Arnold-Chiari malformation type II is a rare neurological disorder caused by a structural defect in the brain. It is characterized by a downward displacement of the cerebellar tonsils through the foramen magnum (the opening at the base of the skull). This displacement can cause a variety of symptoms, including difficulty swallowing, poor coordination, and breathing problems. In some cases, it can also lead to hydrocephalus (an accumulation of fluid in the brain). Treatment typically involves surgery to correct the displacement and relieve symptoms.