a result of improper development of the neural tube during the embryonic state. Under normal conditions there is a balance of pulsatile pressure in the lateral and fourth ventricles from the third ventricle. However, when an imbalance in pulsatile pressure occurs, so that the pressure in the lateral ventricles exceeds the pressure in the fourth ventricle, the development of the posterior fossa is retarded.3,4 As the posterior fossa is too small, the tonsils are forced down below the level of the foramen magnum. This is supported by Badie, et al., who observed that the posterior fossa was smaller in patients with Chiari I; and this small fossa may be the primary cause of tonsillar herniation.6 Some researchers have indicated a familial link to congenital Chiari I malformation, and have suggested that genetics may play a role in improper development of the neural tube.5,7 Syringomyelia can occur concurrently when these pulsatile forces are directed down the central canal of the spinal cord resulting in the formation of syringes. This is known as the “water hammer effect.”3,4 Syringomyelia is often associated with Chiari I, although the rates of occurrence differ from investigator to investigator. Some investigators have reported that syringomyelia occurs in approximately 30-50% of Chiari I patients,4 while others have reported incidences as high as 60-75%3,5 or even 85-100%.8 ,9 Figures 1 and 2 show a Chiari I malformation with associated syringomyelia.

Figure 1: Chiari I malformation with associated syringomyelia (lateral view). A) The arrow points to the cerebellar tonsils herniated through the narrow foramen magnum. B) The arrow points to the syringes in the spinal cord. (Adapted from Syringomyelia and the Chiari Malformations.)