"The Chiari Malformations are a group of defects of posterior fossa anatomy" (a type of defect to the back of the brain stem where it grows in a tapered fashion,growing down the center of the spinal cord)", ranging from minor displacement of cerebellar tissue to gross bony defects or cerebellar hypoplasia."

a congenital defect where "tonsils" from the bottom part of the cerebellum sort of hang out of/in the spinal canal. since it affects the brain stem, it affects the functions that are controlled by the brain stem. chiari malformation is almost always associated with myelomeningocele, spina bifida and hydrocephalus.

some common symptoms are:

  1. clumsiness
  2. inability to adjust to adverse lighting levels, double vision, blurred vision
  3. sudden bouts of vertigo (usually when changing position abruptly)
  4. pressure and/or limited movement in the neck
  5. constant headaches
  6. sudden headaches (from coughing or sneezing)
  7. difficulty walking on uneven surfaces (or walking in general)
  8. weakness and/or loss of feeling in the extremities
  9. possible spinal cord injury (that spells paralysis, kids)
  10. various other problems depending on the individual

sources/further information (especially chip's page. he even has pics from his surgery):
chip's chiari page - http://www.pressenter.com/~chip/chiari.htm
the chiari information exchange - http://www.chiari.com
chiari malformation information - http://www.yvy69.ukgateway.net/
spina bifida association of america - http://www.sbaa.org
world acm association - http://www.pressenter.com/~wacma/
national institute of neurological disorders and stroke - http://www.ninds.nih.gov/health_and_medical/disorders/chiari_doc.htm

Section 3 of the Agent Orange Act of 1991, 
Pub. L. 102-4, 105 Stat. 11, directed the Secretary of Veterans Affairs 
to seek to enter into an agreement with the National Academy of 
Sciences (NAS) for a series of reports to review and summarize the 
scientific evidence concerning the association between exposure to 
herbicides used in support of military operations in the Republic of 
Vietnam during the Vietnam era, and each disease suspected to be 
associated with such exposure

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