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Classification & external resources
Arachnoid cysts are cerebrospinal fluid covered by arachnoidal cells and collagen that may develop between the surface of the brain and the cranial base or on the arachnoid membrane, one of the three membranes that cover the brain and the spinal cord. Arachnoid cysts are a congenital disorder, and most cases begin during infancy; however, onset may be delayed until adolescence.
Additional recommended knowledge
Arachnoid cysts can be found on the brain, or on the spine. Intracranial arachnoid cysts usually occur adjacent to the arachnoidal cistern. Spinal arachnoid cysts may be extradural, intradural, or perineural and tend to present with signs and symptoms indicative of a radiculopathy.
Arachnoid cysts can be relatively asymptomatic or present with insidious symptoms; for this reason, diagnosis is often delayed.
Signs and symptoms
Patients with arachnoid cysts may never show symptoms, even in some cases where the cyst is large. Therefore, while the presence of symptoms may provoke further clinical investigation, symptoms independent of further data cannot -- and should not -- be interpreted as evidence of a cyst's existence, size or location.
Symptoms vary by the size and location of the cyst(s), though small cysts usually have no symptoms and are discovered only incidentally. On the other hand, a number of symptoms may result from large cysts:
- Cranial deformation or macrocephaly (enlargement of the head), particularly in children
- Cysts in the suprasellar region in children have presented as bobbing and nodding of the head called Bobble-Head Doll Syndrome.
- Cysts in the left middle cranial fossa have been associated with ADHD in a study on affected children.
- Headaches. While the most common symptom, a patient experiencing a headache does not necessarily have an arachnoid cyst.
- In a 2002 study involving 78 patients with a migraine or tension-type headache, CT scans showed abnormalities in over a third of the patients, though arachnoid cysts only accounted for 2.6% of patients in this study.
- A study found 18% of patients with intracranial arachnoid cysts had non-specific headaches. The cyst was in the temporal location in 75% of these cases.
- Hydrocephalus (excessive accumulation of cerebrospinal fluid)
- Increased intracranial pressure
- Developmental delay
- Behavioral changes
- Hemiparesis (weakness or paralysis on one side of the body)
- Ataxia (lack of muscle control)
- Musical hallucination
- Pre-senile dementia, a condition often associated with Alzheimer's disease
- In elderly patients (>65 years old) symptoms were similar to chronic subdural hematoma or normal pressure hydrocephalus:
- The following list of location-specific symptoms should be interpreted in the context of what they represent: results from several independent, unrelated studies. As of September 2006
 , no published research comprehensively maps physical and neuropsychiatric symptoms to a specific arachnoid cyst location.
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.
In a few rare cases, intracranial arachnoid cysts may be inherited as an autosomal recessive trait. In recessive disorders, the condition does not appear unless a person inherits the same defective gene for the same trait from each parent. If an individual receives one normal gene and one gene for the disease, the person will be a carrier for the disease, but usually will not show symptoms. The risk of transmitting the disease to the children of a couple, both of whom are carriers for a recessive disorder, is 25 percent. Fifty percent of their children risk being carriers of the disease, but generally will not show symptoms of the disorder. Twenty-five percent of their children may receive both normal genes, one from each parent, and will be genetically normal (for that particular trait). The risk is the same for each pregnancy.
In a few rare cases, spinal intradural arachnoid cysts may be inherited as an autosomal dominant trait....
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.)
Diagnosis is principally by MRI. Frequently, arachnoid cysts are incidental findings on MRI scans performed for other clinical reasons. In practice, diagnosis of symptomatic arachnoid cysts requires symptoms to be present, and many with the disorder never develop symptoms.
Additional clinical assessment tools that may be useful in evaluating a patient with arachnoid cysts include the mini-mental state examination (MMSE), a brief questionnaire-based test used to assess cognition. Myelograms are contraindicated for people with arachnoid cysts.
Treatment for arachnoid cysts occurs when symptoms present themselves. A variety of procedures may be used to decompress (remove pressure from) the cyst.
- Surgical placement of a cerebral shunt:
- An internal shunt drains into the subdural compartment.
- A cystoperitoneal shunt drains to the peritoneal cavity.
- Craniotomy with excision
- Various endoscopic techniques are proving effective, including laser-assisted techniques.
- Drainage by needle aspiration or burr hole. While these procedures are relatively simple, there is a high incidence of recurrence.
- Capsular resection
- Pharmacological treatments may address specific symptoms such as seizures or pain.
A 1994 study found surgery necessary for good outcome in patients >65 years old when the cysts began displaying symptoms.
Untreated, arachnoid cysts may cause permanent severe neurological damage due to the progressive expansion of the cyst(s) or hemorrhage (bleeding). With treatment most individuals with arachnoid cysts do well with the double edge sword of developing surgical arachnoiditis which can cause its own specific problems and can lead to "popping" another cyst.
More specific prognoses are listed below:
- Patients with arachnoid cysts of the left temporal fossa who experienced impaired preoperative cognition had postoperative improvement.
- Surgery can resolve psychiatric manifestations in selected cases.
Arachnoid cysts are seen in 4% of the population. Only 20% of these have symptoms, usually from secondary hydrocephalus.
A study that looked at 2,536 healthy young males found a prevalence of 1.7% (95% CI 1.2 to 2.3%). Only a small percentage of the detected abnormalities require urgent medical attention.
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