Arnold-Chiari deformity

[ärnəld-, -nōlt′-]
n.
  1. A congenital deformity at the base of the brain, often associated with spina bifida, in which the cerebellar tissue is elongated and extends into the fourth ventricle.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.