You are a helpful, respectful, and honest assistant. Always answer as helpfully as possible, while being safe. Your answers should not include any harmful, unethical, racist, sexist, toxic, dangerous, or illegal content. Please ensure that your responses are socially unbiased and positive in nature. If a question does not make any sense or is not factually coherent, explain why instead of answering something not correct. If you don't know the answer to a question, please don't share false information.
What is Arachnoid cyst and explain in detail?
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 meningeal layers that cover the brain and the spinal cord. Primary arachnoid cysts are a congenital disorder whereas secondary arachnoid cysts are the result of head injury or trauma. Most cases of primary cysts begin during infancy; however, onset may be delayed until adolescence.

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 cysts existence, size, location, or potential functional impact on the patient.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.  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.
Seizures
Hydrocephalus (excessive accumulation of cerebrospinal fluid)
Increased intracranial pressure
Developmental delay
Behavioral changes
Nausea
Dysdiadokinesis
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 Alzheimers disease
In elderly patients (>80 years old) symptoms were similar to chronic subdural hematoma or normal pressure hydrocephalus:Dementia
Urinary incontinence
Hemiparesis
Headache
Seizures

Location-specific symptoms
A supratentorial arachnoid cyst can mimic a Ménières disease attack.
Frontal arachnoid cysts have been associated with depression.
Cysts on the left temporal lobe have been associated with psychosis.  A left fronto-temporal cyst showed symptoms of alexithymia.
Cyst on the right sylvian fissure resulted in new onset of schizophrenia-like symptoms at age 61.
A patient with a cyst on the left middle cranial fossa had auditory hallucinations, migraine-like headaches, and periodic paranoia
Patients with left temporal lobe cysts had mood disturbances similar to manic depression (bipolar disorder) and were known to show outward aggression

Causes
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.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.There are some cases where hereditary disorders have been connected with arachnoid cysts.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.Some scientists debate whether arachnoid cysts are a true congenite condition or if this should be separated from secondary cysts. A recent study shows differences in communication between the arachnoid cyst and the subarachnoid space by CT cisternography. A comparison of arachnoid cyst fluid and CSF in a series of patients show differences in chemical composition.Arachnoid cysts can also occur secondary to other disorders such as Marfan syndrome, arachnoiditis, or agenesis of the corpus callosum.

Diagnosis
Diagnosis is principally by MRI