This project assessed distortion product otoacoustic emissions (DPOAE) as a non-invasive measure of intracranial pressure changes. The long-term interaction between intracranial pressure (ICP) and the ocular globe may cause visual acuity changes in spaceflight. Changes in DPOAE responses correlate with changes in ICP, making DPOAEs a useful proxy measure. The technique used here, DPOAE level/phase mapping (DPOAE L/P maps), collects data at multiple sites throughout the cochlea and provides a comprehensive picture of cochlear responses to ICP changes. This work was done in conjunction with an existing National Space Biomedical Research Institute (NSBRI)-funded set of experiments, Cranial Venous Modeling (CA03401). It also leveraged data from our Office of Naval Research and EPSCoR research grants. This study provides a novel way to make detailed DPOAE mapping measurements in association with multiple ocular and cranial vascular measurements.

Specific Aim 1: Create DPOAE level/phase maps to characterize changes as a result of the isolated effects of fluid shifts and alterations in hydrostatic gradients. In the first reporting year, DPOAE level/phase maps were collected in two experiments. Although there were visually apparent changes, we did not have a statistical basis by which to compare the data. Therefore, in conjunction with a project funded by the Office of Naval Research, a normative group of maps were collected on 29 subjects in 4 visits each. Data represented the normal range of variability that could be expected in the maps, and provides a statistical basis to compare the postural and fluid shift data to assess changes. Experimental data was collected--data in an experiment where 16 subjects underwent lower body negative and positive pressure (LBNP and LBPP) in both the supine and prone positions. DPOAE maps were collected on 13 subjects in 7 experimental conditions. In this way, the individual effects of time, gravitational direction, and fluid shifts on the DPOAE maps were isolated.

Specific Aim 2: Determine the DPOAE level/phase map response signature to fluid shifts and hydrostatic gradient changes. This research uses random field theory to calculate regional changes in maps across all subjects. The repeatability cohort described in Specific Aim 1 was used as the population's normal range of variation. Subjects in the experimental conditions were compared to the population averages at each distortion product data point. Regions within the cochlea most sensitive to DPOAE amplitude changes were found, and changes were most pronounced in the prone position under LBPP. These regions, in the 2f1 – f2 region of the map between 6-8 kHz are consistent with regions most sensitive to changes in stapes velocity and basilar membrane stiffness. DPOAE phase data is analyzed with two dimensional Fourier transforms. Due to its cyclical response, phase data may be broken down into characteristic parameters using two-dimensional Fourier transforms. Development of these methods is ongoing.

Specific Aim 3: Explore the relationship between ocular and cranial vascular measurements to changes seen in DPOAE level/phase maps. Tympanometry was measured for each subject to assess changes in middle ear state as an additional explanatory factor. The tympanometry results indicate that middle ear status, particularly the tympanic peak pressure, may be a relevant factor in explaining changes in DPOAE Level maps. Also, in conjunction with each experiment, additional ocular measures were taken, including intraocular pressure (IOP) and ocular geometry such as axial length and aqueous depth. These results indicate changes in fluid shift and in posture cause correlated alterations in the measured parameters. This emphasizes the need for numerical modeling to develop explanatory hypotheses for changes to these integrated physiological systems.

Specific Aim 2: This research uses random field theory to calculate regional changes in maps across all subjects. The repeatability cohort described in Specific Aim 1 was used as the population's normal range of variation. Subjects in the experimental conditions were compared to the population averages at each distortion product data point. Regions within the cochlea most sensitive to DPOAE amplitude changes were found, and changes were most pronounced in the prone position under LBPP. These regions, in the 2f1 – f2 region of the map between 6-8 kHz are consistent with regions most sensitive to changes in stapes velocity and basilar membrane stiffness. Work is ongoing to define the decision criteria, the Euler Characteristic used in the random field theory methodology, to ensure that the DPOAE map's inherent smoothness is accurately accounted for. Several statistical strategies have been identified to analyze DPOAE phase data. Due to its cyclical response, phase data may be broken down into characteristic parameters using two-dimensional Fourier transforms. Development of these methods is ongoing.

Specific Aim 3: Tympanometry was measured for each subject to assess changes in middle ear state as an additional explanatory factor for potential changes in DPOAE maps. The tympanometry results from the repeatability cohort and fluid shift/posture subjects indicate that middle ear status, particularly the tympanic peak pressure, many be a relevant factor in explaining changes in DPOAE Level maps. Also, in conjunction with each experiment, additional ocular measures were taken, including intraocular pressure (IOP) and ocular geometry such as axial length and aqueous depth. These results indicate changes in fluid shift and in posture cause correlated alterations in the measured parameters. This emphasizes the need for numerical modeling to develop explanatory hypotheses for changes to these integrated physiological systems.

This project focuses on assessing distortion product otoacoustic emissions (DPOAE) as a non-invasive measure of intracranial pressure changes. The long-term interaction between intracranial pressure (ICP) and the ocular globe may cause visual acuity changes in spaceflight. However, there is no noninvasive, easy-to-perform, on-orbit measure of ICP to test this hypothesis. Changes in DPOAE responses have been shown to correlate with changes in ICP, potentially making DPOAEs very useful as a proxy measure. The technique used here, DPOAE level/phase mapping (DPOAE L/P maps), collects DPOAE data at multiple sites throughout the cochlea and so provides a comprehensive picture of cochlear responses to ICP changes. We will statistically assess DPOAE L/P maps as a tool to measure ICP noninvasively by isolating the effects of fluid shifts and changes in hydrostatic gradients--two separate response mechanisms--by altering body position (hydrostatic gradients) and lower body pressure (fluid shift). We also consider time as a relevant variable in each of these postures. In conjunction with this work, we will also be collecting measures of cerebrovascular flow, ocular geometry/structures, middle ear status, and cardiovascular function to look for anatomical and physiological predictors for changes in the DPOAE L/P maps. This work will be done in conjunction with an existing National Space Biomedical Research Institute (NSBRI)-funded set of experiments, Cranial Venous Modeling (CA03401). This study will provide a novel way to make detailed DPOAE level/phase mapping measurements in association with multiple ocular and cranial vascular measurements.

Specific Aim 1: Create DPOAE level/phase maps to characterize changes as a result of the isolated effects of fluid shifts and alterations in hydrostatic gradients. During the reporting year, we have collected DPOAE level/phase maps in two experiments. Eight subjects were asked to lie in the supine and prone positions for an hour and two maps were collected (at 30 minutes and 45 minutes). We also performed a parabolic flight experiment with 14 subjects. In this experiment, pared-down maps were collected seated, and immediately upon entering the supine, prone, and microgravity conditions. We are beginning a 3rd experiment where ten subjects will experience lower body negative and positive pressure in both the supine and prone positions. In this way, the individual effects of time, gravitational direction, and fluid shifts on the DPOAE L/P maps can be isolated.

Specific Aim 2: Determine the DPOAE L/P map response signature to fluid shifts and hydrostatic gradient changes. Statistical analysis thus far has focused on amplitude maps. Data from ten subjects in the prolonged posture study were analyzed for differences from the seated baseline. Regions within the cochlea most sensitive to DPOAE amplitude changes were found. These regions are consistent with regions most sensitive to changes in stapes velocity and basilar membrane stiffness. Further analysis is ongoing for parabolic flight experimental data. Several statistical strategies have been identified to analyze DPOAE phase data. Due to its cyclical response, phase data may be broken down into characteristic parameters using two-dimensional Fourier transforms. The coefficients of the characteristic equations in different postures may be analyzed using the F test.

Specific Aim 3: Explore the relationship between ocular and cranial vascular measurements to changes seen in DPOAE level/phase maps. In conjunction with each experiment, additional ocular measures were taken, including intraocular pressure (IOP), choroidal area, and ocular geometry such as axial length and aqueous depth. Data from both the prolonged and acute posture studies indicate hydrostatic gradients, tissue offloading, and aqueous humor dynamics are contributing factors beyond fluid shift to change ocular structures. Future experiments will include data on cerebrohemodynamics taken with MRI (magnetic resonance imaging).

Specific Aim 2: Determine the DPOAE L/P map response signature to fluid shifts and hydrostatic gradient changes. Statistical analysis thus far has focused on amplitude maps. Data from ten subjects in the prolonged posture study were analyzed for differences from the seated baseline. Regions within the cochlea most sensitive to DPOAE amplitude changes were found. These regions are consistent with regions most sensitive to changes in stapes velocity and basilar membrane stiffness. Further analysis is ongoing for parabolic flight experimental data. Several statistical strategies have been identified to analyze DPOAE phase data. Due to its cyclical response, phase data may be broken down into characteristic parameters using two-dimensional Fourier transforms. The coefficients of the characteristic equations in different postures may be analyzed using the F test.

Specific Aim 3: Explore the relationship between ocular and cranial vascular measurements to changes seen in DPOAE level/phase maps. In conjunction with each experiment, additional ocular measures were taken, including intraocular pressure (IOP), choroidal area, and ocular geometry such as axial length and aqueous depth. Data from both the prolonged and acute posture studies indicate hydrostatic gradients, tissue offloading, and aqueous humor dynamics are contributing factors beyond fluid shift to change ocular structures. Future experiments will include data on cerebrohemodynamics taken with MRI.

Upon entering microgravity, astronauts experience a headward fluid shift, which could increase intracranial pressure (ICP) above seated levels. For long duration space flight, the interaction between ICP and intraocular pressure (IOP) is suspected to cause the visual acuity changes found in approximately 50% of astronauts. Distortion product otoacoustic emissions (DPOAEs) may be a noninvasive, easy-to-perform, assessment technique of ICP in-flight, using hardware that will be sent to the International Space Station. But, DPOAEs have not been rigorously evaluated to determine their effectiveness as a proxy measure for ICP. Although studies have shown DPOAEs are altered with postural and ICP changes on Earth, the contribution of the removal of all hydrostatic gradients in microgravity has not been determined. Also, studies to date have focused on a narrow set of test conditions, rather than optimizing the DPOAE testing parameters. This proposal seeks to address these limitations by evaluating subjects under test conditions that isolate the effects of fluid shifts and alterations in hydrostatic gradients. We will use unique measurement hardware that allows us to collect DPOAEs over a broad spectrum of frequencies and frequency ratios to create a response map of both DPOAE amplitude changes and phase shifts over the entire cochlear. We propose to perform this evaluation in conjunction with an existing NSBRI funded grant: Cranial Venous Modeling (CA03401) to gather a richer set of data.

Objective: To evaluate the contribution of fluid shifts and alterations in hydrostatic gradients to changes in DPOAE amplitude and phase across the cochlea to assess DPOAE level/phase mapping as a possible in-flight intracranial pressure assessment technique.

Hypotheses: Both fluid shifts and changes in hydrostatic gradients will alter DPOAE level/phase maps. Each will have different signatures present in map data.

Specific Aim 1: Create DPOAE level/phase maps to characterize changes as a result of the isolated effects of fluid shifts and alterations in hydrostatic gradients. Overview: Subjects will be evaluated under seven experimental conditions to isolate the effects of hydrostatic gradients and fluid shifts: seated baseline, prone, supine, supine with LBNP, prone with LBNP, supine LBPP, and prone with LBPP. DPOAE level/phase maps will be created over a range of frequencies and ratios.

Specific Aim 2: Statistically evaluate changes in DPOAE level/phase mapping to determine response signature of fluid shifts and hydrostatic gradient changes. Overview: Maps from experiment 1 will be statistically analyzed to determine map regions where response is altered under each experimental condition. Regression, machine learning, and spatial analysis statistics will be used. Data will be analyzed within subjects to characterize individual changes and across subjects to determine signatures associated with changes in fluid shifts and alterations in hydrostatic gradients.

Specific Aim 3: Explore the relationship between ocular and cranial vascular measurements to changes seen in DPOAE level/phase maps. Overview: Subjects from experiment 1 will be placed in each of the seven experimental conditions described previously. Magnetic resonance imaging (MRI), optical coherence tomography (OCT), tonometry, and optical biometry will be used to measure cranial fluid flow and subject ocular anatomy. These parameters will be inputs to cerebral and ocular models to calculate several additional metrics, such as ICP. Each of these parameters will be statistically evaluated for correlations to changes in DPOAE level/phase maps.

Conclusion: these set of experiments and statistical analyses will allow us to evaluate the feasibility of DPOAE level/phase maps as an in-flight assessment of ICP by isolating and evaluating the effects of fluid shifts and hydrostatic gradient changes and connecting the results to a rich set of ocular and cerebral hemodynamic measures.