Intracranial and Intraocular Pressure During Various Degrees of Head-Down Tilt

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 degrees, -12 degrees, and -18 degrees HDT, -12 degrees with 1% CO2, and -12 degrees with -20 mmHg LBNP. In Experiment II, IOP was measured in 16 healthy subjects after 5 min tilt at +12 degrees, 0 degrees, -6 degrees, -12 degrees, -18 degrees, and -24 degrees, with and without-40 mmHg LBNP. RESULTS: ICP was only found to increase from supine baseline during -18 degrees HDT (9.2 +/- 0.9 and 14.4 +/- 1 mmHg, respectively), whereas IOP increased from 15.7 +/- 0.3 mmHg at 0 degrees to 17.9 +/- 0.4 mmHg during -12 degrees HDT and from 15.3 +/- 0.4 mmHg at 0 degrees to 18.7 +/- 0.4 mmHg during-18 degrees 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 24 degrees 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.