UC San Diego

Chiari malformation type I (CM-I) is characterized by herniation of the cerebellar tonsils past the foramen magnum into the upper spinal canal. This descent can obstruct cerebrospinal fluid (CSF) flow. A large fraction of CM-I patients develop syringomyelia, in which a fluid-filled cavity, or syrinx, forms within the spinal cord. The mechanisms governing syrinx formation remain incompletely understood, although abnormal CSF dynamics associated with CM-I may play an important role. One leading hypothesis proposes that the pulsating cerebellar tonsils create amplified pressure waves in the spinal subarachnoid space that promote fluid transport into the spinal cord. However, some aspects of altered CSF dynamics in CM-I and the mechanisms underlying syrinx formation remain unexplained. This work characterizes cerebellar tonsillar motion, cervical CSF flow, and craniocervical pressure dynamics before and after posterior fossa decompression in pediatric CM-I patients. Cardiac-gated phase-contrast magnetic resonance imaging (PC-MRI) was used to quantify tonsillar motion and cervical CSF flow. These measurements, together with T2-weighted MRI, were used to construct patient-specific computational fluid dynamics (CFD) models to evaluate pressure fields and longitudinal impedance, a pressure-based measure of resistance to oscillatory flow. Our results reveal that surgical decompression alters the relationship between tonsillar motion and cervical CSF flow. Most notably, postoperative cervical CSF flow exhibits more balanced bidirectional oscillations and becomes less synchronized with tonsillar motion than in the preoperative state, suggesting that postoperative CSF flow is less influenced by piston-like tonsillar displacement. In addition, the maximum longitudinal pressure difference exhibits variable changes across subjects, while longitudinal impedance consistently decreases following decompression, indicating reduced resistance to oscillatory CSF flow. These findings provide insight into the physiological effects of posterior fossa decompression and provide an MRI-informed CFD framework for future investigations of CM-I pressure dynamics and the mechanism underlying syrinx formation.