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Role of the Cranial Venous Circulation in Microgravity-Associated Visual Changes

Principal Investigator:
Jay C. Buckey, M.D.

Geisel School of Medicine at Dartmouth College

Upon entering microgravity, astronauts’ legs become thinner and their faces can look puffy, because of a shift of body fluids toward the head. This headward fluid shift affects the volume and pressure within veins in the head. These pressure and volume changes may underlie microgravity-associated visual symptoms because changes in pressures within the head can also affect the eye.

But, not all astronauts experience changes to their vision in weightlessness. Differences in the anatomy, flow, and compliance of the veins in the head between individuals may explain this discrepancy. Our goal is to develop a numerical model of the cerebral venous circulation that can predict the effects of the fluid shifts. We will validate the model by using magnetic resonance imaging (MRI) of the head to measure changes in venous flow, venous volume, venous pressure, intracranial compliance, cerebrospinal fluid (CSF) volume and flow pulsatility during both fluid shifts and changes in body position. The likely anatomic differences that could alter the responses to a fluid shift will be identified. This model and supporting data will provide a way to develop hypotheses about how microgravity produces visual changes over time and may allow predictions about which subjects may be at risk for the visual deficits associated with microgravity.

NASA Taskbook Entry

Technical Summary

This project has three main aims. The first aim is to develop a numerical model of the cranial venous system and surrounding tissues for predicting changes in intracranial venous flow, volume, compliance, and pressure in response to a fluid shift and changes in hydrostatic gradients (pressure changes due to gravity). This model will provide a quick and easy way to test hypotheses about potential causes of microgravity-induced visual changes. The second aim will measure the cranial venous changes produced by fluid shifts and altered hydrostatic gradients in human subjects. We will use the latest advances in MRI imaging to measure cerebral blood flow, venous outflow, cerebrospinal fluid flow, intracranial compliance, and venous architecture/venography in normal individuals exposed to fluid shifts and alterations in hydrostatic gradients produced by changes in body position. The data collected will serve to develop the numerical model and improve its predictive capabilities. The third aim is to identify cranial venous variants that may influence the response to a microgravity-induced fluid shift. Human subjects with these variants will be studied further to see if the variants may be associated with inter-individual differences in the response to headward fluid shifts.

Earth Applications

On Earth, the anatomy and physiology of the cranial venous system is associated with health problems. Cranial venous insufficiency, where the venous outflow from the head is reduced or obstructed, has been linked with headaches, vision changes, acute mountain sickness, obstructive sleep apnea, multiple sclerosis, and idiopathic intracranial hypertension (IIH). Bilateral transverse sinus stenosis, a common venous variant, is found in 90% of IIH sufferers, and internal jugular vein stenosis occurs in 80% of IIH patients. When the transverse sinus is constricted or blocked, intracranial venous pressure and CSF pressure are increased. The size of the jugular foramen (the opening for the jugular vein) varies between individuals, and can also be asymmetric within a given individual. This asymmetry can affect oxygen levels in the jugular vein. Hypoplasia of the transverse sinuses occurs frequently (49%), with the right side usually being larger than the left side, which may influence pressure within the head. Arachnoid granulations are also common in major venous sinus cavities and can lead to venous hypertension by impeding bloodflow.

Our numerical model will allow both scientists and health care providers to understand the physiology of the cranial venous system, how it might be associated with health problems, and how it might be affected by anatomical differences. Using the model, scientists will be able to test hypotheses quickly and inexpensively, which could provide direction for future research. Health care providers can use the model as an aid when determining the course of treatment with patients.