Arnold-Chiari malformation is a medical condition that refers to the abnormal development of the brain and skull. It occurs when the lower part of the brain, called the cerebellum, extends into the spinal canal due to a structural defect in the skull. This condition can cause a range of symptoms, including headaches, neck pain, dizziness, and balance problems. It is also commonly known as Chiari malformation or hindbrain herniation and can be classified into four types. Treatment for Arnold-Chiari malformation usually involves surgery to alleviate symptoms and prevent further complications. Other synonyms for this condition include tonsillar herniation, cerebellomedullary malformation, and Arnold-Chiari deformity.