What causes hyperviscosity in MM? A list member provided an excellent explanation:
The hyperviscosity seen in myeloma and Waldenstrom’s is not due to the number of cells but the amount of protein in the plasma. Proteins do not dissolve in a solution; they form a colloid. the more protein in a liquid the more likely it is that the liquid will form a gel just like JelloĈ does.
Now the proteins or protein parts found in myeloma can vary in size. Some folks have very little whole proteins in their blood; they usually have small pieces. For them, the chance of hyperviscosity is quite small. Of the five different proteins which can be the issue in MM, IgG, IgE and IgD can be thought of as single units while IgA is a double unit and IgM is a pentamer or 5 unit structure. So, if you have the entire protein present, it is most likely to hyperviscosity if you have excess IgM, more likely to have hyperviscosity if you have an excess in IgA and least likely if you have an excess of IgG, IgE or IgD.