A fossa is an indentation on a bone, while a cyst is a sac-like growth filled with liquid, gas, or solid matter. An arachnoid fossa cyst is the most problematic type, usually found in the brain, and can cause symptoms such as nausea, vomiting, headaches, seizures, imbalance, and blackouts. Treatment depends on the location and severity of the cyst. A popliteal fossa cyst, also known as a baker’s cyst, occurs in the back of the knee and can be treated with cortisone injections.
In anatomy, a fossa is an indentation along a bone, such as behind the knee, in the armpit, or at various points along the skull. A cyst is a sac-like growth that forms in the body and is filled with liquid, gas, or solid matter. A pit cyst is a fluid-filled cyst that forms in a pit area in the body.
The most problematic type of cyst is found in the brain, also known as an arachnoid fossa cyst or cranial cyst. These usually start while the brain and skull are forming in the womb. As the brain is developing, the arachnoid membrane divides and fills with spinal fluid, and a cyst forms around the area that encloses the fluid. A very small number of arachnoid cysts have been linked to brain trauma rather than a congenital condition.
An arachnoid fossa cyst can form anywhere in the brain, although more than half of those diagnosed are found in the fossa in the center of the skull, usually on the left side. These seem to occur more frequently in males than in females. Pit cysts may never be detected unless they grow and start causing 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 brainstem and can cause nausea, vomiting, headaches, seizures, imbalance, and blackouts. The posterior fossa of 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 traveling through this region into the brain.
Treatment of a pit cyst depends on where the cyst is located and may include opening the cyst with a needle to allow fluid to drain. If this procedure fails, a shunt may be installed. Surgery may be possible if the cyst is easily accessible and can be performed without serious complications. Arachnoid fossa cysts are usually removed due to the significant damage they can cause.
A popliteal fossa cyst, also known as a baker’s cyst, occurs in the back of the knee and can result from an injury, gout, arthritis, or other conditions that place stress on the joint. These cysts appear as a lump behind the knee and may shrink or grow, depending on 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 blotchy clot. If the cyst has become painful or causes knee instability, it can be drained and treated with cortisone injections.
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