A mega cisterna magna, or Blake's pouch, refers to a condition in which the CSF retrocerebellar cisterns is enlarged with normal cerebellar morphology, according to Radiopaedia. It is not clear whether the condition is due to cerebellar volume loss or if it is a normal anatomical variant.
Mega cisterna magna occurs in up to 1 percent of imaged brain studies. It is associated with chromosomal abnormalities, including trisomy 18. It is also associated with inflammation, infection and infarction, states Radiopaedia. The prognosis for mega cisterna magna is good when the condition occurs with normal ventricles in an isolated finding. It is usually observed on a CT/MRI or ultrasound.