Lower body negative pressure reduces optic nerve sheath diameter during head-down tilt

Area of Science:

• Space medicine
• Ophthalmology
• Physiology

Background:

• Microgravity ocular syndrome (MOS) causes ophthalmic changes during spaceflight, potentially due to increased cerebrospinal fluid (CSF) pressure.
• Headward fluid shifting and elevated ambient CO2 are hypothesized contributors to MOS.
• Lower body negative pressure (LBNP) is explored as a countermeasure for headward fluid shifts.

Purpose of the Study:

• To assess the effects of head-down tilt (HDT), LBNP, and elevated CO2 on intracranial and periorbital CSF.
• To evaluate LBNP as a countermeasure against headward fluid shifting and its impact on CSF pressure.

Main Methods:

• Nine healthy males underwent five 5-hour head-down tilt (HDT) conditions: -6°, -12°, -18° HDT, -12° HDT with LBNP, and -12° HDT with 1% CO2.
• Three-dimensional MRI scans measured intracranial CSF volume and optic nerve sheath diameter (ONSD) after 4.5 hours.
• Measurements were compared to a supine baseline.

Main Results:

• ONSD significantly increased during all HDT conditions (-6°, -12°, -18°).
• Intracranial CSF volume increased during -12° HDT.
• LBNP mitigated the increases in ONSD and intracranial CSF during HDT; 1% CO2 did not further affect ONSD.

Conclusions:

• LBNP directly influences CSF and optic nerve sheath diameter, indicating its potential as a countermeasure for increased CSF.
• Findings suggest LBNP could help mitigate risks associated with microgravity ocular syndrome (MOS) in spaceflight.
• This study highlights LBNP's potential for managing elevated CSF pressure both in space and on Earth.