any syndrome associated with increased viscosity of the blood; in syndrome of serum hyperviscosity there is spontaneous bleeding and neurologic and ocular disorders; syndromes of polycythemic hyperviscosity is marked by retarded blood flow, organ congestion, reduced capillary perfusion, and increased cardiac effort; syndromes of sclerocythemic hyperviscosity comprise those in which the deformability of erythrocytes is impaired, as in sickle cell anemia.