Hydrodynamic and Longitudinal Impedance Analysis of Cerebrospinal Fluid Dynamics at the Craniovertebral Junction in Type I Chiari Malformation

Abstract

Elevated or reduced velocity of cerebrospinal fluid (CSF) at the craniovertebral junction (CVJ) has been associated with type I Chiari malformation (CMI). Thus, quantification of hydrodynamic parameters that describe the CSF dynamics could help assess disease severity and surgical outcome. In this study, we describe the methodology to quantify CSF hydrodynamic parameters near the CVJ and upper cervical spine utilizing subject-specific computational fluid dynamics (CFD) simulations based on in vivo MRI measurements of flow and geometry. Hydrodynamic parameters were computed for a healthy subject and two CMI patients both pre- and post-decompression surgery to determine the differences between cases. For the first time, we present the methods to quantify longitudinal impedance (LI) to CSF motion, a subject-specific hydrodynamic parameter that may have value to help quantify the CSF flow blockage severity in CMI. In addition, the following hydrodynamic parameters were quantified for each case: maximum velocity in systole and diastole, Reynolds and Womersley number, and peak pressure drop during the CSF cardiac flow cycle. The following geometric parameters were quantified: cross-sectional area and hydraulic diameter of the spinal subarachnoid space (SAS). The mean values of the geometric parameters increased post-surgically for the CMI models, but remained smaller than the healthy volunteer. All hydrodynamic parameters, except pressure drop, decreased post-surgically for the CMI patients, but remained greater than in the healthy case. Peak pressure drop alterations were mixed. To our knowledge this study represents the first subject-specific CFD simulation of CMI decompression surgery and quantification of LI in the CSF space. Further study in a larger patient and control group is needed to determine if the presented geometric and/or hydrodynamic parameters are helpful for surgical planning.

Cerebrospinal Fluid Flow Impedance is Elevated in Type I Chiari Malformation

“…Impedance modulus, Z L , was calculated by computing the ratio of Fourier coefficients of the pressure drop F DP t ð Þ ð Þ and flow waveforms F ðQðtÞÞ at each harmonic, where Z L ¼ FðDPðtÞÞ=F j ðQðtÞÞj. The resulting Z L (dyn-s/cm 5 ) values for each harmonic were integrated from 1-8 Hz to obtain integrated longitudinal impedance (LI) for each subject [29]. The range of 1-8 Hz was chosen because 90-100% of the signal power for all the flow waveforms used as inlet boundary conditions was contained between those frequencies.…”

“…Thus, pressure gradients and, consequently, LI may have been underestimated by this particular CFD methodology. Further, bulk neural tissue displacement in the spinal cord and cerebellar tonsils throughout the cardiac cycle in the rostrocaudal direction ranging from 0 to 0.6 mm has been documented for CMI patients and healthy subjects [10,[24][25][26]29]. This data suggest that elastic elements could be added to the boundary conditions that in turn may impact the pressure and velocity fields.…”

“…Of interest in this study is longitudinal impedance (LI), or the impedance to the pulsatile component of CSF motion, in the cervical SSS near the CVJ [29]. While CTH is typically indicative of a blockage, it only accounts for one linear dimension, depth, and does not consider the three-dimensionality of the stenosis (e.g., the girth of the blockage) at the CVJ (Fig.…”

“…By quantifying the severity of the stenosis in terms of LI, rather than by a single geometric dimension such as CTH, LI may provide a better parameter to assess the pathoanatomy of CMI. Martin et al [29] demonstrated the utility of LI to quantify CMI blockage in a pilot study with one healthy subject and two CMI patients before and after surgery. LI was found to be elevated in patients compared to a healthy subject and to decrease after surgery, but the limited number of subjects analyzed did not permit statistical analysis.…”

Continuous positive airway pressure alters cranial blood flow and cerebrospinal fluid dynamics at the craniovertebral junction

“…A full understanding of this coupling is thought to be important to understand the pathophysiology of cerebrovascular disorders such as stroke, interstitial fluid transport within the brain [4,5], and craniospinal disorders such as type I Chiari malformation, syringomyelia and hydrocephalus [6][7][8][9]. A number of studies have sought to understand the CSF dynamics in the spinal subarachnoid space (SSS) and its importance for the overall intracranial balance between the arterial, venous, and CSF flow pulsations [10][11][12].…”