Posterior Fossa Cystic Malformations Made Easy - How To Tell Them Apart 

Thank you. Cisterna magna is located between the cerebellum above and foramen magum below and back surface of medulla in front, and CSF from foramina Magendie and Luschka drains into it. So if I see enlargement of this CSF space just below the cerebellum and without mass effect, then mega cisterna magna. Arachnoid cyst is typically more posterior and can have some mass effect on cerebellum and vermis. If Arachnoid cyst is in the typical location of mega cisterna magna, then I think it's tough to tell. If I see sign of CSF obstruction, then it's more likely arachnoid cyst (or blake pouch cyst), since mega cisterna magna is not expected to cause obstruction of CSF.

@@neuroradish Makes a lot of sense. Thanks so much 🙏🙏

Sorry but I'd defer any meaningful opinion to your physician. It would be irresponsible to make an assessment without a full clinical picture and images. Having said that, isolated Blake pouch cyst with normal vermis and no hydrocephalus or other significant findings often has a good outcome. Please refer to your clinician and with a follow up ultrasound.