Hyperviscosity syndrome is a group of symptoms triggered by an increase in the viscosity of the blood. Symptoms of high blood viscosity include spontaneous bleeding from mucous membranes, visual disturbances due to retinopathy, and neurologic symptoms ranging from headache and vertigo to seizures and coma.

Hyperviscosity occurs from pathologic changes of either cellular or protein fractions of the blood such as is found in polycythemias, multiple myeloma (particularly IgA and IgG3), leukemia, monoclonal gammopathies such as Waldenström macroglobulinemia, sickle cell anemia, and sepsis.

Types of hyperviscosity syndromes vary by pathology; including serum hyperviscosity, which may cause neurologic or ocular disorders; polycythemic hyperviscosity, which results in reduced blood flow or capillary perfusion and increased organ congestion; and syndromes of hyperviscosity, caused by reduced deformability of red blood cells, often evident in sickle cell anemia.