Before discussing acute radiation syndrome (ARS), the concepts of both cell death and the LD₅₀ must be understood.Cell death is defined as the stopping of cell division and not the killing of the cell outright. Significant doses of radiation can prevent cell division, whereas very high doses are required to kill the cell outright. Rapidly dividing cell populations within the body are very sensitive to radiation such as the hematopoietic cell lines of the bone marrow and the epithelial cells lining the small intestine.

Our body senses cannot detect radiation, so we must rely on instruments to measure any exposure. Experiments were carried out on animals in the late 1940s and early 1950s to determine the acute dose of whole body exposure required to kill 50% of the animals in a set period of time, usually 30 days, known as the LD_50/30. The dose required varied from animal species to species. In general, the smaller the animal, the higher the LD_50/30. For example, the goat requires only 3.5 Gy (350 rads), whereas the frog must receive the higher dose of 7 Gy (700 rads). Other factors influence the susceptibility to radiation, including age, sex, genetic considerations, health, and the nutritional status of the animals. With respect to humans, very limited information is available to be certain
of the LD₅₀. With information from accident data and results from human studies on patients after the dropping of the atomic bombs on Hiroshima and Nagasaki, the LD₅₀ for humans is believed to be in the region of 2.5 to 4.5 Gys (250 to 450 rads). ARS in humans takes longer to express itself compared to animal data and is closer to 60 days. Therefore in humans we talk about the LD_50/60, defined as the lethal dose required to kill 50% of the population within 60 days.

To produce classical ARS, certain parameters are required. Of particular importance is the presence of a high dose delivered very rapidly at a high dose rate. Also the radiation must be penetrating, and it is usually the result of whole body exposure. Note that the source of radiation is not important. It is immaterial what the radioactive source is, whether the source is from a reactor or from sealed or unsealed containers used in industry or medicine. If the dose is high enough, it will produce the same biological damage.

ARS follows a set course, which will express itself within hours to weeks depending on the level of dose received. The syndrome can be divided into four phases: (a) the prodromal phase, (b) the latent phase, (c) the illness phase, and (d) the phase of final outcome, either recovery or death. ARS can be expressed as injury to several major organ systems including the hematopoietic, gastrointestinal, pulmonary, and finally the cardiovascular and central nervous system (CVS/CNS) syndrome. The expression of damage to these organ systems depends on the dose received. The hematopoietic syndrome is seen within the dose range of 2 to 8 Gy (200 to 800 rads). The gastrointestinal syndrome is seen in the dose range of 8 to 30 Gy (800 to 3,000 rads). The CVS/CNS syndrome is in the range of 30 Gy (3,000 rads) and upward. Note that as the dose increases, the survival time decreases. If untreated, death from the hematopoietic syndrome will occur within 60 days. Death from the gastrointestinal syndrome will occur within 1 to 2 weeks, and CVS/CNS syndrome death is the most rapid, usually occurring within 48 hours (Table 16-1).

After exposure to radiation, several vague, nonspecific symptoms begin to occur, known as the prodromal phase. The time to onset of the prodromal phase of signs and symptoms can be from minutes to hours depending on the dose received. Anorexia, vomiting, and diarrhea are the main symptoms and signs. The most important indicator of prognosis is the time of onset of vomiting following irradiation. Vomiting within 2 to 4 hours is an indication of a high dose of radiation. Other signs and symptoms are the development of headache and a rise in core temperature. At doses below 1 Gy (100 rads) there are very few signs and symptoms, and these patients can be sent home and followed up on an outpatient basis. At doses of 1 to 2 Gy (100 to 200 rads), vomiting will develop in a number of patients, and it is advisable to admit these patients for observation. At doses in the range of 2 to 4 Gy (200 to 400 rads), vomiting is noted in the majority of patients. This occurred within the first 2 to 8 hours. In doses of 4 to 8 Gy (400 to 800 rads), vomiting occurs in only 2 hours. At doses above 8 Gy, vomiting is severe, starting much earlier, within the first hour or so.

A word of caution is warranted from the experience of several radiation accidents around the world. The prodromal phase is sometimes misdiagnosed or not even seen. In several cases the patients were thought to have “food poisoning,” and it was only after a fall in the white cell count (lymphocytes, granulocytes, and platelets) that medical personnel realized they were dealing with a radiation incident. Another lesson to be learned was from the Goiania accident in Brazil, where out of the 112,000 people monitored, approximately 8,000, who had no radiation exposure presented with either erythematous rashes or nausea or vomiting. Of course these people were concerned that they, too, might have been exposed (1).

During the latent phase, the patient appears asymptomatic. This phase typically lasts from 2 to 4 weeks. However, if the dose received by the patient is very high, the latent phase can be shortened. Occasionally, the patient goes directly into the illness phase. During the latent phase, the patients are asymptomatic; however, it is during this period that rapidly dividing cell populations become depleted and patients begin to develop infections. These patients will usually require prophylactic antibiotics, antiviral agents, and antifungal agents.

The illness phase is expressed by the damage to the specific organ system and depends on the level of whole body exposure dose received by the patient. Great strides have been made toward treatment of the hematological system, but unfortunately this is not the case for the “full-blown” gastroin-
testinal syndrome or the CVS/CNS syndrome. For the latter two, at present no treatment is available that can confidently bring about a full recovery.

Recovery of these patients can take weeks to months and follow-up will be required for the rest of their lives.

When assessing a casualty from a terrorist event, a number of distinct scenarios are possible. The patient could have injuries resulting from the use of explosives. The patient may have received whole body or partial body irradiation, and the patient may be externally or internally contaminated. Various combinations of these may occur together in a single patient. The time interval between when patients received the dose to when they develop signs and symptoms could range from a few days up to 3 weeks.

Remember that the irradiated patient is not a “medical emergency.” No immediate intervention will make a difference to the survival of the patient. When caring for a traumatized irradiated patient, your first priority is to stabilize the patient for the trauma, not radiation. After the patient has been stabilized, proceed to focus on the problems of irradiation and contamination. Note that radiological contamination differs from chemical decontamination, where early decontamination is the rule. This is because with radiation exposures, the causative agent can be detected immediately with instrumentation. Also, staff members are wearing protective clothing with dosimeters. It is therefore possible to monitor them and have them rotate into or out of the environment if needed. Hence, with respect to radiation, it is often said, “You will be thanked for a live contaminated patient; however, you will not be thanked for a clean corpse.”

Several important variables may be obtained from simply talking to the awake and oriented patient. For example, did the patient develop nausea, vomiting, and diarrhea recently? What time did the symptoms start, how long did they last, and was it diagnosed as food poisoning? Did the patient receive any treatment for the vomiting? Other important information may be obtained by directed questions. Were any other people involved? Was the patient aware of where he or she was on that particular day? If the patient noticed skin redness, when and where did it occur? Did it disappear, has it reoccurred, and are there any areas of dry peeling or blistering of the skin?

The past medical history of the patient is also important. Ask if there is any history of renal disease, for example, because most interventional drugs used for removal of internal radioactive contamination are excreted through the kidneys. Has the patient had any nuclear medicine procedures with radioactive isotopes? If so, when were these performed? There has been the odd case where it was believed the patient was contaminated with radiation, only to learn later the patient had a recent nuclear medicine procedure. Inquire about the family medical history, and then carry out a full medical examination. If anything suggests radiation injury, photograph the involved site (2).