Can Vascular Dynamics Cause Normal Pressure Hydrocephalus?

Abstract

This study investigated the hypothesis that changes in the brain vasculature system can explain the mechanics of normal pressure hydrocephalus (NPH) growth. A generic 3-D mesh of a healthy human brain was created. We used a fully non-linear (geometric, constitutive and boundary conditions) 3-D model of the brain parenchyma. The brain parenchyma was modelled as a nearly incompressible, single-phase continuum. A hyperelastic constitutive law and finite deformation theory described the deformations within the brain parenchyma. The brain vasculature system was modelled biomechanically by modifying the relaxed shear modulus according to the cardiac cycle to produce a time varying shear modulus. As no study currently exists on the effects of vasculature on brain parenchyma material properties, a parametric investigation was conducted by varying the brain parenchyma relaxed shear modulus. It is widely believed that no more than 1 mm of Hg (133.416 Pa) transmantle pressure difference is associated with NPH. Hence, we loaded the brain parenchyma with 1 mm of Hg transmantle pressure difference to investigate this suggestion. Fully non-linear, implicit, quasi-static finite element procedures in the time domain were used to obtain the deformation of the brain parenchyma and the ventricles. The results of the simulations showed that 1 mm of Hg (133.416 Pa) did not produce the clinical condition of NPH, even with brain vasculature effects taken into account. We conclude from our work that it is highly unlikely for a phenomenon, such as the brain vasculature effects due to the cardiac cycle, which occurs many times a minute to be able to influence an event such as NPH which presents itself over a time scale of hours to days. We further suggest that the hypothesis of a purely mechanical basis for NPH growth needs to be revised.