Noninvasive intracranial compliance from MRI-based measurements of transcranial blood and CSF flows: indirect versus direct approach

Authors: Tain RW, Alperin N.

Intracranial compliance (ICC) determines the ability of the intracranial compartment to accommodate an increase in volume without a large increase in intracranial pressure (ICP). The clinical utilization of ICC is limited by the invasiveness of current measurement. Several investigators attempted to estimate ICC noninvasively, from magnetic resonance imaging (MRI) measurements of cerebral blood and cerebral spinal fluid flows, either using indirect measures of ICC or directly by measuring the ratio of the changes in intracranial volume and pressure during the cardiac cycle. The indirect measures include the phase lag between the cerebrospinal fluid (CSF) and its driving force, either arterial inflow or net transcranial blood flow. This study compares the sensitivity of phase-based and amplitude-based measures of ICC to changes in ICC. In vivo volumetric blood and CSF flows measured by MRI phase contrast from healthy volunteers and from patients with elevated ICP were used for the comparison. An RLC circuit model of the craniospinal system was utilized to simulate the effect of a change in ICC on the CSF flow waveform. The simulations demonstrated that amplitude-based measures of ICC are considerably more sensitive than phase-based measures, and among the amplitude-based measures, the ICC index provides the most reliable estimate of ICC.

Improved noninvasive intracranial pressure assessment with nonlinear kernel regression

Authors: Peng Xu, Magdalena Kasprowicz, Marvin Bergsneider, Xiao Hu

The only established technique for intracranial pressure (ICP) measurement is an invasive procedure requiring surgically penetrating the skull for placing pressure sensors. However, there are many clinical scenarios where a noninvasive assessment of ICP is highly desirable. With an assumption of a linear relationship among arterial blood pressure (ABP), ICP, and flow velocity (FV) of major cerebral arteries, an approach has been previously developed to estimate ICP noninvasively, the core of which is the linear estimation of the coefficients f between ABP and ICP from the coefficients w calculated between ABP and FV. In this paper, motivated by the fact that the relationships among these three signals are so complex that simple linear models may be not adequate to depict the relationship between these two coefficients, i.e., f and w , we investigate the adoption of several nonlinear kernel regression approaches, including kernel spectral regression (KSR) and support vector machine (SVM) to improve the original linear ICP estimation approach. The ICP estimation results on a dataset consisting of 446 entries from 23 patients show that the mean ICP error by the nonlinear approaches can be reduced to below 6.0 mmHg compared to 6.7 mmHg of the original approach. The statistical test also demonstrates that the ICP error by the proposed nonlinear kernel approaches is statistically smaller than that estimated with the original linear model (p < 0.05). The current result confirms the potential of using nonlinear regression to achieve more accurate noninvasive ICP assessment.

Using MRI of the optic nerve sheath to detect elevated intracranial pressure

Authors: Heidi Harbison Kimberly and Vicki E Noble

The current gold standard for the diagnosis of elevated intracranial pressure (ICP) remains invasive monitoring. Given that invasive monitoring is not always available or clinically feasible, there is growing interest in non-invasive methods of assessing ICP using diagnostic modalities such as ultrasound or magnetic resonance imaging (MRI). Increased ICP is transmitted through the cerebrospinal fluid surrounding the optic nerve, causing distention of the optic nerve sheath diameter (ONSD). In this issue of Critical Care, Geeraerts and colleagues describe a non-invasive method of diagnosing elevated ICP using MRI to measure the ONSD. They report a positive correlation between measurements of the ONSD on MRI and invasive ICP measurements. If the findings of this study can be replicated in larger populations, this technique may be a useful non-invasive screening test for elevated ICP in select populations.

Cortical spreading depression recorded from the human brain using a multiparametric monitoring system

Authors: Mayevsky A, Doron A, Manor T, Meilin S, Zarchin N, Ouaknine GE.

The number of parameters (i.e., EEG or ICP-intracranial pressure) routinely monitored under clinical situations is limited. The brain function analyzer described in this paper enables simultaneous, continuous on-line monitoring of cerebral blood flow (CBF) and volume (CBV), intramitochondrial NADH redox state, extracellular K+ concentrations, DC potential, electrocorticography and ICP from the cerebral cortex. Brain function of 14 patients with severe head injury (GCS < or = 8), who were hospitalized in the neurosurgical or general intensive care unit was monitored using this analyzer. Leao cortical spreading depression (SD) has been reported in many experimental animals but not in the human cerebral cortex. In one of the patients monitored, spreading depression was observed. This is the first time that spontaneous repetitive cortical SD cycles have been recorded from the cerebral cortex of a patient suffering from severe head injury. Typical SD cycles appeared 4-5 h after the beginning of monitoring this patient. During the first 3-4 cycles the responses of this patient were very similar to the responses to SD recorded in normoxic experimental animals. Electrocorticography was depressed whereas extracellular K+ levels increased. The metabolic response to spreading depression was characterized by oxidation of intramitochondrial NADH concomitant to a large increase in CBF. During brain death, an ischemic depolarization, characterized by decrease in CBF and an irreversible increase in extracellular K+, was recorded.

 

The epidemiology of traumatic brain injury: a review

Authors: Bruns J Jr, Hauser WA

Epilepsia. 2003;44 Suppl 10:2-10.

Traumatic brain injury (TBI) not only has considerable morbidity and mortality, but it is a major cause of epilepsy. We wish to determine the frequency of TBI, special groups at risk for TBI, and mortality from TBI.

Population-based studies in the United States suggest that the incidence of TBI is between 180 and 250 per 100,000 population per year. Incidence may be higher in Europe and South Africa. There are groups at high risk for TBI. This includes males and individuals living in regions characterized by socioeconomic deprivation. There are selective age groups at risk for TBI. This includes the very young, adolescents and young adults, and the elderly. Mortality varies by severity but is high in those with severe injury and in the elderly.

TBI is a major public health problem as well as a major cause of epilepsy. If primary prevention is to be undertaken, we must understand the epidemiology of the condition. The primary causes of TBI vary by age, socioeconomic factors, and geographic region, so any planned interventions must be tailored accordingly.

 

Optic nerve sheath diameter, intracranial pressure and acute mountain sickness on Mount Everest: a longitudinal cohort study

Authors: A I Sutherland1, D S Morris2, C G Owen3, A J Bron4, R C Roach5 

Objective: To investigate the association of optic nerve sheath diameter (ONSD), as a correlate of intracranial pressure (ICP), with acute mountain sickness (AMS).

Conclusions: ONSD increases at high altitude, and this increase is associated with more severe symptoms of AMS. Given the linkage between ONSD and ICP, these results strongly suggest that intracranial pressure plays an important role in the pathophysiology of AMS.

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