Modeling intracranial pressures in microgravity: the influence of the blood-brain barrier

Authors: Lakin WD, Stevens SA, Penar PL.

INTRODUCTION: A majority of astronauts experience symptoms of headache, vomiting, nausea, lethargy, and gastric discomfort during the first few hours or days after entering a microgravity environment. Due to similarities in symptoms and their time evolution, it has been hypothesized that some of these conflicts are related to the development of benign intracranial hypertension in these individuals in microgravity.

METHODS: This hypothesis was tested using a validated mathematical model that embeds the intracranial system in whole-body physiology. This model was used to predict steady-state intracranial pressures in response to various cardiovascular stimuli associated with microgravity, including changes in arterial pressure, central venous pressure, and blood colloid osmotic pressure. The model also allowed alterations of the blood-brain barrier due to factors such as gravitational unloading and increased exposure to radiation in space to be considered.

RESULTS: Simulations predicted that intracranial pressure will increase significantly if, combined with a drop in blood colloid osmotic pressure, there is a reduction in the integrity of the blood-brain barrier in microgravity.

DISCUSSION: These results suggest that in some otherwise healthy individuals microgravity environments may elevate intracranial pressure to levels associated with benign intracranial hypertension, producing symptoms that can adversely affect crew health and performance.

Alteration of Postural Responses To Visual Field Motion in Mild Traumatic Brain Injury

Authors: Slobounov, Semyon Ph.D.; Tutwiler, Rick Ph.D.; Sebastianelli, Wayne M.D.; Slobounov, Elena M.S.

OBJECTIVE: Balance deficits in individuals suffering from mild traumatic brain injury (MTBI) have been documented in numerous recent studies. However, long-lasting balance deficits and specific mechanisms causing these deficits have not been systematically examined. This paper aimed to present empirical evidence showing destabilizing effects of visual field motion in concussed individuals up to 30 days post-injury.

METHOD: 60 student-athletes participated in pilot (n = 12) and major experiments(n = 48) prior to injury. Eight of these 48 subjects who suffered MTBI in athletic events were tested again on Day 3, Day 10 Day 30 after the incident. Postural responses to visual field motion were recorded using a virtual reality (VR) environment in conjunction with balance (AMTI force plate) and motion tracking (Flock of Birds, FOB) technologies.

RESULTS: The area of the center of pressure during upright quite stance basically did not changed from Day 3-30 post-injury with respect to pre-injury status (P > .05). However, balance deficits induced by visual field motion were present up to 30 days post-injury. Destabilizing effect of visual field motion was observed via significant increase of the CP data (P < .05) and reduced coherence value.

CONCLUSION: Our data suggest the presence of residual sensory integration dysfunction in concussed individuals at least 30 days post-injury and may indicate a lower threshold for brain re/injury.

Transcranial Doppler Pulsatility Index: Not an Accurate Method to Assess Intracranial Pressure

Authors: Behrens, Anders MD, MSc; Lenfeldt, Niklas MSc, PhD; Ambarki, Khalid PhD; Malm, Jan MD, PhD; Eklund, Anders PhD; Koskinen, Lars-Owe MD, PhD.

BACKGROUND: Transcranial Doppler sonography (TCD) assessment of intracranial blood flow velocity has been suggested to accurately determine intracranial pressure (ICP).

OBJECTIVE: We attempted to validate this method in patients with communicating cerebrospinal fluid systems using predetermined pressure levels.

METHODS: Ten patients underwent a lumbar infusion test, applying 4 to 5 preset ICP levels. On each level, the pulsatility index (PI) in the middle cerebral artery was determined by measuring the blood flow velocity using TCD. ICP was simultaneously measured with an intraparenchymal sensor. ICP and PI were compared using correlation analysis. For further understanding of the ICP-PI relationship, a mathematical model of the intracranial dynamics was simulated using a computer.

RESULTS: The ICP-PI regression equation was based on data from 8 patients. For 2 patients, no audible Doppler signal was obtained. The equation was ICP = 23*PI + 14 (R2 = 0.22, P < .01, N = 35). The 95% confidence interval for a mean ICP of 20 mm Hg was −3.8 to 43.8 mm Hg. Individually, the regression coefficients varied from 42 to 90 and the offsets from −32 to +3. The mathematical simulations suggest that variations in vessel compliance, autoregulation, and arterial pressure have a serious effect on the ICP-PI relationship.

CONCLUSIONS: The in vivo results show that PI is not a reliable predictor of ICP. Mathematical simulations indicate that this is caused by variations in physiological parameters.

Noninvasive detection of elevated intracranial pressure using a portable ultrasound system

Authors: Prunet B, Asencio Y, Lacroix G, Montcriol A, Dagain A, Cotte J, Esnault P, Boret H, Meaudre E, Kaiser E.

OBJECTIVE: The aim of this study is to prospectively compare the accuracies of transcranial color-coded sonography (TCCS) and transcranial Doppler (TCD) in the diagnosis of elevated intracranial pressure.

METHODS: A prospective, blinded, head-to-head comparison of TCD and TCCS methods using intracranial pressure (ICP) measured continuously via an intraparenchymal catheter as the reference standard in 2 groups of 20 neurocritical care patients each: high ICP (group 1) and normal ICP (group 2). Middle cerebral artery (MCA) pulsatility index (PI) recordings from all patients' sonographic reports were selected based on the highest left or right recorded MCA PI. Transcranial Doppler was performed using a dedicated TCD device, and TCCS was performed using a portable ultrasound system.

RESULTS: The PI values obtained did not differ significantly between the 2 methods (group 1, P = .46; group 2, P = .11). Linear regression analysis identified a significant relationship between PI obtained with both methods (r = 0.897; P < .0001). The duration of PI measurement was statistically longer with TCCS than TCD (group 1, P < .01; group 2, P < .01). Diagnostic accuracies were good and similar for both methods (TCD area under curve, 0.901; TCCS area under curve 0.870; P = .69).

CONCLUSIONS: This work is a pilot study comparing TCCS and TCD in the detection of elevated ICP. This study suggests that a bedside portable ultrasound system may be useful to determine MCA PI with accuracy similar to that of a dedicated TCD device.

Measurement of intraocular and intracranial pressure: Is there a relationship?

Authors: Kirk T, Jones K, Miller S, Corbett J.

OBJECTIVE: To study whether noninvasive, intraocular pressure (IOP) measurements significantly correlate with standard intracranial pressure (ICP) measurements.

METHODS: This prospective, blinded study enrolled 46 patients who were undergoing medically indicated lumbar puncture (LP). IOP was measured by applanation tonometry immediately prior to measuring LP opening pressure. One patient was excluded due to unsuccessful ICP measurement.

RESULTS: In the 45 patients to successfully undergo IOP and ICP measurement, there was no significant relationship between ICP and average IOP for both eyes (r = -0.005). There was no significant relationship between ICP and IOP in either eye, when studied individually(r = 0.03 ocular dexter , r = -0.05 ocular sinister ). There was no significant relationship between ICP and IOP when the eye best correlated to the patient's ICP was chosen (r = -0.01).

INTERPRETATION: No significant relationship between ICP and IOP was observed. Noninvasive IOP measurements do not predict ICP.ANN NEUROL 2011;

A case of gitelman syndrome associated with idiopathic intracranial hypertension

Authors: Tsutsui H, Hamano T, Kawaura Y, Inaba S, Miyamori I, Yasujima M, Yoneda M, Kuriyama M.

An 18-year-old woman with Gitelman syndrome (GS) associated with idiopathic intracranial hypertension (IIH) is described. She was obese and showed a 10 kg gain in body weight over a period of 8 months. She presented with headache, vomiting, and diplopia. She had bilateral papilledema, and right abducens palsy. CSF examination demonstrated high pressure (over 320 mmH(2)O) with normal cytochemistry. Brain MRI was normal. She showed mild alkalosis, hypokalemia, hypomagnesemia, increased plasma renin activity, and normal blood pressure. Two heterozygous mutations in the SLC12A3 gene were identified. Therefore, she was diagnosed as GS with IIH. We should keep in mind the possible occurrence of IIH in GS.

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