A unifying hypothesis for hydrocephalus and the Chiari malformations part two: The hydrocephalus filling mechanism.

It is proposed that negative central nervous system (CNS) pressure is one of the filling mechanisms of the fluid spaces of the CNS. Negative CNS pressure is caused by the combination of gravitational force and body movement. The venous system imposes pressure fluctuations on the CNS due to changes in posture and body cavity pressure. It is proposed here that filling of veins, arteries and cerebrospinal (CSF) spaces are all assisted by negative CNS pressure. Hyperemia in the CNS in response to pressure changes with movement was described in the first part of this hypothesis. By this means parenchyma water levels may increase (Williams, 2008). In the developmental forms of hydrocephalus expansion of CSF spaces is a more prominent feature than parenchyma water changes. This feature is explained by this second part of the hypothesis where the negative pulsatility of pressure that can accompany positive pressure pulsatility, which occurs with body movement, is described as the pathological force that leads to cavity filling. When CNS compliance is lost there is overrepresentation of low and well as high pressure pulsations in response to body movements. Pressure that leads to the development of hydrocephalus can be described as being abnormally labile. Negative CNS pressure causes cavity filling in an analogous way to pleural cavity filling, with water passing from parenchyma tissue. Positive pressure within the pressure profile may cause expansion of regions of the CNS skeletal system that are able to grow such as the cranial vault so that large head size is a frequent accompaniment to hydrocephalus that is caused by this mechanism. Hydrocephalic disorders that are characterised by negative pressure filling mechanism often have a skeletal anatomical abnormality that causes reduced CNS compliance and adversely affect neural development. This is often in the form of obstruction to CSF flow around the base of the brain that then leads to vault expansion by means of high pressure pulsation and ventricle enlargement by means of low pressure pulsation. In health pressure pulsatility does not lead to enduring changes in water distribution within the CNS compartments but it assists physiological water balance.