The goal of surgical decompression in persons with intractable symptoms from the Chiari I malformation is to alleviate crowding caused by cerebellar tonsillar herniation in the region of the foramen magnum. Expanding the posterior cranio-cervical volume can improve CSF flow and alleviate symptoms.
However, the correlation between the amount of CSF space surgically created and clinical improvement has not been well studied. In part, this due to the tedious nature of the current tools used to measure the pre- and post-operative posterior fossa and CSF space volumes.
In an article published online on in the Journal of Neurosurgery: Pediatrics on February 17, 2017, researchers from the Division of Neurosurgery, Children's National Health System in Washington DC, describe the development of a “semiautomated program for calculating the 3D posterior fossa CSF volume.” Using this technique, the authors determined the correlation of the posterior fossa and CSF space volumes (cisterna magna, prepontine cistern, and fourth ventricle) with the clinical outcome in 42 pediatric patients undergoing decompression surgery for CMI.
Posterior fossa and CSF volumes were measured on the axial T2-weighted MRI images on scans taken before and after surgical decompression. The change in these volumes were then correlated with the postoperative outcome in “headache, syrinx, tonsillar descent, cervicomedullary kinking, and overall surgical success.”
The study revealed that greater enlargement of the posterior fossa volume, resulted in greater clinical improvement. In addition, enlargement of the cisterna magna also resulted in improved outcome. Furthermore, enlargement of the lower portion of the posterior fossa correlated with reduction in syrinx size. In authors words:
“A statistically significant association was found between a larger increase in the total posterior fossa volume and the cisterna magna CSF volume after CM-I decompression and improvement in headache, tonsillar descent, and surgical outcome. When the caudal portion of the posterior fossa volume was isolated, larger volume increases were associated with statistically significant increases in syrinx and cervicomedullary kinking in addition to headache, tonsillar descent, and surgical outcome.”
The authors also identified an area of future study: How large is large enough?
“Decompression volumes will need to be correlated with clinical outcomes in a prospective study before conclusions can be made on the optimal decompression size or technique.”
John Oró MD