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 cyst's 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 Alzheimer's 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 Ménière's 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