Acute radiation syndrome
Acute radiation syndrome (ARS), also called radiation sickness, is the result caused by receiving a dose greater than 75 rads (0.75 Gray (Gy)) of ionizing radiation to the body in a short time (usually minutes). For this definition to apply, the dose must be over most or all of the body. The source is usually external, such as a concentrated radioisotope, a beam generator, a nuclear reactor, or a nuclear weapon.
Other radiation syndromes can be caused by localized radiation, or a cumulative whole-body exposure over a length of time (e.g., Goiana incident).
Unless the exposure causing the syndrome was caused by a well-understood incident to a small number of people, patients presenting with the syndrome should cause an emergency management response.
- The radiation dose must be large (i.e., greater than 0.7 Gray (Gy) ( 70 rads). Mild symptoms may be observed with doses as low as 0.3 Gy or 30 rads.
- The dose usually must be external (i.e., the source of radiation is outside of the patient’s body). Radioactive materials deposited inside the body have produced some ARS effects only in extremely rare cases.
- The radiation must be penetrating [i.e., ionizing]. High energy X-rays, gamma rays, and neutrons are penetrating radiations.
- The entire body (or a significant portion of it) must have received the dose. Local exposure, as to the hands in an industrial accident, does not produce ARS.
- The dose must have been delivered in a short time (usually a matter of minutes).
The syndrome has four basic stages. With extremely high exposures, the patient may progress from prodrome to death in hours. If the radiation injury itself is fatal, death will occur in no more than a few months. It is possible, however, to have a fatal outcome from a secondary disorder, such as infection while immunosuppressed.
Called "NVD" for nausea, vomiting, as well as anorexia and possibly diarrhea (depending on dose), which occur from minutes to days following exposure. The time of onset of the various symptoms is a valuable guide to assessing exposure.
Very high exposures, of 3000 rad/30 Gray or more, may cause temporary loss of consciousness; at even higher levels, seen in a few industrial accidents where there was no other trauma, consciousness was never gained after an exposure in the 100 Gray range.
Skin effects can be complex to evaluate. Depending on the mechanism of radiation, burns that appear immediately may be thermal rather than radiation injury, although complex mechanisms, not fully understood, are involved in radiation effects on the skin.
The patient looks and feels generally healthy for a few hours or even up to a few weeks. Laboratory studies, however, may show pathology after hours or days. There may be a gradual transition into the next stage, with progressive hair loss, bleeding, swelling of the mouth and throat, and general loss of energy may follow. Again, the time of occurrence is important.
Manifest illness stage
In this stage the symptoms depend on the specific syndrome and last from hours up to several months. It is characterized by intense immunosuppression and is the most difficult to manage. If the person survives this stage, recovery is likely.
Recovery or death
Most patients who do not recover will die within several months of exposure. The recovery process lasts from several weeks up to two years.
In industrial, laboratory, or medical treatment accidents, where there may have been instrumentation, that is the best source. There are, however, ways to estimate dose based on laboratory studies, preferably with expert consultation.
The most basic screening is to take several complete blood counts (CBC) preferably within 8-12 hours of exposure, taken every 2-3 hours. Useful data can still be obtained from CBCs taken every 4-6 hours for at least two days, with special attention to changes in the lymphocyte count.
A 50% decline in absolute lymphocyte count within the first 24 hours after exposure, followed by a further, more severe decline within 48 hours, characterizes a potentially lethal exposure. If granulocyte counts show a transient increase before decline, termed an abortive rise, this may suggest the exposure is survivable. 
The earliest symptoms are nausea, fatigue, vomiting, and diarrhea. The severity of injury rises with the dose received. In the discussion below, the exposure is expressed in the pathology caused by that dose of gamma radiation; beta and X-ray exposures are comparable. If the exposure was to neutrons or protons, which can cause more damage with lesser exposure, special expertise will be required to assess the severity of exposure, the clinical picture always being paramount.
There are three basic syndromes, of which only the first, and very rarely the second, is considered survivable. Full recovery is possible; the mortality increases with the dose received.
Bone marrow (hematopoietic syndrome)
Susceptible individuals may develop the symptoms with a dose as low as 0.3 Gy/30 rads, but the usual threshold is between 0.7-10 Gy/70-1000 rads. The primary cause of death is destruction of the blood marrow, leading to bleeding or infection. Serial blood studies can assess the damage.
When not dealing in mass casualties, it is appropriate to use short-term treatment with cytokines that stimulate granulocyte production, although their use has not been approved, for acute radiation syndrome, by the U.S. Food and Drug Administration. It is also rational, especially if the patient has survived for an appreciable time, to try erythrocyte stimulating factors if anemia is present. With higher exposures, extended cytokine therapy, blood component transfusion, and even stem-cell transplantation may be appropriate when exposure dose is high (>7 Gy). 
If transfusions are used, the blood components must be reduced in white cells, which, while it may seem counterintuitive, is done by exposing them to 25 Gy of radiation. Remember that a transfusion is a form of organ transplant, and the possibility of graft vs host disease (i.e., rejection) is always present. The factors, in blood products, which predispose to rejection are suppressed by radiation.
Patients showing immunosuppression, without a major marrow failure, may be helped by the same sort of prophylactic antibiotics used for AIDS, etc.
Gastrointestinal (GI) syndrome
This usually takes a dose of 10 Gy/1000 rads. although it has been seen at 6 Gy/600 rads. Note that this syndrome's triggering dosage overlaps the bone marrow syndrome; individuals vary in sensitivity to radiation.
Survival is highly unlikely, but, in other than a mass casualty incident, cases need to be considered individually. Destructive and irreparable changes in the GI tract and bone marrow usually cause infection, dehydration, and electrolyte imbalance. Death usually occurs within 2 weeks.
Cardiovascular (CV)/ Central Nervous System (CNS) syndrome
The usual threshold for this universally fatal syndrome is 50 Gy/5000 rads, but it has developed in people exposed to 20 Gy/2000 rads. Its prodrome ranges from loss of consciousness to confusion and anxiety. At sufficiently high levels, death can be effectively instantaneous, and from minutes to tens of hours at lower dose rates. Unconsciousness may occur within minutes of doses on the order of 100 Gy. There may be skin burns. NVD may develop almost immediately.
Death occurs in no more than 3 days. Death likely is due to collapse of the circulatory system as well as increased intracranial pressure caused increased fluid content caused by edema, vasculitis, and meningitis.
- Anonymous (2023), ionizing radiation (English). Medical Subject Headings. U.S. National Library of Medicine.
- Centers for Disease Control, Acute Radiation Syndrome: A Fact Sheet for Physicians
- Waselenko,JK et al. (15 June 2004), "Medical Management of the Acute Radiation Syndrome: Recommendations of the Strategic National Stockpile Radiation Working Group", Annals of Internal Medicine 140 (12): 1037-1051
- Anonymous (2023), graft vs host disease (English). Medical Subject Headings. U.S. National Library of Medicine.
- Strom, D.J. (September 2003), "Health Impacts from Acute Radiation Exposure", Pacific Northwest National Laboratory, PNNL-14424, p. 28