Banana sign (cerebellum)
The banana sign is one of the many notable fruit inspired signs.
In Chiari II malformation, the banana sign describes the way the cerebellum is wrapped tightly around the brain stem as a result of spinal cord tethering and downward migration of the posterior fossa contents. This results in obliteration of the cisterna magna and subsequently, the shape of the cerebellum has the appearance of a banana. The sign persists into the second and third trimesters.
The banana sign is also seen in the majority of fetuses with spina bifida. The ultrasound appearance of the banana sign is secondary to the abnormality of the cerebellar shape. In patients with spina bifida, this appearance disappears after 24 weeks. There is frequently concurrent hydrocephalus.
The same condition also gives rise to the lemon sign.