How is an arachnoid cyst diagnosed?
Arachnoid cysts are almost always developmental abnormalities, but some are caused by infection or trauma. They are usually identified readily by CT or MRI scan. Detection is even possible prenatally by ultrasound. Increasingly, scans of the brain (done for other reasons, such as assessment of head injury) discover them as "incidental findings." Some never cause any problems, and some even go away on their own. Others expand, for uncertain reasons, and cause neurological symptoms. Routine evaluation with CT or MRI scan is usually satisfactory. CT scan shows a smooth bordered cystic mass composed of a density similar to cerebrospinal fluid. There is no enhancement with contrast administration. Expansion of the nearby bone by remodeling is usually seen, confirming their chronic nature.