Arachnoid cysts are sacs that are filled with cerebrospinal fluid and form in the surface region of the brain around the cranial base, or on the arachnoid membrane (one of three membranes that covers the brain and spinal cord).
An arachnoid cyst forms when the two lipid (fatty) layers of the arachnoid membrane split apart to form a cavity. Like most membranes, the arachnoid membrane is comprised of two layers (leaflets) of lipid molecules. The hydrophilic (water attracting) region of the lipids is oriented towards an environment rich in water. The hydrophobic (water repelling) portion of the lipids will spontaneously partition away from water, in the interior of the membrane. When an arachnoid cyst forms, the two leaflets of the membrane split apart. Cerebrospinal fluid then fills the cavity.
Arachnoid cysts can be classified according to their location and by the type of tissue making up the cyst wall (arachnoid connective tissue or glioependymal tissue). Cysts that are found in the area of the cerebrum and in the spinal cord tend to be composed of arachnoid tissue, while cysts found in the supracollicular or retrocerebellar regions of the brain tend to be composed of either arachnoid connective tissue or glioependymal tissue.
The expansion of arachnoid cysts may occur when pulses of cerebrospinal fluid become trapped in the cyst cavity. The increasing volume of fluid causes the cyst to grow in size. However, the exact nature of cyst growth is not yet well understood. Arachnoid cysts tend to form on the left side of the brain, where the spinal canal intersects. Typically, a cyst makes up about one percent of the mass of the brain. Arachnoid cysts are also known as intracranial cysts.
Infants are most susceptible to developing arachnoid cysts, although cyst formation can occur up through adolescence. Arachnoid cyst development in adults occurs much less frequently. Arachnoid cysts occur predominantly in males. The ratio of affected males to females is 4:1. The true rate of occurrence of arachnoid cysts is unknown, as many people with the disorder do not develop symptoms and the cyst remains undiagnosed.
Causes and symptoms
Arachnoid cysts arise mainly because of an abnormality occurring in development, sometimes as a result of a neonatal (newborn) infection. Other cysts are congenital (present at birth) and presumably result from abnormal formation of the subarachnoid space during embryological development. Cysts can also result from tumors, and complications of surgery or trauma (bleeding).
The symptoms of an arachnoid cyst are related to the size of the cyst and its location. For example, a small cyst may not cause any symptoms at all, and can be discovered accidentally during an unrelated examination. Large cysts can cause the head to change shape or to become enlarged (a phenomenon called macrocephaly). Symptoms associated with a larger cyst include headaches, seizures , accumulation of a pronounced amount of cerebrospinal fluid (hydrocephalus ), increased pressure inside the cranial cavity, delay in mental and physical development, and altered behavior.
Other symptoms can include weakness or complete paralysis along one side of the body (hemiparesis), and the loss of control of muscles (ataxia ).
Arachnoid cysts are most commonly diagnosed followed a complaint of headaches, disruption of vision, or delayed development in a child. Even then, the discovery of a cyst is often incidental to another examination. The cysts can also be visualized using computerized tomography (CT) scanning, magnetic resonance imaging (MRI) , and cranial ultrasonography . Overall, MRI is the preferred diagnostic technique, although cranial ultrasonography is an especially useful technique for newborns.
Arachnoid cysts have also been documented in people who have maladies such as Cockayne syndrome and Menkes disease. However, it is unclear whether this association is typical (and so of diagnostic importance) or merely coincidental.
Treatment can involve medical specialists such as neurosurgeons, imaging technicians, as well as nursing and other care providers. Physical therapists are also often involved.
Typically, treatment is for the symptoms caused by the presence of the cyst, rather than for the cyst itself. However, when symptoms warrant, surgery is performed to relieve symptoms of increased intracranial pressure caused by the accumulation of fluid within the arachnoid cyst. Often, a device (shunt) is implanted within the cyst that drains the fluid away from the cyst and into the ventricles of the brain, or into the peritoneum (abdominal space), thus relieving the pressure. An alternative surgery called endoscopic fenestration uses an endoscope (an operative tool with an attached camera) to cut a small hole in the cyst, allowing the fluid to escape into the normal cerebrospinal fluid pathway.
Recovery and rehabilitation
Recovery from either surgical treatment is usually rapid, with symptoms resolving quickly after the excess fluid is redirected, assuming no permanent neurological damage occurred prior to treatment. An active infant or young child often wears a protective helmet during the recovery phase. Physical and mental developmental milestones are usually monitored for infants and children. Follow-up monitoring of the implanted shunt and overall assessment of the cyst are normally required.
As of January 2004, the National Institute of Neurological Diseases and Stroke (NINDS) was recruiting patients for a study of syringomyelia . The malady arises when cerebrospinal fluid is blocked from its normal circulation, as by an arachnoid cyst. As well, NINDS and other agencies support research that seeks to understand the basis of arachnoid cyst formation.
While many arachnoid cysts cause no symptoms and require no treatment, others, if left untreated, can grow and cause pressure or severe bleeding within the brain (hemorrhage). The result can be permanent neurological damage. However, with treatment, the outlook for most persons with an arachnoid cyst is encouraging and permanent damage can be avoided.
"Arachnoid Cysts Information Page." National Institute of Neurological Disorders and Stroke. <http://www.ninds.nih.gov/health_and_medical/disorders/aracysts_doc.htm> (January 30, 2004).
Khan, A. N. "Arachnoid Cyst." eMedicine. <http://www.emedicine.com/radio/topic48.htm> (January 30, 2004).
National Institute for Neurological Diseases and Stroke (NINDS). 6001 Executive Boulevard, Bethesda, MD 20892. (301) 496-5751 or (800) 352-9424. <http://www.ninds.nih.gov>.
Brian Douglas Hoyle, Ph.D.