In this issue of The American Journal of Medicine, Lipman et al report on therapy for a serious environmental medical condition.1 My commentary below describes an even more serious environmental condition with multiple dangerous and even life-threatening consequences.
Recently, one of my colleagues and I at the University of Arizona heard a lecture by one of the retired US astronauts. The lecture was truly fascinating, but one of the points made was that astronauts noted multiple small light flashes when they closed their eyes during space flight. These were subsequently shown to be the result of cosmic radiation striking their retinas. The speaker also related that spending time in zero gravity markedly disturbs the body’s baroreceptor function, which can result in severe orthostatic hypotension upon return to the earth. As I pondered this information, it occurred to me that other physiologic systems might be altered by time spent in zero gravity. I consulted my usual reliable sources of information, PubMed and the Internet, and was surprised to find a wealth of information involving alterations in a variety of bodily functions that result from space flight.2, 3, 4, 5, 6, 7, 8, 9, 10, 11 This should not be a surprise because approximately 2 billion years of evolutionary pressure have adapted all of us living here on Planet Earth to the gravitational forces associated with our home.
As you might imagine, the National Aeronautics and Space Administration has been very interested in the health consequences of space travel and has performed numerous clinical investigational studies on our astronauts before, during, and after varying periods of time spent in space flight. A large variety of abnormal physiologic and biochemical changes have been observed, some of which constitute potential and real health problems for the returning astronauts. For example, the tremendous gravitational forces applied during takeoff (4 times normal earth gravity) push blood volume to the periphery, thereby lowering systemic arterial pressure. Some astronauts have even experienced transient syncope during takeoff because of this redistribution of blood volume during ascent.2, 3, 5, 7 Once the astronauts arrive in zero gravity and float around in their capsule or space station, there is almost invariably a period of nausea and even vomiting related to vestibular disturbances. Our balance system is geared for earth gravity and not for zero gravity. Time and medication usually results in improvement with this unpleasant set of symptoms. Other bodily sensations are also affected: a number of the astronauts reported loss of proprioception with resultant inability to ascertain where their limbs were located. Once in zero gravity, blood volume redistributes again, this time with a larger than normal percentage going to the upper body, including the head and the brain. A variety of central nervous system abnormalities have been documented, including changes in vision, with some astronauts experiencing retinal edema.4, 5 Moreover, abnormalities in cognitive function, circadian rhythms with associated sleep deprivation, nasal congestion, and facial edema have been noted. Intracranial pressure is increased, which in some cases resulted in compression of the nerves connecting the visual cortex to the retina. In addition, psychological effects have been noted, such as anxiety, depression, and insomnia, perhaps related to prolonged containment in a small space.6
One of the most prominent effects of space travel is loss of bone and skeletal mass. Astronauts lose between 1% and 2% of their bone mass per month while in space. Bone osteoclast activity is markedly accelerated. Consequently, blood calcium levels rise, leading to a heightened risk for kidney stones.3, 5 Upon return to earth’s gravitation, bone calcification resumes but may take as much as 2 to 3 years before returning to normal. Lower extremity muscles atrophy because no muscular effort is required to stand because individuals float during space travel. Upon return to the earth a considerable amount of exercise and physical therapy is required to restore some if not all muscle function. In addition, upon return to earth, astronauts experience considerable fatigue and muscle weakness. Heart muscle also atrophies because the work of the heart is reduced in space. This is also a reversible problem over time once the astronauts return to earth.
One of the biggest risks with space travel is the large dose of radiation that space travelers encounter.8, 10 Close to the earth, for example on the International Space Station, astronauts encounter mostly cosmic rays, which can damage DNA but are not as dangerous as particles with increased atomic numbers. These latter particles are both heavy and highly charged.10 They can cause serious disruption to DNA. Heavy charged particles are present in outer space once astronauts are no longer near earth’s protective environment. There is definitely an increased risk of developing a malignancy when individuals are exposed to these heavy particles for a substantial period. Because the trip to Mars will take 6 months getting there and 6 months returning, space travelers making this journey will receive a substantial amount of dangerous radiation. Another genomic problem has been encountered in rodent experiments carried out by astronauts during space flight. Examination of the thymus gland in “astronaut mice” revealed an increase in cancer-related genes, suggesting that space flight is indeed a risk for carcinogenesis.5
The immune system and healing properties of the body are also negatively affected by zero gravity, making space travelers more prone to infection.5, 9 A particularly disturbing observation was that pathogenic bacteria thrive in zero gravity and may even remain viable in the hostile space environment. Wound-healing experiments in rodents during space travel also demonstrated delayed healing and lower levels of trophic molecules associated with the healing process.
Thus, it seems clear that space travel involves a great number of health risks above and beyond the possibilities for technical failure. Given the ease with which characters in “Star Wars” and “Star Trek” handle space travel, it is likely that many individuals who are planning to sign up for the trip to Mars may not fully understand the risks and the discomfort involved. Recently Scott Kelly, an experienced US astronaut, returned from spending 1 year on the International Space Station.11 He graphically describes the problems he encountered while on the space station and, more importantly, how painful it was physically and emotionally when he returned to Mother Earth. I think space travel is definitely an area in which there is sound advice in the saying, “caveat emptor, let the buyer beware.”
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-Joseph S. Alpert, MD (Editor-in-Chief, The American Journal of Medicine)
This article originally appeared in the February 2018 issue of The American Journal of Medicine.
