Chiari II malformation is characterized by which anatomical change and its consequence?

• A. Elongation of brainstem and cerebellum through the foramen magnum into the cervical canal; contributes to hydrocephalus by blocking CSF flow in the fourth ventricle
• B. Displacement of brainstem into the spinal canal
• C. Herniation of cerebellar tonsils through foramen magnum; causes scoliosis
• D. Elongation of brainstem and cerebellum through the foramen magnum into cervical canal; increases CSF flow

Answer: (A)

Chiari II malformation involves downward displacement of the hindbrain, specifically the brainstem and cerebellum, through the foramen magnum into the cervical canal. This descent often accompanies a myelomeningocele and can obstruct CSF flow at the level of the fourth ventricle, leading to hydrocephalus. That combination—downward herniation with CSF flow obstruction causing hydrocephalus—best matches the described anatomy and consequence. In contrast, herniation of cerebellar tonsils through the foramen magnum is characteristic of Chiari I, not II, and stating that CSF flow increases would be incorrect. A vague displacement of the brainstem into the spinal canal does not capture the typical Chiari II pattern or its CSF-related consequence.