The Chiari I Malformation is considered to be a congenital malformation, although there have been some reported cases of an acquired form. It is characterized by a small or misshapen posterior fossa (the compartment in the back of the skull), a reduction in cerebrospinal fluid pathways and a protrusion of the cerebellar tonsils through the bottom of the skull (foramen magnum) into the spinal canal. The tonsils would normally be round but often become elongated as they protrude down the spinal canal. Diagnosis can be difficult because not all patients will have the classical sign of deeply herniated tonsils. Since the advent of MRI, the incidence of the Chiari I Malformation has risen dramatically. MRI is safe and painless and currently the most reliable means available for diagnosing Chiari Malformations. Chiari Malformations are also known as herniation of the cerebellar tonsils, cerebellar ectopia, hindbrain herniation and Arnold-Chiari malformations.