A Type I Chiari malformation is a congenital anomaly of the brain in which the cerebellar tonsils are elongated and pushed down through the opening of the base of the skull (see foramen magnum), blocking the flow of cerebrospinal fluid (CSF) as it exits through the medial and lateral apertures of the fourth ventricle. Also called cerebellar tonsillar ectopia, or tonsillar herniation.
A Type I Chiari malformation is a congenital brain disorder where the cerebellum's tonsils protrude into the spinal canal, disrupting the normal flow of cerebrospinal fluid and affecting the brain's ability to function properly.