ClĂnica de DiagnĂłstico por Imagem (CDPI) & Instituto Fernandes Figueira (IFF) â FIOCRUZRio de Janeiro â Brazil
Chiari II malformation is a congenital defect in the posterior fossa that is present in most fetuses with spina bifida. It results from herniation of the spinal cord. There is a reduction of intraspinal pressure allowing the hind brain to be displaced downwards. The posterior fossa compartment is too small because reduced intraspinal pressure in early pregnancy. The cerebellum grows into this small compartment and becomes compressed. This leads the observed abnormalities such as obliterated cisterna magna, banana sign (the cerebellum is compressed from a bi-lobed structure into the abnormal form) and lemon sign.These are ultrasound images of a case from 17 to 37 week gestation.
Left image:Â The spine looks normal at 17 weeks (sagittal view)