Astronaut

During long-term missions, some astronauts experience structural and functional changes of the eyes and brain which resemble signs/symptoms experienced by patients with intracranial hypertension. Weightlessness prevents the normal cerebral volume and pressure “unloading” associated with upright postures on Earth, which may be part of the cerebral and ocular pathophysiology. By placing the lower body in a negative pressure device (LBNP) that pulls fluid away from cranial compartments, Petersen et al. from the Department of Biomedical Sciences, Faculty of Health Sciences, University of Copenhagen, Department of orthopedic surgery, University of California, San Diego, Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Dallas, Institut für Sportwissenschaft, Universität Innsbruck, Department of Neurosurgery, Rigshospitalet, Copenhagen, Baker Heart and Diabetes Institute, Melbourne, Department of Internal Medicine, Division of Cardiology. University of Colorado Anschutz Medical Campus, Aurora, CO, University of Washington School of Medicine, Departments of Neurology and Neurological Surgery, Seattle, simulated effects of gravity and significantly lowered pressure within the brain parenchyma and ventricle compartments. Application of incremental LBNP demonstrated a non-linear dose-response curve suggesting 20 mmHg LBNP as the optimal level for reducing pressure in brain without impairing cerebral perfusion pressure. This non-invasive method of reducing pressure in the brain holds potential as a countermeasure in space as well as treatment potential for patients on Earth with traumatic brain injury or other pathology leading to intracranial hypertension.

Patients with elevated intracranial pressure (ICP) exhibit neuro-ocular symptoms including headache, papilledema, and loss of vision. Some of these symptoms are also present in astronauts during and after prolonged space-flight where lack of gravitational stress prevents daily lowering of ICP associated with upright posture. Lower body negative pressure (LBNP) simulates the effects of gravity by displacing fluid caudally and we hypothesized that LBNP would lower ICP without compromising cerebral perfusion. Ten cerebrally intact volunteers were included: 6 ambulatory neurosurgical patients with parenchymal ICP-sensors and 4 former cancer patients with Ommaya-reservoirs to the frontal horn of a lateral ventricle. We applied LBNP while recording ICP and blood pressure while supine, and during simulated intracranial hypertension by 15° head-down tilt. LBNP from 0-50 mm Hg at increments of 10 mmHg lowered ICP in a non-linear dose-dependent fashion; when supine (N = 10), ICP was decreased from 15 ± 2 mmHg to 14 ± 4, 12 ± 5, 11 ± 4, 10 ± 3, 9 ± 4, respectively (P < 0.0001). Cerebral perfusion pressure (CPP), calculated as mean arterial blood pressure at midbrain-level minus ICP, was unchanged (from 70 ± 12 mmHg to 67 ± 9, 69 ± 10, 70 ± 12, 72 ± 13, 74 ± 15; P = 0.02). 15° head-down tilt (N = 6) increased ICP to 26 ± 4 mmHg, while application of LBNP lowered ICP (to 21 ± 4, 20 ± 4, 18 ± 4, 17 ± 4, 17 ± 4; P < 0.0001) and increased CPP (P < 0.01). Twenty mmHg LBNP may be the optimal level to lower ICP without impairing CPP to counteract spaceflight associated neuro-ocular syndrome in astronauts. Furthermore, LBNP holds clinical potential as a safe, non-invasive method for lowering ICP and improving CPP for patients with pathologically elevated ICP on Earth ¹⁾.

Among possible causes of visual impairment or headache experienced by astronauts in microgravity or post-flight and that hamper their performance, elevated intracranial pressure (ICP) has been invoked, but never measured for lack of non-invasive methods. The goal of this work was to test two noninvasive methods of ICP monitoring using in-ear detectors of ICP-dependent auditory responses, acoustic and electric, in acute microgravity afforded by parabolic flights. The devices detecting these responses were hand-held tablets routinely used in otolaryngology for hearing diagnosis, customized for ICP extraction and serviceable by unskilled operators. These methods had been previously validated against invasive ICP measurements in neurosurgery patients. The two methods concurred in their estimation of ICP changes with microgravity, i.e., 11.0 {plus minus} 7.7 mmHg for the acoustic method (n = 7 subjects with valid results out of 30, auditory responses being masked by excessive in-flight noise in 23 subjects), and 11.3 {plus minus} 10.6 mmHg for the electric method (n = 10 subjects with valid results, out of 10 tested despite the in-flight noise). These results agree with recent publications using invasive access to cerebrospinal fluid in parabolic flights and suggest that acute microgravity has a moderate average effect on ICP, similar to body tilt from upright to supine, yet with some subjects undergoing large effects while others seem immune. The electric in-ear method would be suitable for ICP monitoring in circumstances and with subjects such that invasive measurements are excluded ²⁾.