Foramen Magnum Decompression for Symptomatic Chiari Type 1 Malformation: Analysis of Clinical and Radiological Factors

Background Chiari 1 malformation is thought to develop as a result of the failure of the proper development of the occipital part of the clivus. This tends to form a smaller posterior fossa and overcrowding of the normally developed hindbrain leading to tonsillar herniation. We tried to see if there is any correlation between the degree of tonsillar ectopia, syrinx diameter, and bony posterior fossa characteristics with the clinical course and outcome following posterior fossa decompression.

Methods Fourteen patients with symptomatic Chiari type 1 malformation were included in this study. For headache or neck pain, a symptomatic scoring scale of 1 to 10 was used; for cerebellar symptoms, the SARA scale (scale for the assessment and rating of ataxia) was used; for lower cranial nerve involvement, we used an objective grading system; for numbness/paresthesias, a subjective scale of mild, moderate, or severe was used; and for weakness, the minimum power in a group was recorded. The clinical assessment of these parameters was repeated 6 months postoperatively. The following MR parameters were noted both in preoperative and/or postoperative imaging: degree of tonsillar herniation; syrinx—diameter; the syrinx to spinal cord ratio; posterior fossa bony characteristics: namely, foramen magnum diameter; length of the clivus; length of the supraocciput; angle α—between clivus and Mac Rae line; angle β—between Mac Rae line and the supraoccipital line; angle gamma—between supraoccipital line and the tentorium and slope of the tentorium

Results

1. Patients of Chiari 1 malformation with a higher degree of tonsillar ectopia presented earlier, but the degree of tonsillar ectopia did not correlate with symptoms.

2. The syrinx and syrinx cord ratio was more likely to be indicative of spinal manifestations of the disorder.

3. The tentorium was steeper in patients who had associated syringomyelia and a shorter length of clivus correlated with the severity of suboccipital headache/neck pain.

4. The more the tendency toward platybasia, the more was the likelihood of patients developing motor symptoms, and the narrower the Beta angle (angle between Mcrae line and supraoccipital line) the more likely the patient was to have cerebellar symptoms.

5. The retrocerebellar CSF thickness improved significantly postdecompression and was predictive of the resolution of headache/neck pain.

6. Patients with a smaller posterior fossa were more likely to benefit from decompressive surgery.

Conclusion Our study demonstrated that degree of tonsillar ectopia correlated with the age of presentation but not with symptomatology. The morphometric bony parameters did not show any correlation with symptomatology except a reduced length of clivus. Postdecompression, there was a definite clinical improvement in symptoms. The MRI images showed an increase in posterior fossa space as well as a decrease of syrinx diameter. The clinical improvement also tended to correlate with the degree of reestablishment of retrocerebellar CSF space. This study thus validates that posterior fossa decompression is effective in restoring CSF flow with concomitant relief of symptoms.