Chiari I Malformation (CMI) is one of several identified malformations of the hindbrain, which consists of the cerebellum, pons, and medulla oblongata. Additionally, the back part of the skull (posterior fossa) is too small, and the cerebellar tonsils (part of the cerebellum) are pushed downward through the hole at the base of the skull (the foramen magnum). Chiari I malformation is TYPICALLY defined as a 3 to 5mm or even higher herniation of the cerebellar tonsils below the foramen magnum. However, there have been a significant number of diagnosed cases with no herniation. The result can include compression of the brain stem, the part of the brain which controls involuntary body responses such as the heart beating and breathing; compression of the cranial nerves; and disruption of the flow of cerebrospinal fluid.  The compression on the brainstem can block the flow of cerebrospinal fluid (the liquid that surrounds and protects the brain and spinal cord) and can cause a range of symptoms including dizziness, muscle weakness, numbness, vision problems, headache, and problems with balance and coordination.