Also known as Arnold Chiari Malformation. Most children with these malformations who do not have Spina-Bifida will have the form known as type I (ACM 1). Essentially the cerebal tonsils crowd the outlet of the brainstem/spinal cord from the skull on its way to the spinal canal. 

Known symptoms that result from this crowding can include headaches, neck pain or pressure, funny feelings in the arms and/or legs, stiffness, drop attacks, difficulties with swallowing, gagging, reflux, vomiting, seizures.  Chiari may be responsible for frequent  ear and sinus infections.  There is evidence to suggest a Chiari is the culprit for developmental disabilities  including Autism in some children. When the diagnosis is suspected a MRI scan can confirm the diagnosis. These malformations will not be seen on on plain x-rays and usually can't be seen on CT's. 

In general the symptoms of the type I malformations are less severe than that of the type II malformation. Left untreated, the crowding of the brainstem and spinal cord can lead to very serious consequences including paralysis and death.  It can also lead to the development of syrixes which may further injure the child's spinal cord and function.

There are several surgical options in treating Chiari malformations. Basically the operation is one of un-crowding the area at the base of the cerebellum where it is pushing against the brainstem and spinal cord.  Most children who have the surgery do quite well and have an improvement in their symptoms.