Validation of Davson’s equation in patients suffering from idiopathic normal pressure hydrocephalus

Introduction The so-called Davson’s equation relates baseline intracranial pressure (ICP) to resistance to cerebrospinal fluid outflow (Rout), formation of cerebrospinal fluid ( I f ) and sagittal sinus pressure ( P SS ) There is a controversy over whether this fundamental equation is applicable in patients with normal pressure hydrocephalus (NPH). We investigated the relationship between Rout and ICP and also other compensatory, clinical and demographic parameters in NPH patients. Method We carried out a retrospective study of 229 patients with primary NPH who had undergone constant-rate infusion studies in our hospital. Data was recorded and processed using ICM+ software. Relationships between variables were sought by calculating Pearson product correlation coefficients and p values. Results We found a significant, albeit weak, relationship between ICP and Rout ( R = 0.17, p = 0.0049), Rout and peak-to-peak amplitude of ICP (AMP) ( R = 0.27, p = 3.577e-05) and Rout and age ( R = 0.16, p = 0.01306). Conclusions The relationship found between ICP and Rout provides indirect evidence to support disturbed Cerebrospinal fluid circulation as a key factor in disturbed CSF dynamics in NPH. Weak correlation may indicate that other factors—variable P SS and formation of CSF outflow—contribute heavily to linear model expressed by Davson’s equation.