Hyperviscosity syndrome

Hyperviscosity syndrome is a group of blood disorders that cause increased serum density of the blood, making it more viscous and less likely to flow. The extra density is usually from proteins, including increased numbers of white blood cells. This results in reduced blood flow to most of the body's organs, including the brain. It's generally characterized by increasingly severe neurological symptoms.
There are several underlying disorders that can result in hyperviscosity syndrome, including myeloma, leukemia, and polycythemia. If the underlying disorder is not treated, hyperviscosity is very likely to reoccur.
Hyperviscosity can be fatal on its own no matter what the underlying condition that causes it. Although blood viscosity can be measured directly, the test for it takes time and patients will often die waiting for the results to come back. As a result, treatment should start if it's even suspected. Initially, a patient should be given fluids to lower blood density. At that point, plasmapheresis should be started as soon as possible to remove the extra protein in the blood. Transfusion is contraindicated as it can temporarily increase blood density.
Hyperviscosity syndrome at Wikipedia
Hyperviscosity syndrome