Syringomyelia is the term used to refer to a cavity of fluid in the spinal cord. Other terms commonly used to describe this condition are hydromyelia or syringohydromyelia. Syringobulbia is the occurrence of a syrinx that extends into the medulla oblongata. The actual cavity is called a syrinx, or syringes if there is more than one. Syringomyelia can occur as the result of trauma, tumors, arachnoiditis, or it can be associated with Chiari malformations, specifically Chiari I and Chiari II. But in all cases, a partial obstruction or the subarachnoid space occurs in conjunction with syringomyelia. Chiari malformations (or deformations) are defined as a hindbrain herniation. These can be congenital or acquired, as a result of arachnoiditis, or excessive lumbar shunting or drainage. 1 The severity of herniation or malformation is broken down into 4 distinct classifications. Chiari I is most commonly known for herniation of the cerebellar tonsils below the level of the foramen magnum. Chiari II also has tonsillar herniation, but the abnormalities are more severe. Both Chiari I and Chiari II are associated with syringomyelia. Chiari III and IV are associated with severe medulla and cerebellar malformations. Both of these are very rare and will not be discussed in this paper.

Chiari I malformation is characterized by the downward displacement of the cerebellar tonsils through the foramen magnum into the cervical canal. The MRI is the modality of choice for imaging hindbrain and spinal cord anomalies.2 ,3 ,4 Using an MRI, tonsillar displacement can be measured. Aboullez, et al. observed that the cerebellar tonsil did not extend more that 3mm below the foramen magnum in normal patients. But tonsillar herniation was not less than 5 mm below the foramen magnum in patients with Chiari I.4 This was substantiated by Milhort, et al.5 Chiari I malformations can occur as a result of a congenital defect or excessive lumbar shunting.

Replacing the LP shunt with a VP shunt can usually reverse tonsillar herniation resulting from lumbar shunting.1,3,4 The origin of chiari malformations is widely debated and many theories exist. The most known theory on the cause of congenital chiari malformations is Gardner’s hydrodynamic theory. He suggests that chiari malformations occur as