MODULE 9
The Nervous System and Radiation
After studying this module, the reader will be able to
Some people believe there is a higher-than-usual rate of nervous system disorders among individuals exposed to Hanford's releases of radioactive materials. This module summarizes research about disorders of the nervous system. Researchers know more about how high-dose radiation affects the nervous system than about how low-dose radiation affects it. Researchers also know more about exposures from external sources than from radioactive substances acting within the body. Hanford's releases, however, resulted in low whole-body doses from mainly internal exposure, according to estimates of the Hanford Environmental Dose Reconstruction Project (HEDR). Nonetheless, to understand the range of potential health effects, it is useful to look at what is known about the health effects of high-dose radiation from external exposures.
High doses of radiation can be defined as greater than 50 rem to the whole body. High-dose radiation is used to treat cancer. In the past it was used to treat benign conditions such as ringworm of the scalp and enlargement of the thymus gland in the neck. Some Japanese atomic bomb survivors, nuclear industry workers, and survivors of nuclear accidents also received high doses of radiation.
Health effects in people exposed to high radiation doses include effects on the brain, spinal cord, and peripheral nerves.
People treated with radiation for brain tumors often receive doses of a few thousand rad to the head, usually over a period of days or weeks. Health effects that can occur within days or weeks of treatment include swelling of the brain, seizures, paralysis, and confusion. Long-term effects that may occur include the destruction of brain cells, changes in the blood vessels of the brain, seizures, and confusion [1].
Ron et al conducted a study of Israeli children treated with radiation for ringworm of the scalp. The researchers estimated the dose to the brain for this group as between 100 and 200 rad. This group had a higher rate of cancerous and benign brain tumors than unexposed groups [2].
Other studies have also linked radiation exposure to the development of brain tumors [3]. These were studies of people treated with radiation for conditions of the head and neck, arthritis of the spine, and acute lymphocytic leukemia.
However, studies of Japanese atomic bomb survivors have not reported a link between high-dose radiation exposure and brain tumors [4]. While these studies found no evidence of a radiation effect for brain tumors, there is evidence of an increased risk for other nervous system tumors in people exposed to the atomic bomb explosions before they were 20 years old [5].
These atomic bomb studies have, however, found measurable nervous system effects on some children born to Japanese women who were pregnant during the bombings of Hiroshima and Nagasaki. The women received whole-body radiation doses ranging from 50 to 100 rad between eight and 25 weeks after conception. The children had an increased risk for small brain size and mental retardation, lower intelligence test scores, and decreased school achievement [6].
The spinal cord is generally more sensitive to the acute (short-term) effects of radiation than is the brain. It also takes less time for radiation-induced damage to the spinal cord to show up than similar injury to the brain.
Myelitis, or inflammation of the spinal cord, can occur within two to four months after a patient being treated with radiation is exposed to thousands of rad. Myelitis may have symptoms of tingling, prickling, and shock-like sensations.
Myelitis is sometimes delayed, not occurring until four months to three years after radiation exposure. This delayed effect is due to scarring of the spinal cord and is not a direct effect on spinal cord nerve cells. When delayed, a person may experience more severe problems such as paralysis and lack of bladder control [7].
Another delayed effect of high-dose radiation exposure to the spine is the development of spinal cord tumors years after the radiation exposure [8].
Peripheral nerves (which connect the brain and spinal cord to other parts of the body) are among the parts of the body most resistant to radiation [9]. Nonetheless, the Israeli children discussed above in the study by Ron and her colleagues had a higher rate of cancerous and benign tumors of peripheral nerves than people not exposed to radiation [10].
Localized treatment with high doses of radiation has also been reported to injure a group of nerves that extends from the lower neck to the underarm area. This group of nerves is called the brachial plexus.
Lowell Sever and others conducted a case-control study of children born in the Hanford area [11]. The researchers investigated if there was a link between a parent's work exposure to low-level external radiation and birth defects in that worker's children. The study investigated births between 1957 and 1980 since only a few hospital records were available for earlier years.
The researchers found that the higher the radiation dose received by parents before their children were born, the more likely the children were to be born with neural tube defects. Defects of the neural tube--which develops into the spinal cord and brain--occur when the tube fails to close completely during the early stages of pregnancy.
Researchers also reported a link between parental employment at Hanford and two non-neurologic birth defects: congenital dislocation of the hip and tracheoesophageal fistula, an abnormal connection between the trachea (the windpipe) and the esophagus (the part of the digestive tract that connects the mouth to the stomach). However, these two birth defects were not linked with parental exposure to radiation as not all Hanford workers were exposed to radiation at work. Many worked in offices and did not receive any occupational radiation exposure. This study did not include parents' exposure to radioactive materials outside of work, including environmental releases from Hanford, medical procedures, or background radiation.
Sever et al also conducted a study of the rate of birth defects in communities near the Hanford Site [12]. The researchers investigated whether rates of birth defects among infants in the Hanford area were higher than expected, potentially because of exposure to radioactivity from Hanford operations.
The researchers determined the rates of certain birth defects for Benton and Franklin counties in Washington state between 1968 and 1980. The Hanford Site covers part of both Benton and Franklin counties. These rates were compared to rates from the Centers for Disease Control's Birth Defects Monitoring Program for Washington, Oregon, and Idaho. While the overall rate of birth defects in Benton and Franklin counties was not greater than expected, the rate of certain neural tube defects was increased.
The researchers compared the study group to other radiation exposed groups. They considered the Hanford doses too low to account for the elevated rates of neural tube defects as being caused by radiation exposure. The researchers did not have an explanation for the elevated rates but proposed that exposure to agricultural chemicals be considered.
The study was conducted before any dose estimates were available from HEDR. Also, the study's dose estimate for the public includes only the years 1974 through 1980, during which there were limited Hanford operations and emissions.
Deaths from brain tumors are unusual in the general population (4.1 deaths per 100,000 people). In a work force of several thousand, even a small number of cases (two to five) points to a higher number of such deaths than would ordinarily be expected.
Alexander reviewed studies of workers at 10 nuclear facilities [13]. Radiation doses were available for workers at eight of the facilities. For three of the groups, doses included both external and internal exposure. Cumulative average whole-body doses ranged from 0.67 rem to 4.75 rem. There is a small group of workers who had higher doses, perhaps as high as 100 rem. However, individual doses for the workers who developed brain cancer were not provided.
The researcher concluded that there was a higher-than-expected number of deaths from brain cancer among the nuclear industry workers studied. While chemical exposure may contribute to the risk of cancer, the only common factor among the workers was exposure to radiation.
In summary, some researchers have linked certain nervous system health effects to radiation exposure. Health effects linked to high-dose exposure include cancerous and benign brain tumors in people exposed, and small brain size and mental retardation in children of women who were exposed during pregnancy. Health effects linked to low-dose exposure include brain cancer among nuclear industry workers.
Unfortunately, most of what is known about radiation's effects on the nervous system cannot be directly compared with the Hanford situation. Current knowledge comes from studies of people exposed to high organ doses or high whole-body doses of radiation received over minutes, hours, or days. In contrast, Hanford's releases resulted in some high radiation doses to the thyroid, some low doses to other organs, and low whole-body doses. These doses occurred over weeks, months, or years.
While most studies of radiation's effect on the nervous system involve external exposure, most of the dose from Hanford's releases came from internal radiation exposure. People were internally exposed when they drank contaminated milk or water, or ate contaminated foods. Because the circumstances of the studies discussed above differ from the Hanford situation, the conclusions of those studies do not necessarily apply to Hanford-exposed people.
Some people believe there is a higher-than-usual occurrence of nervous system diseases among people exposed to Hanford's releases of radioactive materials. Determining the occurrence of nervous system diseases can be difficult. For most of these diseases, only estimates are available of the number of cases in the United States, since there are no requirements to report nervous system diseases to health officials. Little or no information has been collected about the number of cases of many of these diseases among people exposed to Hanford's releases of radioactive materials. As a result, it is not possible, with the information currently available, to determine if the occurrence of nervous system diseases among Hanford downwinders is higher than usual.