Intracranial and intraocular pressure during various degrees of head-down tilt

Abstract

BACKGROUND: More than half of astronauts develop ophthalmic changes during long-duration spaceflight consistent with an abnormal intraocular and intracranial pressure (IOP, ICP) difference. The aim of our study was to assess IOP and ICP during head-down tilt (HDT) and the additive or attenuating effects of 1% CO2 and lower body negative pressure (LBNP). METHODS: In Experiment I, IOP and ICP were measured in nine healthy subjects after 3.5 h HDT in five conditions: -6°, -12°, and -18° HDT, -12° with 1% CO2, and -12° with -20 mmHg LBNP. In Experiment II, IOP was measured in 16 healthy subjects after 5 min tilt at +12°, 0°, -6°, -12°, -18°, and -24°, with and without -40 mmHg LBNP. RESULTS: ICP was only found to increase from supine baseline during -18° HDT (9.2 6 0.9 and 14.4 6 1 mmHg, respectively), whereas IOP increased from 15.7 6 0.3 mmHg at 0° to 17.9 6 0.4 mmHg during -12° HDT and from 15.3 6 0.4 mmHg at 0° to 18.7 6 0.4 mmHg during -18° HDT. The addition of -20 mmHg LBNP or 1% CO2 had no further effects on ICP or IOP. However, the use of -40 mmHg LBNP during HDT lowered IOP back to baseline values, except at 224° HDT. DISCUSSION: A small, posterior intraocular-intracranial pressure difference (IOP . ICP) is maintained during HDT, and a sustained or further decreased difference may lead to structural changes in the eye in real and simulated microgravity.