The term acute radiation syndrome (ARS) is used to describe the onset and progression of symptoms after exposure to ionizing radiation, and the clinical course is divided into four phases [1] [2]:

• Prodromal phase - Gastrointestinal complaints, such as nausea, vomiting, diarrhea, weakness and weight loss due to profound damage of the intestinal mucosa are main features. In doses of 1-2 grays (Gy), symptoms appear two or more hours after exposure, whereas very severe and lethal exposures (6-8 and > 8 Gy, respectively) produce symptoms in less than 30 minutes [3]. Headaches, altered consciousness, and fever may be observed as well [3]. Cutaneous radiation syndrome (CRS) is the term used to describe skin-related changes that range from mild erythema in this period to more severe lesions encountered later on [4].
• Latent phase - Myelosuppression is the hallmark of latent phase, developing between 1 and 5 weeks after initial exposure, again depending on the severity of exposure [5]. Lymphopenia, granulocytopenia, and thrombocytopenia are most prominent findings, accompanied by anorexia and generalized fatigue [1]. Pneumonitis, pulmonary edema, and varying degrees of skin depilation are other notable symptoms [1] [3].
• Illness manifestation phase - Considered as the "critical phase" of radiation exposure, it is characterized by the dose-dependent occurrence of impaired central nervous system activity (headaches, impaired cognitive function, disorientation, ataxia, and convulsions), the progression of vomiting and fever to life-threatening bloody diarrhea, cardiovascular collapse, and shock. Additionally, acute respiratory distress syndrome (ARDS), purpura, electrolyte disturbances and infection due to severe myelosuppression that may lead to sepsis are frequently reported [1] [2].
• Final outcome - Patients either recover from the deleterious effects induced by radiation, or suffer from fatal organ damage, and the most significant predictors of outcome are the dosage received and heterogeneity of dosage, as either single or multiple organs may be affected [3].

Physicians must be careful when interpreting symptoms, as gastrointestinal and central nervous system complaints are nonspecific and may arise in a myriad of conditions. For this reason, patient history is the vital component of the diagnostic workup. The detailed information regarding the onset of symptoms, potential exposure to radioactive substances at the workplace, as well as presence of similar findings in other coworkers is vital in making a presumptive diagnosis [6]. The use of a Geiger-Muller probe attached to a Geiger counter to inspect patients for contamination is pivotal for confirming radiation exposure, and the entire body should be examined [6]. External contamination is performed by surveying the skin, while internal contamination requires sampling of the ears, the mouth, as well as urine, feces and vomit [6]. Laboratory studies should include complete blood counts (CBC) performed on a daily basis after admission, with a particular emphasis on total lymphocyte count, which is considered to be one of the most valid indicators of the dose of radiation received [6]. If a total lymphocyte count is > 1500/mL, the radiation dose is expected to be around 0.4 Gy, and the chance of survival is very high, whereas counts of < 500/mL indicate exposure to > 4 Gy and carry a poorer prognosis [6]. A lymphocyte count of < 100/mL is universally fatal [6]. In addition to lymphocyte count, serum amylase, and C-reactive protein (CRP) levels should be evaluated, as they are elevated in proportion to the dose received [3] [6]. In the setting of profound skin changes, procedures such as bone scintigraphy, capillary microscopy and nuclear magnetic resonance imaging should be performed, if possible [4].

1. Heslet L, Bay C, Nepper-Christensen S. Acute radiation syndrome (ARS) – treatment of the reduced host defense. Int J Gen Med. 2012;5:105-115.
2. Longo DL, Fauci AS, Kasper DL, Hauser SL, Jameson J, Loscalzo J. eds. Harrison's Principles of Internal Medicine, 18e. New York, NY: McGraw-Hill; 2012.
3. Macià i Garau M, Lucas Calduch A, López EC. Radiobiology of the acute radiation syndrome. Rep Pract Oncol Radiother. 2011;16(4):123-130.
4. Peter RU, Gottlöber P. Management of cutaneous radiation injuries: diagnostic and therapeutic principles of the cutaneous radiation syndrome. Mil Med. 2002;167(2):110-112.
5. Dörr H, Meineke V. Acute radiation syndrome caused by accidental radiation exposure - therapeutic principles. BMC Med. 2011;9:126.
6. Porter RS, Kaplan JL. Merck Manual of Diagnosis and Therapy. 19th Edition. Merck Sharp & Dohme Corp. Whitehouse Station, N.J; 2011.