Lesson 2. Acute radiation syndrome

The acute radiation syndrome (ARS) occurs after whole-body exposure to a large dose of ionizing radiation. This syndrome includes a number of characteristic signs and symptoms whose severity depends on magnitude of dose and duration of exposure. ARS, by definition, does not occur at doses less than 1 Gy and is uniformly fatal at doses greater than 10 Gy. The estimated LD_50/60 (the dose at which 50% of those exposed die within 60 days) is 3.5 Gy for humans without medical treatment and roughly 7.0 Gy with treatment.

Frequently, the exact details of an accidental exposure are not known, leaving uncertainty in dose assessments. Clinical presentation, symptomatology, and laboratory measures, especially in the early period, can be used to indirectly determine dosage of exposure and prognosis.

Stages of ARS

ARS has been described according to progression of illness through 4 stages: (1) prodrome, (2) clinical latency, (3) manifest illness, and (4) recovery or death. The prodromal symptoms occur shortly after irradiation, with the dose of exposure determining severity, duration, and onset. Common prodromal symptoms include nausea, vomiting, anorexia, fatigue, diarrhea, abdominal cramping, and dehydration. At doses of greater than 10 Gy, those exposed show symptoms within 5-15 minutes; at lower doses such as 2-3 Gy, symptoms can take up to 12 hours to present. Immediate diarrhea, hypotension, and fever indicate a supralethal exposure. Severe and early onset of prodromal symptoms indicates higher dosage of exposure and a poor prognosis. Progression through the other phases depends on dosage of exposure.

Classic ARS syndromes

ARS is further described by its 3 classic subsyndromes: the hematopoietic syndrome, the gastrointestinal syndrome, and the cerebrovascular syndrome. The hematopoietic syndrome typically occurs after exposures of 2-5 Gy. At these doses, lymphocytes die from radiation-induced apoptosis, and precursor cells in the bone marrow are destroyed preventing new production of leukocytes and platelets. During the period of a few weeks (clinical latency), circulating cells die off with no replacements; it is at this nadir that the full syndrome becomes clinically apparent with development of infections and possible hemorrhage. Anemia from red cell depression usually does not occur alone in the absence of hemorrhage. Early supportive care, treatment and prevention of infections, and the consideration of cytokine therapy are all important aspects of care for this subsyndrome. However, even if the hematopoietic syndrome is treated, death commonly still occurs from multiorgan failure.

The gastrointestinal syndrome usually occurs after exposures of more than 5-12 Gy. Irradiation leads to death of intestinal mucosal stem cells in the crypts. After loss of mucosal cells at the villi through normal functioning, the stem cells are unable to produce new cells, leading to denudation of the GI tract. As the normal GI boundary is compromised, bacterial growth proliferates increasing the risk of sepsis. Common symptoms include anorexia, nausea, vomiting, prolonged bloody diarrhea, abdominal cramps, dehydration, and weight loss. Often the prodrome onset is rapid, followed by a latent period of roughly 1 week then return of symptoms. The mainstays of treatment are fluid and electrolyte balance and infection prevention, but death often follows in 3-10 days.

The cerebrovascular syndrome occurs after exposures of very high doses (>30 Gy) and is uniformly fatal. At doses of greater than 100 Gy, death occurs within hours. Although the exact mechanism of death is not fully understood, vascular damage is thought to lead to significant cerebral edema, producing neurologic and cardiovascular collapse. Immediate symptoms include nausea, vomiting, hypotension, ataxia, and convulsions, and death follows in a few days.

Date: 2015-01-12; view: 892