Effects of Gravity and Lower Body Negative Pressure on Intracranial Pressure; Implications for Astronauts and Patients on Earth
Authors: Petersen Lonnie G , Hargens Alan , Petersen Johan , Cyron Alexander , Juhler Marianne 
Published: November 2023

Introduction

Up to 70% of astronauts on long‐term space‐missions experience loss of visual accuracy along with some degree of optic disc edema, increased optic nerve sheath diameter, and choroidal folds. This is referred to as the Visual Impairment and Intracranial Pressure (VIIP) syndrome, because the structural and functional changes resample that of patients on Earth suffering from elevated intracranial pressure (ICP). A possible mechanism is the microgravity induced cephalic fluid‐shift and loss of normal diurnal and postural changes in pressure. However, owing to the gab in knowledge regarding both normal physiology and pathology of regulatory mechanisms of ICP, we currently no effective countermeasures for VIIP in space, nor reliable effective treatments for intracranial hypertension for patients on Earth. This study aimed to characterize effects of fluid‐shift by changes in posture and application of lower‐body negative‐pressure (LBNP) on ICP and cerebral perfusion pressure (arterial blood pressure at brain‐level minus ICP).

Methods

During a three‐year period, from a total of some 250 patients, we identified fifteen ambulatory neurosurgical patients who were “as normal as possible”. 24hrs invasive ICP monitoring were performed as part of their diagnostic work‐up at the Neurocernter, Rigshospitalet, Copenhagen, Denmark. The strict inclusion criteria included a “normal” 24‐hour ICP‐profile, normal cognitive function, and absence of focal lesions, global edema, or shunts. ICP was measured via tip‐transducer catheter in the brain‐parenchyma. Cardiovascular variables were measured non‐invasively with a cuff around a finger. In (N=9) passive fluid‐shift was induced by posture changes from standing to 20 degrees head‐down tilt and in additionally (N=6) graded LBNP at increments of 10 mmHg were applied during head‐down tilt and supine position.

Results

Compared to the supine position ICP increased during 10° and 20° of head‐down tilt (from 9.4 ± 3.8 to 14.3 ± 4.7 and 19 ± 4.7 mmHg; P<0.001). Conversely, 10° and 20° of head‐up tilt reduced ICP to 4.8 ± 3.6 and 1.3 ± 3.6 mmHg and ICP reached −2.4 ± 4.2 in the standing posture (P<0.05). Concordant changes in mean arterial pressure maintained cerebral perfusion pressure unchanged 77 ± 7 mmHg.

In N= 6; application of LBNP from 0 to 50 mmHg at increments of 10 mmHg, decreased ICP from a supine value of 15.2 ± 2.9 mmHg to 13.7 ± 4.3; 12.9 ±5.4; 11.8 ± 4.8; 10.8 ± 4.9; 10 ± 4.74 mmHg. Similarly, the same increments of LBNP during 15° of head‐down tilt decreased ICP from 26.1 ± 4.3 mmHg to 21.4 ± 3.8; 19.7 ± 4; 18.3 ± 3.9; 16.8 ± 3.4; 16.6 ± 3.9 mmHg.

Discussion

Positional and gravitational effects on ICP are greater than previously described. Although currently not ascribed a significant role for maintaining cerebral health, data from space suggest that the brain is “calibrated to the gravitational circadian pressure rhythm on Earth. Mimicking the effects of gravity by LBNP could be an important countermeasure for astronauts in space as well as providing a new treatment perspective for patients on Earth.

Support or Funding Information

Funding Sources The Novo Nordic Foundation, Denmark