Arachnid cyst:spider webs in the brain (Part I)

Most of my life I have had a "lump" on the back of my head just above my neck. My neurosurgeon originally thought that it was herniation of my third and fourth ventricles due to my hydrocephalus. Earlier in the week, however, she had a CT scan done in preparation for an upcoming surgery I need and that indicated it wasn't a herniation at all, but, rather, an Arachnoid cyst. For those who might not be familiar, an arachnoid cyst is usually congenital in nature - meaning it's present at birth - but it can also be caused by trauma to the head. (NORD, N.D.) The cyst is a fluid-filled sack that occurs on the arachnoid membrane which covers both the brain and the spine. The most common locations for the cyst to occur are the middle fossa (near the temporal lobe), the suprasellar region (near the third ventricle), and the posterior fossa which contains the cerebellum, pons, and medulla oblongata. In many cases persons with an arachnoid cyst are asymptomatic. In those cases were symptoms are present, they include headache, seizures, and an abnormal accumulation of cerebrospinal fluid. The exact cause of the cyst is unknown.

What causes arachnoid cysts

While the exact cause of an arachnoid cyst is unknown, however, researchers believe that most cysts are developmental malformations that occur when unexplained splitting or tearing of the arachnoid membrane. In some cases, the arachnoid cysts occurring in the middle fossa are accompanied by underdevelopment (hypoplasia) and/or compression of the temporal lobe. The exact role that temporal abnormalities play in the development of cyst is unknown.

Some complications associated with an arachnoid cyst occur as the result of damage to the cyst caused by minor head trauma. The trauma causes fluid within the cyst to leak into another area such as the subarachnoid space. Head trauma can also cause the blood vessels on the surface of the cyst to tear and then bleed into the cyst (intracystic hemorrhage) causing it to increase in size. This can also result in increased intracranial pressure. 

An arachnoid cyst can also occur secondary to other disorders such as Marfan's syndrome or agenesis of the corpus callosum.

BIBLIOGRAPHY

National Organization for Rare Disorders (NORD) (N.D.). Arachnoid cyst. Retrieved on August 31, 2019 from https://rarediseases.org/rare-diseases/arachnoid-cysts/