October 31, 2017
Years published: 1994, 2002, 2004, 2012, 2015
NORD gratefully acknowledges Hrayr K. Shahinian, MD, FACS, Director, and Christina Kim, Clinical Manager, Skull Base Institute, for assistance in the preparation of this report.
Arachnoid cysts are fluid-filled sacs that occur on the arachnoid membrane that covers the brain (intracranial) and the spinal cord (spinal). There are three membranes covering these parts of the central nervous system: the dura mater, arachnoid, and pia mater. Arachnoid cysts appear on the arachnoid membrane, and they may also expand into the space between the pia mater and arachnoid membranes (subarachnoid space). The most common locations for intracranial arachnoid cysts 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, arachnoid cysts do not cause symptoms (asymptomatic). In cases in which symptoms occur, headaches, seizures and abnormal accumulation of excessive cerebrospinal fluid in the brain (hydrocephalus) are common. The exact cause of arachnoid cysts is unknown. Arachnoid cysts are classified according to their specific location.
In most cases, arachnoid cysts are present at birth (congenital), but usually do not cause any symptoms (asymptomatic) throughout an individual’s life. Less often, arachnoid cysts may develop because of head injury, the presence of a tumor, infection or surgery on the brain
Whether symptoms develop depends upon the size and the specific location of the cyst within the brain. Small cysts usually do not cause symptoms. However, cysts can increase in size causing symptoms to appear, especially if they press against a cranial nerve, the brain, or the spinal cord.
Most cases of arachnoid cysts that are associated with symptoms occur in childhood. The specific symptoms present vary from case to case. It is important to note that affected individuals will not have all of the symptoms listed below.
The most common symptoms associated with arachnoid cysts are usually nonspecific and include headaches, nausea, vomiting, dizziness and the accumulation of excessive cerebrospinal fluid in the brain (hydrocephalus), resulting in increased intracranial pressure In rare cases, in some children, an arachnoid cyst can cause malformation of certain cranial bones, resulting in an abnormally enlarged head (macrocephaly). .
A variety of additional symptoms occur in some individuals with arachnoid cysts depending upon the size and location of the cyst. Most cysts occur near the middle fossa region of the brain. Such symptoms include lethargy, seizures, vision abnormalities and hearing abnormalities. Neurological signs may be present because arachnoid cysts may cause increased pressure on structures of the brain. Such neurological findings may include developmental delays, behavioral changes, an inability to control voluntary movements (ataxia), difficulties with balance and walking and cognitive impairment. Weakness or paralysis on one side of the body (hemiparesis) has also been reported.
In addition to hydrocephalus, cysts located in the suprasellar region may be associated with vision disturbances, continuous bobbing of the head, and abnormalities affecting certain hormone-producing glands that help to regulate the rate of growth, sexual development, and certain metabolic functions (endocrine system).
Although they occur much less often than those found within the skull (intracranial), arachnoid cysts may also arise near the spine (spinal arachnoid cysts). Spinal arachnoid cysts may be associated with progressive weakness of the legs, tingling or numbness in the hands or feet, abnormal side-to-side curvature of the spine (scoliosis), back pain, and involuntary muscle spasms (spasticity) that result in slow, stiff movements of the legs. In rare cases, these cysts may cause paralysis of the legs (paraplegia). Urinary tract infections may also occur in individuals with spinal arachnoid cysts.
Other symptoms and physical findings have been reported to be associated with arachnoid cysts including migraine headaches, attention-deficit disorder and difficulties understanding or expressing language (aphasia). The exact cause and effect relationship between these findings and arachnoid cysts is not clear.
The exact cause of arachnoid cysts is not known. Researchers believe that most cases of arachnoid cysts are developmental malformations that arise from the unexplained splitting or tearing of the arachnoid membrane. According to the medical literature, cases of arachnoid cysts have run in families (familial cases) suggesting that a genetic predisposition may play a role in the development of arachnoid cysts in some individuals.
In some cases, arachnoid cysts occurring in the middle fossa are accompanied by underdevelopment (hypoplasia) or compression of the temporal lobe. The exact role that temporal lobe abnormalities play in the development of middle fossa arachnoid cysts is unknown.
Some complications of arachnoid cysts can occur when a cyst is damaged because of minor head trauma. Trauma can cause the fluid within a cyst to leak into other areas (e.g., subarachnoid space). Blood vessels on the surface of a cyst may tear and bleed into the cyst (intracystic hemorrhage), increasing its size. If a blood vessel bleeds on the outside of a cyst, a collection of blood (hematoma) may result. In the cases of intracystic hemorrhage and hematoma, the individual may have symptoms of increased pressure within the cranium and signs of compression of nearby nerve (neural) tissue.
Arachnoid cysts can also occur secondary to other disorders such as Marfan’s syndrome, arachnoiditis, or agenesis of the corpus callosum. (For more information on these disorders, choose the specific disorder name as your search term in the Rare Disease Database).
Arachnoid cysts affect males more often than females. These cysts may occur at any age and have been found in all races and geographic locations. They are the most common type of intracranial cyst. Because many cases of arachnoid cysts have no symptoms, it is difficult to determine the true frequency of this disorder in the general population.
A diagnosis of arachnoid cysts is often made incidentally, often during examination of an individual with seizures. A diagnosis may be suspected based upon a detailed patient history, a thorough clinical examination, and a variety of specialized tests, especially advanced imaging studies such as computed tomography (CT scan) and magnetic resonance imaging (MRI). CT scans and MRIs can reveal or confirm the presence of arachnoid cysts. During CT scanning, a computer and x-rays are used to create a film showing cross-sectional images of the brain’s tissue structure. During MRI, a magnetic field and radio waves are used to create cross-sectional images of the brain.
Most arachnoid cysts are found incidentally and remain constant in size, leading many physicians to recommend conservative treatment. When no symptoms are present, no treatment may be necessary and affected individuals may be periodically monitored. If symptoms arise, a cyst can be reevaluated.
When treatment is necessary the specific therapy used depends upon whether symptoms are present, the size of the cyst and the specific location of the cyst within the skull.
In cases where treatment is recommended, therapy traditionally consists of one of two procedures – an open craniotomy fenestration or ventriculoperitoneal shunting.
During a craniotomy fenestration, a portion of the skull is removed to give a surgeon access to the cyst, where multiple openings are made in the cyst wall, (fenestrations), to allow cerebrospinal fluid to drain into the subarachnoid space where the fluid is reabsorbed into the surrounding tissue. Alternatively, some cases may be treated by surgically inserting a device (shunt) into the cyst to provide drainage either into the ventricular system of the brain or into the abdominal cavity. This will drain the cyst and provide an adequate passageway for cerebrospinal fluid to circulate.
More recently, advancements in minimally invasive brain and skull base surgery have evolved these traditional procedures into fully endoscopic techniques, distinguished by shorter operating times, fewer complications, excellent outcomes with faster recovery and overall decreased patient morbidity. While the approach varies depending on the size and location of the arachnoid cyst, fully endoscopic surgical management has provided the surgeon with superior access for either fenestration, or in other cases, resection of the cyst without the complications and risks associated with brain manipulation or retraction. Few facilities provide minimally invasive, endoscope assisted fenestrations, endoscopic shunt placement and endscope assisted or fully endoscopic resection of arachnoid cysts as treatment when indicated.
Spinal arachnoid cysts may be treated by the complete surgical removal (resection) of the cyst, if possible. Surgery generally leads to a resolution of symptoms. In some cases, complete surgical removal of a spinal cyst is not possible. In such cases, fenestration or shunting of the cyst to drain the fluid may be necessary.
Other treatment is symptomatic and supportive.
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