Increased serum viscosity usually results from increased circulating serum immunoglobulins and can be seen in Waldenstrom's macroglobulinemia and multiple myeloma. The complications most commonly associated with this syndrome include mucus membrane bleeding, retinopathy, and neurological symptoms.
Viscosity is the property of fluid to resist flow.
As serum proteins increase, the blood becomes more viscous, leading to the clinical symptoms of hyperviscosity syndrome. The normal relative serum viscosity ranges from 1.4-1.8 units. Symptoms usually are not seen before reaching a viscosity of 4 units, and the hyperviscosity syndrome typically requires a viscosity above 5 units. Hyperviscosity syndrome is associated most commonly with plasma cell dyscrasias and is due to the large size of the excess IgM paraproteins. Waldenstrom's macroglobulinemia is the most common cause.