In recent years, intracranial pressure (ICP) monitoring has gained an important role in the diagnosis and treatment of patients affected by chronic cerebrospinal fluid (CSF) dynamics disturbances, such as normal pressure hydrocephalus (NPH), idiopathic intracranial hypertension (IIH), Chiari malformation type I (CM-I), and various forms of hydrocephalus.1 Compared to patients in the acute setting, patients with chronic CSF dynamics disorders are most commonly mobile and provide a unique opportunity to study the changes of ICP and pulse amplitude (PA) in different body positions.
Most studies on ICP monitoring describe ICP in the supine position, despite the fact that humans spend most of their time in an upright posture. Better insight into the effect of position on ICP and intracranial compliance is important for several reasons. First of all, compared to our understanding of physiology in other medical fields (e.g., cardiology), the available knowledge on ICP physiology is limited, and this is probably attributable to the invasive nature of the investigations needed to measure this important physiological parameter. More importantly, our treatments may not be able to restore the physiological CSF dynamics until we know how they work. Previous studies have demonstrated that ICP is lower in the upright position than while supine.2-7 A better understanding of the effect of position on ICP dynamics could also allow improvement of CSF shunt technologies toward the creation of the long-awaited “smart” shunt system.8
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Intracranial compliance is the ability of the intracranial compartment to accommodate an increase in volume without a large increase in ICP. PA, the difference between systolic and diastolic ICP, is considered a reliable, indirect marker of compliance: the higher the PA, the lower the brain compliance. Intracranial compliance is recognized to be a very useful marker in clinical practice;1 however, the effect of position on markers of intracranial compliance is not clear. For example, Farahmand et al. noticed a slightly higher ICP waveform amplitude (indicating lower brain compliance) in the upright posture than in the supine posture.5 In contrast, Raabe et al. analyzed markers of intracranial compliance in 13 patients with intraventricular sensors and concluded that different levels of head-up tilts do not affect intracranial compliance.9 Noninvasive studies using MRI or mathematical models have concluded that brain compliance is increased in the upright position compared to the supine.10,11
The intracranial compliance model proposed by Marmarou et al. in 1975 demonstrates that higher ICP levels correspond to lower intracranial compliance.12 Since the supine position is associated with higher ICP levels, we hypothesized that intracranial compliance would be reduced in this position compared to that in an upright posture. The specific aims of this cross-sectional study were 1) to test the hypothesis that PA, as a marker of intracranial compliance, is correlated to body position; 2) to test the hypothesis that PA is correlated to the day/night cycle since night ICP is recorded in the supine position, whereas day ICP is mostly measured in an upright position; and 3) to describe the mean ICP and PA in different body positions for patients considered to have “near-normal” ICP dynamics based on a post hoc analysis.
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