What defines Arnold-Chiari malformation?

Arnold-Chiari malformation is primarily characterized by the herniation of brain tissue, specifically the cerebellum, through the foramen magnum, which is the opening at the base of the skull where the spinal cord passes. This condition typically involves the downward displacement of the cerebellar tonsils and sometimes parts of the brainstem into the spinal canal, leading to potential obstruction of cerebrospinal fluid flow and various neurological symptoms.

Understanding the pathophysiology behind this malformation is crucial. When the cerebellar tonsils herniate, they can compress the brainstem and spinal cord, resulting in a range of issues such as headaches, balance problems, and other neurologic dysfunctions. This herniation is a key feature that distinguishes Arnold-Chiari malformation from other conditions and helps inform diagnosis and treatment options.