Chiari malformation type 1 occurs when the section of the skull containing a part of the brain (cerebellum) is too small or is deformed, thus putting pressure on and crowding the brain. The lower part of the cerebellum (tonsils) is displaced into the upper spinal canal.

What causes cerebellar tonsils?

Chiari I begins with the underdevelopment of the fetal skull during pregnancy. During childhood, the brain continues to grow and the skull hardens. However, the small size or shape of the Chiari skull is mismatched to the size of the brain. Thus, a crowding of the brainstem, cerebellum, and tonsils occurs.

When your brain is too big for your skull?

Chiari malformation is a condition that causes brain tissue to settle into the spinal canal. It develops where your skull and neck (cervical spine) come together; when part of the skull is either too small or misshapen, part of the brain can settle into the foramen magnum.