Abstract
A tight relationship exists between pressure and volume within the craniospinal compartment, given the rigidity of its boundaries (i.e., the skull and the vertebral column). Space-occupying neurological disorders, such as hydrocephalus, alter the physiological craniospinal pressure-volume state. Therefore, in patients suffering from these pathologies, the measurement of intracranial pressure (ICP) and craniospinal compliance (CC) can be informative during both the diagnostic and the clinical management phases of patient care. However, the only clinically accepted approaches to obtain ICP and CC are invasive. Their invasiveness puts the patients undergoing them at risk of complications, and it limits the overall number of patients who can benefit from such assessment. Consequently, extensive efforts were made in the last decades to develop noninvasive alternatives to derive ICP and CC surrogates with clinically acceptable accuracy and reliability. So far, none of these noninvasive methods accomplished the goal of replacing the invasive gold standard.
In this PhD thesis the continuous measurement of the head’s dielectric properties was investigated as a novel approach to noninvasively estimate CC. These properties depend on the intracranial volume composition, which, in turn, affects CC. The apparatus necessary for such measurement was developed, through which the electric signal W is acquired. After showing that W has an intracranial component, the aim was to collect data supporting that this noninvasively obtained signal contains information about CC. To this end, physiological testing was performed on healthy volunteers, primarily consisting of body position changes through tilting.
Cardiac and respiratory action determine periodic oscillations in the head’s dielectric properties and, consequently, in the W signal. In the first and second part of this thesis, the analysis of W focused on AMP, the peak-to-valley amplitude in W due to cardiac activity. In 18 healthy young (< 30 years) volunteers, AMP diminished during head-up tilting (HUT), and it increased during head-down tilting (HDT). The observation of AMP decreasing with increasing intracranial compliance, when going from a HDT to a HUT position, is consistent with the variation due to tilting in how blood and cerebrospinal fluid (CSF) volumes within the head change during the cardiac cycle. Notably, the results from computational electromagnetics simulations showed the same AMP behavior with intracranial compliance changes. AMP was then assessed in 13 healthy older (> 60 years) volunteers. These tests were performed since aging is expected to influence the head’s dielectric properties, due to its impact on the cardiovascular and CSF systems. A further reason is that space-occupying neurological disorders are more prevalent in the older population. A higher AMP was observed in the older cohort. This agrees with the anticipated reduction of CC with aging. The decrease in AMP during HUT and its increase during HDT noted also in the older subjects further support the robustness of this metric. Finally, in the last part of this thesis, technical improvements were made to the measurement apparatus, which allow the acquisition of W between 0 and 0.1 Hz. Such enhancement was tested by tilting and manual jugular vein compression on healthy volunteers. The measured changes in the mean of W align with the expected intracranial blood volume variations due to these tests. This additional capability broadens the potential clinical applications of the noninvasive monitoring of W.
Overall, the data presented in this PhD thesis suggest that the W signal may contain information about CC, with AMP as potential CC surrogate. Clinical studies with concomitant measurement of invasive CC and W are now required to confirm the possible association between the two.
Boraschi, Andrea