Impedance to Cerebrospinal Fluid Flow in the Cervical Spinal Canal is Dominated by Geometric Complexity

In a fluid system, longitudinal impedance (LI) is the impedance per unit length of a conduit. Its magnitude depends on conduit geometry and the mechanical properties of both the fluid and conduit. In the context of vein grafts, LI has been shown to correlate with vein graft size and graft patency at one year from surgery [1]. More generally, LI has been shown to be consistent over a wide range of physiological flows [2]. Due to its ability to characterize the effect of geometry on flow in a conduit, LI may be useful in examining differences between a healthy spinal canal and one affected by Type I Chiari malformation (CMI). CMI is a complex disorder of the craniospinal system classically characterized by herniation of the cerebellar tonsils of 3–5 mm past the foramen magnum [3], which has proven difficult to properly diagnose as the associated neurological symptoms may overlap with the symptomatology of other disorders. Current methods of quantifying CMI severity, such as cerebellar tonsil herniation depth, do not necessarily correlate with symptom severity. Likewise, studies that have sought to hydrodynamically analyze CMI by measuring cerebrospinal fluid (CSF) velocity have yielded mixed results. We hypothesized that the severity of obstruction to CSF flow in the CMI-affected cervical spinal canal can be quantified and compared by calculation of LI.