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Blood viscosity and other rheological properties of blood have important effects on blood circulation. In preterm infants, blood viscosity (at given hematocrit), plasma viscosity and red cell aggregation are decreased compared to term neonates and adults. In addition, the Fahraeus-Lindqvist effect (i.e., decrease in blood viscosity with decreasing vessel diameter) is more pronounced in preterm infants. This suggests that blood flow through arteries, arterioles and veins is facilitated in preterm infants. The large neonatal red cells are less filterable and less able to enter small capillaries than adult red cells. This suggests that the passage of neonatal red cells through narrow capillaries and splenic pores is impaired. The high risk of intracranial hemorrhage in small preterm infants may in part result from poor red cell aggregation, impeded passage of their large red cells through narrow cerebral capillaries and raised blood viscosity due to rapid transfusion of adult blood.

In leukemia patients with extremely high leukocytosis the great number of poorly deformable lymphoblasts compared to normally deformable red cells greatly influences the flow properties of leukemic blood. The increased blood viscosity implies a great risk of disturbance of the microcirculation by leukostasis and bleeding. Removal of large amounts of leukemic cells by exchange transfusion with fresh blood diminished leukemic cell burden and reduced the initial elevated leukocyte counts by more than 50% in 3 patients. In addition, anemia and thrombocytopenia improved and the disturbed plasma coagulation returned to normal. One of the patients with additional risk factors treated by exchange transfusion died 8 months after diagnosis in hematologic release. The two other patients perform well without relapse six and nine months after diagnosis, respectively. Exchange transfusion with 150 ml/kg of fresh blood is considered to be of value to avoid severe early complications as e.g. massive intracerebral hemorrhage observed in 3 other patients and to correct hematological and rheological abnormalities in childhood leukemia with extreme leukocytosis. Possible favourable effects as to long term prognosis have to be awaited.

The article reports on a case of visceral leishmaniasis in a 3-year-old child of German residents in Rome who had passed two vacations on the isle of Ischia. Initial signs were intermittent temperatures, marked anorexia, hepatosplenomegaly and pancytopenia, with spontaneous recovery after three weeks. At that time, leishmania serology was already positive, whereas no leishmania were found in several bone marrow preparations despite a most thorough search. During the following eight weeks, the patient had chickenpox and mumps. During the mumps, relapse of the visceral leishmaniasis occurred, associated with a dramatic increase of the hepatosplenomegaly and recurring pancytopenia. It was only now that we could discover multiple leishmania infection of the bone marrow. Subsequent therapy with sodium stibogluconate (Pentostam) was effective without any complications, and eventually cured the patient.