Arnold-Chiari Malformation

An Arnold-Chiari malformation describes any malformation of the brain and skull, where the bottom of the cerebellum passes through the opening at the base of the skull. As a result, cerebrospinal fluid cannot drain properly through the opening and pressure builds up in the skull. Arnold-Chiari can cause a constellation of symptoms and, although it's fairly common, it's difficult to diagnose.
The malformation can either be congenital or can develop over time, due to a number of genetic diseases that affect connective tissue. It is often accompanied by hydrocephalus, syringomyelia, Ehlers-Danlos Syndrome, and Marfan syndrome.
The condition is usually diagnosed with a medical history, a neurological examination, and finally, an MRI. It is generally corrected with surgery to enlarge the opening at the base of the skull. This usually provides relief, but where the condition arises secondary to another condition, ongoing treatment is usually required. 
Arnold-Chiari malformation at Wikipedia
Chiari malformation at Mayo Clinic