Background: Post-traumatic hydrocephalus (PTH) is potentially under-diagnosed and under-treated, generating the need for a more efficient diagnostic tool. We aim to report CSF dynamics of patients with post-traumatic ventriculomegaly. Materials and methods: We retrospectively analysed post-traumatic brain injury (TBI) patients with ventriculomegaly who had undergone a CSF infusion test. We calculated the resistance to CSF outflow (Rout), AMP (pulse amplitude of intracranial pressure, ICP), dAMP (AMPplateau-AMPbaseline) and compensatory reserve index correlation coefficient between ICP and AMP (RAP). To avoid confounding factors, included patients had to be non-decompressed or with cranioplasty > 1 month previously and Rout > 6 mmHg/min/ml. Compliance was assessed using the elasticity coefficient. We also compared infusion-tested TBI patients selected for shunting versus those not selected for shunting (consultant decision based on clinical and radiological assessment and the infusion results). Finally, we used data from a group of shunted idiopathic Normal Pressure Hydrocephalus (iNPH) patients for comparison. Results: Group A consisted of 36 patients with post-traumatic ventriculomegaly and Group B of 45 iNPH shunt responders. AMP and dAMP were significantly lower in Group A than B (0.55 ± 0.39 vs 1.02 ± 0.72; p < 0.01 and 1.58 ± 1.21 vs 2.76 ± 1.5; p < 0.01. RAP baseline was not significantly different between the two. Elasticity was higher than the normal limit in all groups (average 0.18 1/ml). Significantly higher Rout was present in those with probable PTH selected for shunting compared with unshunted. Mild/moderate hydrocephalus, ex-vacuo ventriculomegaly/ encephalomalacia were inconsistently reported in PTH patients. Conclusions: Rout and AMP were significantly lower in PTH compared to iNPH and did not always reflect the degree of hydrocephalus or atrophy reported on CT/MRI. Compliance appears reduced in PTH.

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 and formation of CSF outflow-contribute heavily to linear model expressed by Davson's equation.

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 peakto-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 Pss and formation of CSF outflow contribute heavily to linear model expressed by Davson's equation.