What shapes pulse amplitude of intracranial pressure?

The pulsatile component of intracranial pressure (ICP) has been shown to be a predictor of outcome in normal pressure hydrocephalus (NPH) and traumatic brain injury (TBI). Experimental studies have demonstrated that the pulse amplitude of ICP (AMP_ICP) is dependent on the mean ICP (mICP), and on the pulse amplitude of the cerebral arterial blood volume (AMP_CaBV), according to the exponential craniospinal compliance curve. In this study, we compared the influence of mICP and AMP_CaBV on AMP_ICP in patients with NPH (infusion study) and TBI (spontaneous recording). We retrospectively analyzed 25 NPH and 43 TBI patients with continuous monitoring of ICP and cerebral blood flow velocity (CBFV), as assessed with transcranial doppler. AMP_CaBV was extracted from the CBFV waveform. The influence of mICP and AMP_CaBV on AMP_ICP were determined using partial coefficients a, b, and c of the multiple regression model: AMP _ICP=a * mICP+b * AMP_CaBV+c. AMP_ICP was more dependent on mICP in NPH patients than in TBI patients (partial coefficient a=0.93 versus -0.03; p<0.001). On the contrary, AMP_ICP was more dependent on AMP_CaBV in patients with TBI than in those with NPH (b=0.86 versus 0.10; p<0.001). This study shows that AMP _ICP depends mostly on changes in mean ICP during cerebrospinal fluid (CSF) infusion studies in patients with NPH, and on changes in cerebral arterial blood volume (AMP_CaBV) in TBI patients. Further clinical studies will reveal whether AMP_ICP is a better indicator of clinical severity and outcome than mICP in TBI and NPH patients.