Leukostasis is an extreme form of leukocytosis (an excess of white blood cells) that is dangerously symptomatic. It is therefore also called symptomatic hyperleukocytosis. It is a white blood cell count above 100,000/μL. It is characterized by abnormal intravascular leukocyte aggregation and clumping. It is most often seen in leukemia patients. The brain and lungs are the two most commonly affected organs. Occluded microcirculation causes local hypoxemia and hemorrage manifesting as headache, blurred vision, transient ischemic attacks, cerebrovascular accidents and dyspnea. It is an acute syndrome requiring aggressive cytoreductive modalities including chemotherapy and/or leukapheresis to both reduce the number of circulating leukocytes and to break apart any aggregates that have already formed. Such rapid and massive lysis of tissue poses risk of complications (tumor lysis syndrome), but it is necessary to avoid stroke.
Leukostasis is different from leukemic infiltration which is a neoplastic process where leukemic cells invade organs.