Pineal gland cyst
A pineal gland cyst is a usually benign cyst in the pineal gland, a small endocrine system gland in the brain. Historically, these fluid-filled bodies appeared on 1-4% of magnetic resonance imaging (MRI) brain scans, but were more frequent at death, seen in 21-41% of autopsies. But a 2007 study by Pua et al. found a frequency of 23% in brain scans (with a mean largest inner cross-sectional diameter of 4.3 mm), with an insignificantly higher frequency for women over men.
These smaller cysts (less than 5.0 mm) are usually asymptomatic, but for larger cysts (greater than 5.0 mm), possible symptoms could include headache, unexpected seizures, visual disturbances, muscle fasciculations, light sensitivity, circadian rhythm dysfunction, or hydrocephalus if the cyst impinged on the superior colliculi or caused obstruction of the cerebral aqueduct. In some cases this is normal and will not affect the human body. In most cases, it will need to be removed before more, life threatening situations occur. Despite the pineal gland being in the center of the brain, due to recent advancements in endoscopic medicine, endoscopic brain surgery to drain and remove the cyst can be done with the patient only spending one night in the hospital, and being fully recovered in a week, rather than a year as is the case with open-skull brain surgery.[Reference?]
The National Organization for Rare Disorders says that pineal cysts larger than 5.0 mm are "rare findings" and are possibly symptomatic. If narrowing of the cerebral aqueduct occurs, many neurological symptoms may exist, including headaches, vertigo, nausea, muscle fasciculations, eye sensitivity, and ataxia. Continued monitoring of the cyst might be recommended to monitor its growth, and surgery may be necessary.
- Y. Pua, S. Mahankalia, J. Houa, J. Lia, J.L. Lancastera, J.-H. Gaoa, D.E. Appelbaumb and P.T. Fox. "High Prevalence of Pineal Cysts in Healthy Adults Demonstrated by High-Resolution, Noncontrast Brain MR Imaging" American Journal of Neuroradiology 28:1706-1709, October 2007. doi:10.3174/ajnr.A0656 
- Pineal Cysts, Symptomatic, National Organization for Rare Disorders
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