I feel in the thigh and calf are giving me sleepless nights. My GP gave me diclofenac sodium but this just doesn't seem to be doing anything at all. Is there anything you can recommend until this is sorted for me?

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