Background
Sickle hemoglobin (hemoglobin S, HbS) is a structural variant of adult hemoglobin. HbS polymerizes when oxygen tensions are low, leading to red blood cell (RBC) deformation, so-called "sickling". In sickle cell anemia (SCA), loss of RBC deformability is considered to be a primary factor responsible for vaso-occlusion and hemolysis. Until recently no laboratory tests to measure RBC deformability in SCA have been readily available.
Study Aims
In this study we examine RBC deformability, measured with the oxygenscan module of the Laser Optical Rotational Red Cell Analyzer (Lorrca) ektacytometer, in children with SCA treated with or without hydroxyurea (HU). Furthermore, we investigate the relationship between RBC deformability and pain frequency, as well as genetic and laboratory measures known to be associated with disease severity in SCA.
Methods
We included children aged 0-16 years with a confirmed diagnosis of SCA (HbSS) from the pediatric sickle cell clinic at King's College Hospital in London. Children were excluded if they had received any blood transfusions within 3 months of study inclusion. Children on HU were only included if treatment had been initiated >3 months prior to recruitment and the dose was stable. Children and their parents or guardians reported frequency of pain as: daily, weekly, monthly, yearly, or never. Laboratory measurements, including total hemoglobin (hb), hemoglobin F (HbF), and reticulocyte percentage, were performed on the same day as a sample was taken for oxygenscan analysis. Data on co-inheritance of α-thalassemia was recorded if available.
EDTA blood samples were kept at approximately 4°C and transported from King's College London to Copenhagen University Hospital (Herlev and Gentofte Hospital), where they were analyzed within 48 hours of sampling using the Lorrca oxygenscan (RR Mechatronics, the Netherlands). The oxygenscan measures RBC deformability expressed as an elongation index (EI) during deoxygenation and reoxygenation, with EImax expressing RBC deformability at normal oxygen concentrations, EImin expressing RBC deformability after deoxygenation, and the point of sickling (POS) expressing the point at which >5% decrease in EI is observed, representing the pO2 at which sickling begins.
All statistical analyses were performed in Stata V16.0 (StataCorp. 2019, USA), using the two-sided t-test, one-way ANOVA, and Pearson's correlation when appropriate.
Results
We included 47 children aged 0-16 years (mean age 7.9 years) in the study, 24 (51%) receiving HU. Children in the HU group presented with significantly higher HbF percentage compared to the non-HU group (15.6% and 10.9%, p=0.03). Children receiving HU had higher EImax and EImin, and lower POS values, compared to children in the non-HU group, although results were not significant (Table 1). There was a positive correlation between HbF and EImax (r= 0.57, p=0.0001) and HbF and EImin (r= 0.56, p=0.0001), and a negative correlation between HbF and POS (r=-0.37, p=0.01), as well as a positive correlation between total hb and EImax (r=0.35, p=0.02). There was no significant correlation between any oxygenscan parameters and reticulocyte percentage. Data on α-thalassemia was available for 23 children. EImax and EImin values were higher in heterozygous children compared to children without co-inherited α-thalassemia, and POS values were lower, but results were not significant (Table 2). We found no significant association between any oxygenscan parameters and pain frequency (Table 3).
Conclusion
In this study we identified a strong correlation between all oxygenscan parameters and HbF percentage, as has been reported previously. We found higher EImax and EImin and lower POS values in children receiving HU treatment and children with co-inherited heterozygous α-thalassemia, suggesting increased RBC deformability in these children. These results were not significant, however, which may in part be due to lack of power in the study. Also, it is possible that children in the HU group would have presented with lower EImax and EImin and higher POS values prior to HU initiation, with treatment response leading to results similar to those found in the non-HU group. Finally, our results suggest that there is no association between oxygenscan parameters and self-reported frequency of pain in children with SCA.
Disclosures
No relevant conflicts of interest to declare.
Hematological and Genetic Predictors of Daytime Hemoglobin Saturation in Tanzanian Children with and without Sickle Cell Anemia