Hemorrhage typically occurs in small venules associated with increased viscosity in areas with minimal supporting tissue, such as the nose and oral cavity. The predominant symptoms of HVS are mucocutaneous bleeding, ophthalmologic, and neurologic ( Table 1). Symptoms and Management of Hyperviscosity Syndrome It should be noted that, although the likelihood of HVS increases at higher levels of immunoglobulin, there is no discrete cutoff in any circumstance, and HVS should be considered for patients with characteristic symptoms and evidence of a paraprotein-secreting hematologic malignancy. Even in the context of a paraprotein-excreting hematologic malignancy, the risk of HVS is relatively low until the serum viscosity increases above 4 cP ( 9). IgG is relatively small (180 kDA) and can require levels as high as 10 g/dL to produce significant changes in viscosity, except for the IgG3 subtype, which has a tendency to aggregate and can increase viscosity at lower levels, perhaps due to its elongated hinge region and resultant increases in Fc-Fc interaction ( 9, 12, 13). IgA is smaller (320 kDA) than IgM but circulates as a dimer and is associated with increased viscosity at levels of 6 g/dL or greater ( 5, 11). IgM is pentameric and very large in size (970 kDA), and serum viscosity can increase significantly with IgM levels as low as 3 g/dL, and IgM levels of 6 g/dL or higher are associated with rapid development of hyperviscosity, with a median time to symptomatic HVS of 3 months ( 9, 10). Serum viscosity is correlated with immunoglobulin levels for patients both with and without monoclonal gammopathies ( 8), but the concentration of immunoglobulins required to significantly increase viscosity depends on the specific type of paraprotein. In contrast to most serum proteins, which are spherical in shape, immunoglobulins are relatively large but also linear in shape thus, when they travel through the serum, they spin around their longitudinal axis, increasing serum viscous drag, and therefore viscosity. The increased viscosity of serum relative to water relates primarily to its protein content. A typical serum viscosity for a healthy patient is 1.5 cP, or 1.7 relative to water. Viscosity can be measured in absolute terms in centipoise (cP), or in relative terms compared to the viscosity of water (0.894 cP). Paraprotein Levels and Hyperviscosity Syndrome
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