To the Editor: There is a need for better knowledge about the role of microgravity in physiological response in astronauts. Microgravity has relevant repercussions that lead to structural remodeling of the neural tissue and changes in brain autoregulation. In the present article, Marshall-Goebel et al1 offered an important analysis of the effect of microgravity on the central nervous system. Their analysis provides an important insight into the need to know more about intracranial physiology. There are certain aspects that need to be understood while discussing neurological changes in outer space. Spaceflight-associated neuro-ocular syndrome (SANS) is a condition of concern in space travel.2 The structural changes (optic disc edema, hyperopic shifts, and choroidal folds) that are not related to vision impairment of the crew and that have an anatomical substratum can correspond to adaptive changes. Countermeasure strategies have been postulated for the management of this condition; among the multiple measures proposed is the use of acetazolamide to decrease intracranial pressure (ICP). Scott et al3 report using swimming goggles for artificially increasing the intraocular pressure. EFFECT OF HYPOGRAVITY The force of gravity is reduced in the outer space, and, hence, sensory perception, and vestibular and proprioceptive functions recalibrate and lead to a wasting of muscles and loss of bone mass. Along with this, there is a headward shift of fluids resulting in cardiovascular decompensation. This cephalad shift of fluids with a reduced venous outflow leads to an increased ICP. This is in addition to the metabolic changes and vasodilatory effects of carbon dioxide on the ocular system. When these astronauts return to earth, they develop the "visual impairment and intracranial pressure (VIIP) syndrome," a term coined by NASA, which has recently been termed SANS. This usually develops after a minimum duration of 6 mo in space and results in papilledema, posterior globe flattening, hyperopic shift, and choroidal fold formation as a result of optic nerve sheath distention. Other factors that have been postulated to play a role are the body weight of the patient, the "ocular glymphatic system," and alterations in folate- and vitamin B12-dependent one-carbon metabolism.4 Optic coherence tomography (OCT) is a noninvasive method to quantify retinal thickness and was done onboard the International Space Station and has demonstrated an increase in the optic nerve head diameter, thus objectively demonstrating optic nerve head edema. Fundus photographs have also demonstrated cotton-wool spots, which are markers of retinal exudates, and flame-shaped hemorrhages, which have been confirmed on OCT angiographies. These are all markers of an increased intracranial pressure, but the real question is: What is normal pressure in a spaceflight? ICP has never actually been measured in a human in a spaceflight, and the only reference available is the ICP measured invasively in a Macaque monkey in 1992, during a 10-d Bion satellite flight, with an intracranial (epidural) probe, which showed a rise on day 1 of the spaceflight.5 This is in addition to another rarely talked about condition of "Space headache," which was recently proposed by Vein et al6 to be considered as a separate entity among the secondary headaches in the International Classification of Headache Disorders (ICHD-II). Space motion sickness is another aspect that has been attributed to changes in the neurovestibular system and includes symptoms of dizziness, vertigo, hypersalivation, vomiting, fatigues, and sleeplessness. These symptoms are probably due to the angular acceleration suffered during re-entry after the microgravity environment of space. LONG-TERM EFFECTS Following the return of the astronauts, magnetic resonance imaging (MRI) scans showed a certain permanent alternation in these cases, such as the narrowing of the central sulcus, upward shift of the brain after all long-duration flights, narrowing of cerebrospinal fluid spaces at the vertex, optic disc edema, white matter hyperintensities on MRI along with a reduced neurocognitive function are the changes that these "superhealthy" individuals will have.7 They also have a 53% to 68% increased risk of low back pain and a 4-fold increased risk of developing herniated intervertebral discs due to atrophy and reduced motor control of the lumbar multifidus and transversus abdominis muscles. All these changes are a result of deconditioning as a result of microgravity, and to counter these, astronauts have to exercise for a minimum of 2.5 h, which includes running, cycling, and strength training. After their stay at the International Space Station, these individuals develop changes in posture, which include an increased flexion with a loss of normal spinal curvature and lengthening of the spine.8 These are all changes that were experienced in a controlled and monitored environment of the space station and were shown in an exponentially drastic situation of an abandoned astronaut in the wildly popular motion picture "The Martian," in which the protagonist lost most of his body mass and developed skin ulcerations and nonhealing after an extremely long hypothetical stay on Mars on a budget diet with little to no exercise or monitoring. There is an increasing need to understand these remarkable, individuals and their journey as a spaceflight is now a reality and the near future may require neurosurgeons and neurologists to evaluate cases for space tourism, which will breed a special branch of "Space Medicine." We congratulate the authors1 for highlighting an important contributor to a relevant aspect of futuristic medicine, which is going to be an integral component in human evolution. Disclosures The authors have no personal, financial, or institutional interest in any of the drugs, materials, or devices described in this article.