The four original varieties of Chiari malformation all represent some type of hindbrain abnormality. However, the underlying cause and the neurological and anatomic consequences are very different. In truth, the only real similarity between the Chiari malformations is that they involve the hindbrain and they were described by Hans Chiari.
The most common Chiari malformation is Chiari I, represented by cerebellar tonsillar position outside the foramen magnum. This is often asymptomatic and without significant clinical consequence. However, in some cases, Chiari I can cause symptoms or neurologic dysfunction. When the position of the cerebellar tonsils impairs the movement of cerebrospinal fluid (CSF) at the craniocervical junction, one consequence may be syringomyelia. This is a pathological accumulation of fluid within the parenchyma of the spinal cord that causes spinal cord expansion. Syringomyelia may cause symptoms of spinal cord dysfunction, including pain and numbness in the hands, loss of dexterity, hyperreflexia, etc.
In 1998, a group led by Dr. Jerry Oakes1 identified five patients with syringomyelia and no evidence of tonsillar herniation (i.e. no Chiari I malformation). MR imaging of the entire neuraxis ruled out other causes of a syrinx (e.g. tumor, post-traumatic). Ultimately, abnormal CSF flow at the posterior fossa or foramen magnum was identified as the suspected cause. The label “Chiari 0” was used to categorize these patients by virtue of the fact that they behaved as though they had fourth ventricular outlet obstruction.
In Oakes’s series, he performed posterior fossa decompression and frequently found physical barriers to CSF movement, despite position of the cerebellar tonsils within the foramen magnum (without displacement). All underwent a posterior fossa decompression and duraplasty with surgical technique identical to that used to treat patients with a Chiari I malformation. No attempt was made to directly treat the syrinx. Significant syrinx and symptom resolution was observed in all patients. Interestingly, two of these patients had undergone earlier shunting of the syrinx. While the syrinx shunting did not provide long-term improvement, posterior fossa decompression was effective. Their response to surgery suggests “Chiari-like” pathophysiology can be present in the absence of tonsillar herniation. This contention is supported by the identification of a crowded foramen magnum in two patients, multiple arachnoid adhesions in two others and a 4th ventricular arachnoid veil in one. Each of these findings can alter CSF flow at the craniocervical junction. This group went on to publish details of another 15 patients treated with Chiari 0 with similar results.4
Morphological studies of the posterior fossa in the Chiari 0 group have found the sagittal anteroposterior diameter of the foramen magnum to be greater than controls and inferior descent of the obex (i.e. caudal displacement of the brainstem) was found to be three standard deviations below normal.2 These findings suggest caudal displacement of the medulla and compensatory enlargement of the foramen magnum may result from a small posterior fossa.
Similarly, Kyoshima et al.3 have reported four patients with what they termed a “tight cistern magna” and syringomyelia. These patients did not have hindbrain hernias and each underwent posterior fossa decompression with improvement of the syrinx in three. We would consider these patients as examples of the Chiari 0 malformation.
It should be emphasized that Chiari 0 is a very rare entity. As originally described, Chiari 0 applies only to individuals with crowding at the craniocervical junction, likely resulting in CSF movement abnormalities, and with syringomyelia. The original authors considered the diagnosis of Chiari 0 only in patients with syringomyelia of no apparent cause. Careful patient selection is critical when making the diagnosis of Chiari 0 malformation. Without an obvious Chiari I malformation, other etiologies of a spinal syrinx must be conclusively ruled out. Only then can one reasonably expect to ameliorate the clinical course of these patients with a posterior fossa decompression.