Abnormal blood viscosity in Raynaud's phenomenon

The exact cause of Raynaud's phenomenon is still unknown. Since the principal feature of this condition is the precipitation of the attacks by cold, blood viscosity was measured in ten patients and in ten normal controls at 27 degrees C and 37 degrees C. The viscosity of the patients' blood was significantly higher (p less than 0.05) at 27 degrees C than that of the controls when measured at a low shear-rate (0.77 s-1). This, as well as the higher yield stress of the patients' blood, suggests that Raynaud's phenomenon may be caused by an abnormal local increase in the viscosity and yield stress of the blood in response to a fall in temperature.