Chiari Malformation: A congenital defect which causes part of the brain (the cerebellar tonsils) to be pushed through the opening in the base of the skull (foramen magnum) into the spinal canal.  If the malformation puts significant pressure on the lower brainstem or cervical spinal cord, symptoms can occur including headache, neck pain, numbness and tingling of the extremities (especially of the shoulders and arms), weakness, difficulty urinating and balance or incoordination.  In more severe cases, the Chiari malformation is associated with a syrinx (a fluid-filled cavity within the spinal cord) which can also contribute to the neurological problems described above.  Treatment of a symptomatic Chiari I malformation typically involves removal of bone at the skull base (sub-occipital craniectomy and upper cervical laminectomy) and expansion of the dura in the posterior fossa.  This operation has a success rate of alleviating symptoms and halting neurological deterioration of approximately 80%.