Radiation-induced cognitive decline describes the possible correlation between radiation therapy and cognitive impairment. Radiation therapy is used mainly in the treatment of cancer. Radiation therapy can be used to cure, care or shrink tumors that are interfering with quality of life. Sometimes radiation therapy is used alone; other times it is used in conjunction with chemotherapy and surgery. For people with brain tumors, radiation can be an effective treatment because chemotherapy is often less effective due to the blood–brain barrier. Unfortunately for some patients, as time passes, people who received radiation therapy may begin experiencing deficits in their learning, memory, and spatial information processing abilities. The learning, memory, and spatial information processing abilities are dependent on proper hippocampus functionality. Therefore, any hippocampus dysfunction will result in deficits in learning, memory, and spatial information processing ability. The hippocampus is one of two structures of the central nervous system where neurogenesis continues after birth. The other structure that undergoes neurogenesis is the olfactory bulb. Therefore, it has been proposed that neurogenesis plays some role in the proper functionality of the hippocampus and the olfactory bulb. To test this proposal, a group of rats with normal hippocampal neurogenesis (control) were subjected to a placement recognition exercise that required proper hippocampus function to complete. Afterwards a second group of rats (experimental) were subjected to the same exercise but in that trial their neurogenesis in the hippocampus was arrested. It was found that the experimental group was not able to distinguish between its familiar and unexplored territory. The experimental group spent more time exploring the familiar territory, while the control group spent more time exploring the new territory. The results indicate that neurogenesis in the hippocampus is important for memory and proper hippocampal functionality. Therefore, if radiation therapy inhibits neurogenesis in the hippocampus it would lead to the cognitive decline observed in patients who have received this radiation therapy. In animal studies discussed by Monje and Palmer in "Radiation Injury and Neurogenesis", it has been proven that radiation does indeed decrease or arrest neurogenesis altogether in the hippocampus. This decrease in neurogenesis is due to apoptosis of the neurons which usually occurs after irradiation. However it has not been proven whether the apoptosis is a direct result of the radiation itself or if there are other factors that cause neuronal apoptosis, namely changes in the hippocampus micro-environment or damage to the precursor pool. Determining the exact cause of the cell apoptosis is important because then it may be possible to inhibit the apoptosis and reverse the effects of the arrested neurogenesis.
Radiation therapy Ionizing radiation is classified as a neurotoxicant. A 2004 cohort study concluded that irradiation of the brain with dose levels overlapping those imparted by computed tomography can, in at least some instances, adversely affect intellectual development. Radiation therapy at doses around "23.4 Gy" was found to cause cognitive decline that was especially apparent in young children who underwent the treatment for cranial tumors, between the ages of 5 and 11. Studies found, for example, that the IQ of 5-year-old children declined each year after treatment by additional several IQ points, thereby the child's IQ decreased and decreased while growing older though may plateau at adulthood. Radiation of 100 mGy to the head at infancy resulted in the beginning appearance of statistically significant cognitive-deficits in one Swedish/radiation-therapy follow-up study. Radiation of 1300-1500mGy to the head at childhood was similarly found to be roughly the threshold dose for the beginning increase in statistically significant rates of schizophrenia. From soliciting for participants in a study and then examination of the prenatally exposed at Hiroshima & Nagasaki, those who experienced the prompt burst of ionizing radiation at the 8-15 and 16–25 week periods after gestation were to, especially in the closest survivors, have a higher rate of severe intellectual disability as well as variation in intelligence quotient (IQ) and school performance. It is uncertain, if there exists a threshold dose, under which one or more of these effects, of prenatal exposure to ionizing radiation, do not exist, though from analysis of the limited data, "0.1" Gy is suggested for both.
Warfare
Adult humans receiving an acute whole body incapacitating dose (30 Gy) have their performance degraded almost immediately and become ineffective within several hours. A dose of 5.3 Gy to 8.3 Gy is considered lethal within months to half of male adults but not immediately incapacitating. Personnel exposed to this amount of radiation have their cognitive performance degraded in two to three hours. Depending on how physically demanding the tasks they must perform are, and remain in this disabled state at least two days. However, at that point they experience a recovery period and can perform non-demanding tasks for about six days, after which they relapse for about four weeks. At this time they begin exhibiting symptoms of radiation poisoning of sufficient severity to render them totally ineffective. Death follows for about half of males at approximately six weeks after exposure. Nausea and vomiting generally occur within 24–48 hours after exposure to mild (1–2 Gy) doses of radiation. Headache, fatigue, and weakness are also seen with mild exposure. Exposure of adults to 150−500 mSv results in the beginning observance of cerebrovascular pathology, and exposure to 300 mSv results in the beginning of the observance of neuropsychiatric and neurophysiological dose-related effects. Cumulative equivalent doses above 500 mSv of ionizing radiation to the head, were proven with epidemiological evidences to cause cerebrovascular atherosclerotic damage, thus increasing the chances of stroke in later life. The equivalent dose of 0.5 Gy (500 mGy) x-rays is 500 mSv.
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