Mega cisterna magna refers to a normal variant characterised by a truly focal enlargement of the subarachnoid space in the inferior and posterior portions of the posterior fossa. It is an incidental finding on neuroimaging, and no imaging follow up is necessary.
A mega cisterna magna is thought to occur in ~1% of all brains imaged postnatally.
Especially if noted antenatally, a mega cisterna magna has been associated with: Infarction
Inflammation/infection: particularly cytomegalovirus
Chromosomal abnormalities: especially trisomy 18
However, when a mega cisterna magna occurs as an isolated finding with normal ventricles the prognosis is good.
There are no specific symptoms related to this condition. Adults with isolated mega cisterna have an overall normal cognitive functioning but may score inferior to controls on some parameters of memory and verbal fluency. Although application of adult cases to the fetuses is not well established, this information might be of value in parental counseling in cases of a fetus with this anomaly 1).
Some authors have proposed that mega cisterna magna is a result of a delayed Blake's pouch fenestration; when this fenestration does not occur it results in a Blake's pouch cyst.
In antenates, mega cisterna magna refers to an enlarged retrocerebellar CSF space:
usually >10 mm in antenatal imaging (some consider up to 12 mm within normal limits)
septa may be seen within a mega cisterna magna, which are thought to be Blake pouch vestigial remnants
vermis should be closely evaluated to exclude Dandy-Walker continuum abnormalities
Typically seen as prominent retrocerebellar cerebrospinal fluid (CSF) appearing space with a normal vermis, normal 4th ventricle, and normal cerebellar hemispheres. The cisterna magna usually measures >10 mm on midsagittal images. An enlarged posterior fossa can sometimes be present.
The term was first coined by Gonsette et al in 1962, in patients with cerebellar atrophy.
Mega cisterna magna needs to be distinguished from other causes of an enlarged retrocerebellar CSF space:
Epidermoid cyst: often shows a heterogeneous/dirty signal on FLAIR and restricted diffusion
Cerebellar atrophy / cerebellar hypoplasia
Dandy Walker malformation: vermis not intact
Blake's pouch cyst: usually a hydrocephalus is present
pilocytic astrocytoma: if very posterior and predominantly cystic.
A 21-year-old female had a 3-year history of refractory psychotic symptoms despite adequate antipsychotic treatment. CT and MRI scans disclosed 'mega-cisterna magna'. Thorough neuropsychological testing recorded extensive deficits. Treatment with amisulpride 1200 mg/day resulted in a 30% decrease in PANSS score within 2 months. Then galantamine 8 mg/day was added and PANSS score decreased further by 27% within 2 weeks. Cognitive and social functioning was overall much improved. The effect was sustained in a 24 months follow-up. It is postulated that even a less extended cerebellar lesion, such as mega-cisterna magna, can be associated with psychosis, and in some cases with treatment refractoriness or cognitive dysfunction. Adjuvant galantamine may improve cognitive and psychosocial functioning in these patients 2).