Acute radiation syndromes (ARS) After the whole body, or a large portion of the body, receives a high dose of penetrating radiation, several distinct syndromes may occur: Cerebrovascular syndrome Gastrointestinal (GI) syndrome Hematopoietic syndrome These syndromes have 3 different phases: Prodromal phase (minutes to 2 days after exposure): Lethargy and GI symptoms (nausea, anorexia, vomiting, diarrhea) are possible.