Exposure to high levels of penetrating radiation can involve the whole body (uniformly or non-uniformly), a significant portion of the body, or a small, localised part. The exposure can be acute, protracted, or fractionated (in divided doses) over time.
Acute radiation syndrome (ARS) is an acute illness caused by irradiation of the whole body (or a significant portion of it). It follows a somewhat predictable course and is characterised by signs and symptoms that are manifestations of cellular deficiencies and the reactions of various cells, tissues, and organ systems to ionising radiation.
Immediate, overt manifestations of the acute radiation syndrome require a large (i.e., at least a few grays (Gy), usually whole-body) dose of penetrating radiation delivered over a short period of time. Penetrating radiation comes from a radioactive source or machine that emits gamma rays, X-rays, or neutrons. Acute radiation syndrome may also be rarely seen with extremely high levels of internal contamination. The signs and symptoms of this syndrome are non-specific and may be indistinguishable from those of other injuries or illness.
The ARS is characterised by four distinct phases: a prodromal period, a latent period, a period of illness, and one of recovery or death. During the prodromal period patients might experience loss of appetite, nausea, vomiting, fatigue, and diarrhoea; after extremely high doses, additional symptoms such as fever, prostration, respiratory distress, and hyperexcitability can occur. However, all of these symptoms usually disappear in a day or two, and a symptom-free, latent period follows, varying in length depending upon the size of the radiation dose. A period of overt illness follows, and can be characterised by infection, electrolyte imbalance, diarrhoea, bleeding, cardiovascular collapse, and sometimes short periods of unconsciousness. Death or a period of recovery follows the period of overt illness.
In general, the higher the dose the greater the severity of early effects and the greater the possibility of late effects.
Depending on dose, the following syndromes can be manifest:
- Haematopoietic syndrome - characterised by deficiencies of leucocytes, especially lymphocytes, and platelets, with immunodeficiency, increased infectious complications, bleeding, anaemia, and impaired wound healing.
- Gastrointestinal syndrome - characterised by loss of cells lining intestinal crypts and loss of mucosal barrier, with alterations in intestinal motility, causing vomiting and diarrhoea, fluid and electrolyte loss. There is loss of normal intestinal bacteria, and damage to the intestinal microcirculation resulting in sepsis; in addition to the haematopoietic syndrome.
- Cerebrovascular/Cardiovascular syndrome - primarily associated with effects on the vasculature and resultant fluid shifts. Signs and symptoms include vomiting and diarrhoea within minutes of exposure, confusion, disorientation, cerebral oedema, hypotension, and hyperpyrexia. Fatal in a short time.
- Skin syndrome - can occur with other syndromes; characterised by loss of epidermis (and possibly dermis) with "radiation burns."
DiagnosisConsider acute radiation syndrome in the differential diagnosis if any of the following are present:
- history of a known or possible radiation exposure (for example, entering an irradiation chamber when the source is unshielded);
- history of proximity to an unknown (usually metallic) object with a history of nausea and vomiting, especially if nausea and vomiting are unexplained by other causes;
- tendency to bleed (epistaxis, gingival bleeding, ptechiae) and/or respiratory infection with neutropaenia, lymphopaenia, and thrombocytopenia, with history of nausea and vomiting two to three weeks previously; or
- epilation, with a history of nausea and vomiting two to three weeks previously.
Obtain an immediate FBE with differential. Repeat in 4-6 hours, then every 6 to 8 hours for 24 to 48 hours. Look for a drop in the absolute lymphocyte count if the exposure was recent (see diagram). If the initial WBC and platelet counts are abnormally low, consider the possibility of exposure a few days to weeks earlier.
Figure. Curves 1-4 correspond roughly to the following whole-body doses: curve 1 - 3.1 Gy; curve 2 - 4.4 Gy; curve 3 - 5.6 Gy; curve 4 - 7.1 Gy. From Goans, Ronald E., Holloway, Elizabeth C., Berger, Mary Ellen, and Ricks, Robert C. "Early Dose Assessment Following Severe Radiation Accidents," Health Physics 72(4): 1997.
Acute Radiation Syndrome: Dose Less than 2 GrayNausea and vomiting due to radiation are seldom experienced unless the exposure has been at least 0.75 to 1 Gray of penetrating gamma or X-rays and exposure occurred within a matter of a few hours or less. The prospective patient who has been asymptomatic within the past 24 hours will most certainly have had less than 0.75 Gray of whole-body exposure. Hospitalisation generally will be unnecessary if the dose has been less than 2 Gray.
Management of ARS (dose <2 Gray)
- Close observation and frequent FBE with differential
- Outpatient management may be appropriate
- Provide instructions regarding home care
Acute Radiation Syndrome: Dose Greater than 2 GraySigns and symptoms become increasingly severe with dose.
- In the prodromal phase nausea, vomiting and anorexia occur within a few hours at higher radiation dose levels or after 6 to 12 hours at lower dose levels. The prodrome lasts 24 to 48 hours, after which time the patient is asymptomatic and may feel well. The absolute lymphocyte count will fall; however a stress response may be present with a transient neutrophilia.
- The latent phase lasts a few days to as long as 2 to 3 weeks at the lower radiation dose levels. The patient is asymptomatic but full blood examination will show characteristic changes, with lymphocyte depression and gradual decrease in neutrophil and platelet counts.
- The bone marrow depression phase requires sophisticated treatment. Infection and haemorrhage could occur when white cell and platelet counts become critically low.
- At 2 to 10 Gray stem cells in the bone marrow are never completely eradicated; some may replicate and eventually produce sufficient blood elements. Supportive therapy is required during the recovery phase.