A Chiari malformation is a condition in which brain tissue, specifically the cerebellar tonsils and sometimes the brainstem, extends below the foramen magnum, the opening at the base of the skull, into the upper spinal canal. There are several types, with Chiari II being the most commonly detected on prenatal ultrasound because it is almost always associated with a neural tube defect called myelomeningocele. On fetal ultrasound, signs of Chiari II include the lemon sign, where the frontal bones appear indented, and the banana sign, where the cerebellum appears curved and small due to its herniation. These signs on a routine anatomy scan prompt further evaluation for spina bifida.
Sonography Term