see also:

• CT brain

• arachnoid cysts are incidental findings on CT brain or MRI scans of the brain
• prevalence is ~1.4%
• rarely they may be symptomatic - usually in childhood
• they are thought to arise from a congenital splitting of the arachnoid layer and thus form very thinly lined cystic spaces which are filled with CSF
• most arise in the Middle Cranial Fossa where they may encroach on the Sylvian fissure, but they can arise anywhere in the CSF space

• old infarcts
• subdural hygroma
• chronic subdural haemorrhage
• mega cisterna magna
• epidermoid cyst
• other cysts
• cysticercosis
• low grade glioma

• these are rare!
• mass effect causing hydrocephalus or raised intracranial pressure (ICP)
• rupture
• bleeding into the cyst
• increased risk of traumatic subdural haematomas

• usually nil
• larger cysts may warrant serial imaging or neurosurgical referral for opinion