Increased Red Blood Cell Adhesion Is Associated with Priapism in Sickle Cell Disease.

The enhanced adhesion of sickle RBCs likely plays a role in the pathogenesis of vaso-occlusive complications in patients with sickle cell disease (SCD). In preliminary studies, we have previously found that RBCs from patients with overt stroke associated with large vessel disease have an abnormally enhanced adhesive phenotype. Priapism, another common vaso-occlusive complication that occurs in approximately 30% of males with SCD, has been associated with the development of stroke as well as pulmonary hypertension. The exact pathophysiology of priapism in the context of SCD is not completely understood. Recently, a potential mechanism of hemolysis-associated NO resistance, endothelial dysfunction and end-organ vasculopathy, including priapism, has been described. In this study, we hypothesized that increased RBC adhesion will also be linked to the development of priapism in patients with SCD. After obtaining informed consent, citrated blood for RBC adhesion analysis was obtained from greater than 3 year old males with homozygous HbSS disease during routine clinic visits (steady state) to the Wisconsin Sickle Cell Comprehensive Center. Female patients, those less than 3 years of age, or those with a history of stroke or recent blood transfusion were excluded. Washed RBCs were perfused through flow chambers previously coated with thrombospondin (TSP) (2μg/cm²) at a wall shear stress of 1 dyne/cm² that mimics the forces in post-capillary venules and optimizes sickle RBC adhesion. After rinsing, adherent RBCs per unit area were counted in four random fields in each of two duplicate wells by direct microscopic visualization. Mean adhesion levels were compared between patients with a history of priapism and those patients without a history of priapism using an independent samples t-test. We studied 27 male patients who met the above inclusion criteria. The priapism group contained 8 patients and the control group 19 patients. The priapism group was older (18.7 vs. 12.9 years). The two groups had comparable laboratory data (Hb, retic, LDH), except for a trend toward a higher total bilirubin in the priapism group (6.4 vs. 3.1 μg/dL), which may be a marker of increased hemolysis. The level of adhesion to TSP for RBCs from patients with priapism was significantly higher compared to the patients without priapism (1336 ± 65 vs. 672 ± 93 RBCs/mm²; mean ± SEM; p= <.0001). Of the 8 patients with priapism, 2 were taking hydroxyurea (HU) at the time of adhesion analysis. Because HU has been shown to decrease sickle cell adhesion, we also analyzed our data excluding all patients taking HU at the time of the adhesion assay. After excluding patients on HU, we still found increased levels of adhesion for RBCs from patients with priapism compared to RBCs from patients without priapism (p= <.0001). In summary, sickle RBCs from patients who develop priapism have increased adhesion to TSP when compared to patients who have not developed priapism. Enhanced RBC adhesion may be an important marker of male patients at increased risk for priapism and perhaps for other severe vaso-occlusive complications associated with SCD.