Honored guest presentation: lifetime experiences and where we are going: Chiari I with syringohydromyelia--controversies and development of decision trees.

The hindbrain herniation syndrome (51), often referred to as the Chiari I malformation, is a disorder that has traditionally been defined as a downward herniation of the cerebellar tonsils through the foramen magnum (8, 9, 36). The extent of tonsillar herniation is recognized as being more than 4 to 5 mm below the plane of foramen magnum on sagittal magnetic resonance images (11, 13, 28, 30, 33, 38). This anomaly is associated with syringohydromyelia in 45 to 68% of cases (17, 30, 38). It occurs in conjunction with osseous abnormalities of the craniovertebral junction (12, 16, 22, 24, 25, 30, 42, 43, 49). This is distinguished from the more familiar Chiari II malformation, which is present at birth and consists of downward herniation of the cerebellum and medulla into the spinal canal in association with complex anomalies such as aqueductal forking, polymicrogyria, and myelodysplasia (7). It is now felt that the Chiari I malformation, or the hindbrain herniation, is inherently different from the Chiari II malformation. The Chiari I occurs sporadically, and there is no inherent nervous system abnormality except for that which is the result of crowding of the posterior fossa, which predisposes to the hindbrain herniation (21, 27, 30, 33). Changes in cerebrospinal fluid (CSF) dynamics contribute to the symptoms and the clinical syndrome characterized by occipital Valsalva-type headaches, lower cranial nerve abnormalities, and a spinal cord myelopathy that may be overshadowed by the syringohydromyelia.