These cystic lesions are congenital.
Arachnoid cysts comprise 1% of all intracranial space-occupying lesions 1).
Arachnoid cysts can be classified as primary developmental cysts or secondary cysts. Primary cysts arise from the splitting of the arachnoid membranes in utero, resulting in the development of anomalous collections of cerebrospinal fluid (CSF). Secondary cysts are less common, often appearing after trauma, surgery, infection, or intracranial hemorrhage.
Intracranial cysts are classified as supratentorial, infratentorial, and supra-infratentorial (tentorial notch) 5).
50-66% occurring within the middle cranial fossa in most series. Ten percent occur in the suprasellar and quadrigeminal regions and approximately 5% each in the posterior fossa and over the frontal convexities
Supratentorial are divided into:
Quadrigeminal plate cistern
Many theories have been postulated for etiopathogenesis of arachnoid cysts. Various hypotheses are: entrapment of CSF in a diverticulum; CSF flow changes leading to arachnoid cell layer tears during the formation of various cisternae; during embryological separation (at around 15th week of gestation) of arachnoid from the dura mater 6).
Spontaneous intracystic hemorrhage in an arachnoid cyst is a rare complication (although hemorrhagic arachnoid cyst is well known in the clinical setting of trauma), which may present with symptoms of raised intracranial pressure (headache, vomiting, altered sensorium) or focal neurological deficits depending on the location and is an indication for surgery 7) 8).
Although complications such as intracystic, subdural, and extradural hematomas are well known after a trauma, spontaneous hemorrhage in an arachnoid cyst is a rare and serious complication with atypical imaging features on cross-sectional imaging and only less than ten cases are documented in the literature 9).