The Role of Intracranial Pressure in Space Adaptation Syndrome

1. The Role of Intracranial Pressure in Space Adaptation Syndrome Edward F. Good, Alan R. Hargens #, Gregory C. Steinbach #, Robert J. Marchbanks ^, William T. Yost †, Daniel Perey †, Daniel L. Feeback * Bay Area Neurology, Webster, TX; # University of California, San Diego, California; ^ Department of Medical Physics and Bioengineering, Southampton University Hospitals NHS Trust,Southampton, England; † NASA Langley Research Center, Hampton, Virginia; * NASA Johnson Space Center, Houston, TX • Introduction • Intracranial pressure (ICP) monitoring is an important diagnostic tool on Earth and in Space • Clinically important measurement for patients with conditions including trauma, hemorrhage, hydrocephalus, cerebral lesions, and severe liver disease • Physiological fluid shifts, such as those that occur during exposure to microgravity or with changes in posture, alter intracranial pressure • Symptoms of space adaptation syndrome are similar to those associated with elevated ICP • Elevated ICP impairs cerebral autoregulation and typical symptoms include headache, malaise, lethargy, and vomiting • Space adaptation syndrome affects flight crews through loss of productivity, physical discomfort, and potentially degraded performance on orbit • New noninvasive techniques to detect ICP changes, and their dynamics, have been developed • The current technique for monitoring ICP is highly invasive and costly, requiring the insertion of a transducer-tipped catheter into the skull • Pulsed phase-lock loop (PPLL) technology can measure intracranial distance changes and can detect skull movements that occur in association with changes in ICP and cardiac cycles in-vivo • Cerebral and Cochlear Fluid Pressure (CCFP) Analyzer can measure the dynamics of structures within the ear that have been shown to correlate with ICP • Despite the apparently rigid nature of the skull, respiration and blood pressure pulses produce detectable skull bone movements on the order of 1.3 - 2.5 µm (micrometers) in cats When a reflecting target (opposite side of the skull) moves, a phase difference comes to exist between emitted and received ultrasonic signals. A square point on the wave represents the point of initial ultrasonic phase (left figure). PPLL alters ultrasonic frequency to keep the initial point on the reflecting target. The equation represents the theoretical relationship between ultrasonic frequency, sound velocity, and distance. The PPLL ground-based instrumentation system (laptop, oscilloscope, PPLL instrument) is shown on the right. Status of Research  Effects of whole -body tilt on non-invasive ICP measurements • Purpose • Non-invasively measure ICP before, during, and after Space Shuttle flights • Evaluate the relationship between ICP measurements and time course and extent of SAS symptoms • If ICP & SAS are correlated, consider application of current countermeasures used on Earth to alleviate SAS symptoms in orbit CCFP measurement of the tympanic membrane displacement (mean +/- SE) at various tilt angles for 6 subjects (from Murthy et al. 1992). († : p < 0.05 relative to -6 degrees, *: p < 0.05 relative to 90 degrees, triangle: p < 0.05 relative to 0 degrees) ICP is computed from this data based on a calibration curve that relates displacement to ICP. • Current Status of Research • Complementary nature of PPLL and CCFP techniques • Together the CCFP and PPLL cover the expected ICP range from low to high pressure • They overlap in a critical region where raised ICP starts to reduce cerebral compliance • In some cases the region of overlap, the compliance ‘knee-point’, may more closely correlate to symptoms than the absolute ICP. PPLL measurement of the change in skull displacement at various tilt angles for 6 subjects (from Ueno et al. 2003) Skull pulsations are largest when the subject is in head-down tilt position and decreases as subject moves to head-up tilt position. ICP is computed from this data based on a calibration curve that relates displacement to ICP. Future Plans Development of non-invasive flight instrumentation Low ICP Normal ICP High Very High CCPF - nominal ICP range of -10 to +35 cmH20 PPLL - nom. ICP of > 25 cmH20 • Current Status of Research • Use of Cerebral and Cochlear Fluid Pressure (CCFP) Analyzer • As the intracranial pressure increases, the resting position of the stapes footplate moves outwards. • Directly measure movements of the tympanic membrane (ear drum) to assess intracranial pressure dynamics • Acoustically stimulate the stapedial muscle to assess the baseline intracranial pressure - i.e. the resting position of the stapes footplate. Shown above is a CAD illustration of the planned flight ICP instrument. Proposed measurement sessions for Space Shuttle flight • Conclusion • Non-invasive instruments have been developed and validated for the measurement of ICP • Instruments have been redesigned for use in Space • Preliminary Design Review (PDR) has been successfully completed • Awaiting approval/funding to develop and test instrument to study the correlation between intracranial pressure and space adaptation syndrome • Acknowledgements • Preliminary funding of this study (through PDR) provided by NASA. Project is awaiting further • funds for development and implementation for use on future Space Shuttle flights. Acknowledgement to Harold F. Schucknecht MD • Use of Pulsed Phase-Lock Loop (PPLL) technology • The measurement system consists of an ultrasound transducer attached to the pulser/receiver module of the PPLL instrument. • The transducer is placed on the skin in the temporal region of the head and emits a 500 kHz tone burst that travels through the cranium and is reflected off the temporal bone on the opposite side of the skull. • Skull displacement changes are recorded based on a phase comparison of transmitted and received waves. The PPLL device adjusts the frequency of the subsequent wave to maintain a constant phase. Changes in path length are calculated from frequency changes by the equation shown below.