Noninvasive indicators of intracranial pressure before, during, and after long-duration spaceflight

Abstract

Weightlessness induces a cephalad shift of blood and cerebrospinal fluid that may increase intracranial pressure (ICP) during spaceflight, whereas lower body negative pressure (LBNP) may provide an opportunity to caudally redistribute fluids and lower ICP. To investigate the effects of spaceflight and LBNP on noninvasive indicators of ICP (nICP), we studied 13 crewmembers before and after spaceflight in seated, supine, and 15° head-down tilt postures, and at ∼45 and ∼150 days of spaceflight with and without 25 mmHg LBNP. We used four techniques to quantify nICP: cerebral and cochlear fluid pressure (CCFP), otoacoustic emissions (OAE), ultrasound measures of optic nerve sheath diameter (ONSD), and ultrasound-based internal jugular vein pressure (IJVp). On flight day 45, two nICP measures were lower than preflight supine posture [CCFP: mean difference -98.5 -nL (CI: -190.8 to -6.1 -nL), P = 0.037]; [OAE: -19.7° (CI: -10.4° to -29.1°), P < 0.001], but not significantly different from preflight seated measures. Conversely, ONSD was not different than any preflight posture, whereas IJVp was significantly greater than preflight seated measures [14.3 mmHg (CI: 10.1 to 18.5 mmHg), P < 0.001], but not significantly different than preflight supine measures. During spaceflight, acute LBNP application did not cause a significant change in nICP indicators. These data suggest that during spaceflight, nICP is not elevated above values observed in the seated posture on Earth. Invasive measures would be needed to provide absolute ICP values and more precise indications of ICP change during various phases of spaceflight.NEW & NOTEWORTHY The current study provides new evidence that intracranial pressure (ICP), as assessed with noninvasive measures, may not be elevated during long-duration spaceflight. In addition, the acute use of lower body negative pressure did not significantly reduce indicators of ICP during weightlessness.

Keywords: intracranial pressure; noninvasive intracranial pressure; spaceflight.

Graphical abstract

Figure 1.

Testing timeline of each data collection session and posture tested. All four noninvasive intracranial pressure indicator techniques were used for each testing session, except IJVp was not used during spaceflight with LBNP. Preflight (pre), flight day 45 (FD45), flight day 150 (FD150), postflight day 10 (R + 10), postflight day 30 (R + 30), postflight day 180 (R + 180). HDT, head-down tilt; IJVp, internal jugular vein pressure; LBNP, lower body negative pressure.

Figure 2.

Noninvasive indicators of intracranial pressure including cerebral and cochlear fluid pressure-derived tympanic membrane displacement (TMD; −nL; A), otoacoustic emission-derived phase (OAE; degrees; B), optic nerve sheath diameter (ONSD; cm; C), and internal jugular vein pressure (IJVp; mmHg; D) from all subjects (n = 13) preflight in various postures and inflight on flight days 45 (FD45) and 150 (FD150). Black round symbol connected by black line is the crewmember identified with Frisèn grade 1 optic disc edema. In multiple instances the individual data point appears similar to the mean. Dark horizontal bar is the mean with 95% confidence interval, and gray data points show individual subject data. *P < 0.05 vs. FD45 Spaceflight. HDT, head-down tilt.

Figure 3.

Spaceflight (FD45 and FD150) and spaceflight with LBNP (FD45 LBNP and FD150 LBNP) nICP indicator measures from all subjects (n =13). The use of lower body negative pressure (LBNP) during spaceflight did not have an effect on nICP indicators. Measures include cerebral and cochlear fluid pressure-derived tympanic membrane displacement (TMD; −nL; A), otoacoustic emission-derived phase (OAE; degrees; B), and optic nerve sheath diameter (ONSD; cm; C). Black round symbol connected by black line is the crewmember identified with Frisèn grade 1 optic disc edema. In multiple instances, the individual data point appears similar to the mean. Dark horizontal bar is the mean with 95% confidence interval, and gray data points show individual subject data. *P < 0.05 vs. FD45 Spaceflight. Note that nICP indicator measures with and without LBNP occurred approximately 1 wk apart.

Figure 4.

Noninvasive indicators of intracranial pressure including cerebral and cochlear fluid pressure-derived tympanic membrane displacement (TMD; −nL; A), otoacoustic emission phase (OAE; degrees; B), optic nerve sheath diameter (ONSD; cm; C), and internal jugular vein pressure (IJVp; mmHg; D) from all subjects (n = 13) preflight and postflight [return to Earth (R+) R + 10, R + 30, R + 180 days] in various postures. The omnibus interaction between posture and time was not significant for any nICP outcome. Black round symbol connected by black line is the crewmember identified with Frisèn grade 1 optic disc edema. In multiple instances, the individual data point appears similar to the mean. Dark horizontal bar is the mean with 95% confidence interval, and gray data points show individual subject data. Note that in some cases the symbol representing the individual with Frisèn grade edema appears similar to the horizontal bar at the mean. The dotted vertical line separates preflight from postflight. HDT, head-down tilt; nICP, noninvasive indicators of intracranial pressure.