Changes in the gravitational vector by postural changes or weightlessness induce fluid shifts impacting ocular hemodynamics and regional pressures. This investigation explores the impact of changes in direction of the gravitational vector on intraocular pressure (IOP), mean arterial pressure at eyelevel (MAPeye), and ocular perfusion pressure (OPP), which is critical for ocular health. Thirteen subjects underwent 360° of tilt (including both prone and supine positions) at 15º increments. At each angle, steady-state IOP and MAPeye were measured and OPP calculated as MAPeye-IOP. Experimental data were compared to a 6-compartment lumped parameter model of the eye. Mean IOP, MAPeye, and OPP significantly increased from 0º supine to 90º head down tilt (HDT) by 20.7±1.7 mmHg (ᵅD; < 0.001), 38.5±4.1 mmHg (ᵅD; < 0.001), and 17.4±3.2 mmHg (ᵅD; <0.001), respectively. Head up tilt (HUT) significantly decreased OPP by 16.5±2.5 mmHg (ᵅD; < 0.001). IOP was significantly higher in prone vs. supine position for much of the tilt range. Our study indicates that OPP is highly gravitationally dependent. Specifically, data show that MAPeye is more gravitationally dependent than IOP, thus causing OPP to increase during HDT and to decrease during HUT. Additionally, IOP was elevated in prone position compared to supine position due to the additional hydrostatic column between the base of the rostral globe to the mid-caudal plane, supporting the notion that hydrostatic forces play an important role in ocular hemodynamics. Changes in OPP as a function of changes in gravitational stress and/or weightlessness may play a role in the pathogenesis of spaceflight-associated neuro-ocular syndrome.

OBJECTIVE To evaluate the effect of soccer heading on intracranial pressure (ICP) in trained athletes. METHODS Eight subjects (all male, 21.7±1.7 years) were recruited from the UC San Diego soccer team. ICP and cardiovascular variables were recorded before and after 6 repeats of heading a soccer ball kicked from 35 yards away, into a goal. ICP was estimated non‐invasively from acoustical evoked tympanic membrane displacement (Vm), where a more negative volume displacement is correlated with higher ICP. Mean arterial pressure (MAP), heart rate (HR; Nexfin, Netherlands), and ICP were recorded in three different postures, 15° head up tilt (HUT), 0° supine, −15° head down tilt (HDT), in a randomized order. RESULTS Mean Vm at 15° HUT was 37.81±133.21 nL at baseline and −20.82±158.27 nL immediately post soccer heading (P=0.0482, paired t test), analogous to an increased ICP. At 0° supine, mean Vm was −46.19±178.50 nL at baseline and −84.27±183.55 nL after impact (P=0.0321, paired t test). At −15° HDT, mean Vm was −148.48±196.07 nL at baseline and −187.02±186.19 nL immediately post soccer heading (P=0.1176, paired t test). MAP and HR did not significantly change between the baseline and post soccer heading measurements. DISCUSSION After repeated soccer ball heading, ICP significantly increased at 15° HUT and 0° supine, while ICP at −15° HDT, MAP and HR did not significantly change. The relative increase in ICP associated with HDT is comparable to previously described trends. Although the measurement technique currently employed does not provide an absolute ICP baseline, the technique has been established as useful in evaluating relative changes. Under that interpretation, these results reflect what may be an elevated ICP related to soccer heading impacts. Accordingly, the results of this study suggest a potential health risk of participation in head contact sports. Support or Funding Information NASA (80NSSC19K0020) and Novo Nordic Foundation (NNF16OC0019196) Soccer Heading Increases Intracranial Pressure at Head Up Tilt and Supine Positions Volume displacement of tympanic membrane (Vm), measured non‐invasively from acoustical evoked tympanic membrane displacement, indicated an increase in intracranial pressure (ICP) after soccer heading at 15° head up tilt (HUT) and 0° supine (n = 8, paired t test, *P < 0.05). No significant increase in ICP at −15° head down tilt (HDT) was observed after soccer heading.