Noninvasive in-ear monitoring of intracranial pressure during microgravity in parabolic flights

Abstract

Among possible causes of visual impairment or headache experienced by astronauts in microgravity or postflight and that hamper their performance, elevated intracranial pressure (ICP) has been invoked but never measured for lack of noninvasive methods. The goal of this work was to test two noninvasive methods of ICP monitoring using in-ear detectors of ICP-dependent auditory responses, acoustic and electric, in acute microgravity afforded by parabolic flights. The devices detecting these responses were handheld tablets routinely used in otolaryngology for hearing diagnosis, which were customized for ICP extraction and serviceable by unskilled operators. These methods had been previously validated against invasive ICP measurements in neurosurgery patients. The two methods concurred in their estimation of ICP changes with microgravity, i.e., 11.0 ± 7.7 mmHg for the acoustic method ( n = 7 subjects with valid results out of 30, auditory responses being masked by excessive in-flight noise in 23 subjects) and 11.3 ± 10.6 mmHg for the electric method ( n = 10 subjects with valid results out of 10 tested despite the in-flight noise). These results agree with recent publications using invasive access to cerebrospinal fluid in parabolic flights and suggest that acute microgravity has a moderate average effect on ICP, similar to body tilt from upright to supine, yet with some subjects undergoing large effects whereas others seem immune. The electric in-ear method would be suitable for ICP monitoring in circumstances and with subjects such that invasive measurements are excluded. NEW & NOTEWORTHY In-ear detectors of intracranial pressure-dependent auditory responses allow intracranial pressure to be monitored noninvasively during acute microgravity. The average pressure increase during 20-s long sessions in microgravity is 11 mmHg, comparable with an effect of body tilt. However, intersubject variability is large, with subjects who repeatedly experience from nothing to twice the average effect. A systematic in-flight use would allow the relationship between space adaptation syndrome and ICP to be established or dismissed.