Hyperviscosity syndrome is a group of symptoms triggered by increase in the viscosity of the blood. The term hyperviscosity describes those conditions which cause increased viscosity, or thickness, of a dog's blood. Viscosity is the state of being thick, sticky, and semifluid in consistency, due to internal friction. Hence, hyperviscosity is the thickening of the blood, which typically occurs in response to high protein levels in the blood. Rarely, a high concentration of erythrocytes (a type of red blood cells) can also bring about hyperviscosity. Besides higher levels of proteins in the blood, other reasons for hyperviscosity are large molecular size, abnormal polymerization, and abnormal shape of immunoglobulin molecules.
Hyperviscosity is most frequently seen as a paraneoplastic syndrome (the consequence of the presence of cancer in the body). Most frequent precipitating diseases include multiple myeloma (plasma cell tumor) and other lymphoid tumors or leukemias. In less frequent incidents, several other conditions can cause high protein levels in the blood, making it thicker than usual. These include polycythemia (a net increase in the total number of blood cells), autoimmune disease (e.g., systemic lupus rheumatoid arthritis) and chronic atypical inflammation with monoclonal gammopathy (in which an abnormal protein has been detected in the blood [tick fever can cause this in dogs]).
Signs and symptoms associated with hyperviscosity are due to reduced blood flow through smaller vessels, high plasma volume and associated coagulopathy (a defect in the body's mechanism for blood clotting).
Signs and symptoms of hyperviscosity are not necessarily consistent. A dog with this condition may display signs that include weakness, lethargy, depression, weight loss, poor appetite, increased thirst and increased urination (polydypsia & polyuria), loss of vision, unsteadiness (ataxia), lameness, bone or joint pain, vomiting, diarrhea, bleeding tendencies, seizures and disorientation, rapid heartbeat and rapid breathing if congestive heart failure present owing to volume overload, nosebleed or other bleeding in the mucus membranes and visual deficits associated with engorged retinal vessels, retinal hemorrhage or detachment, and optic swelling.
Any dog of any breed can be affected by this condition. However, hyperviscosity is more likely to occur in middle-aged and older animals.