A North American Experience Of Hemoglobin SC Disease, Its Complications, and Management
Authors: Naessens Veronique , Ward Richard , Kuo Kevin H.M. 
Published: October 2019

Background
The phenotype of hemoglobin SC (HbSC) disease is distinct from sickle cell anemia (SCA) (HbSS and S/b0) but management of adults is mostly derived from studies of the latter group. Longitudinal observational studies on the complications and outcomes of hemoglobin SC disease are largely confined to centers outside North America. The unique ethnic composition of our cohort, consisting of mostly Western Africans and West Indians, permits further characterization of the HbSC phenotype.

Objective
to describe the baseline characteristics and long-term complications of a cohort of adult HbSC patients followed in a Canadian sickle cell comprehensive care center.

Methods
A retrospective observational cohort study was conducted on all adult patients with HbSC disease attending a sickle cell comprehensive care center in Toronto, Canada from 1994 to 2013. Baseline demographics, acute and chronic complications attributable to sickle cell disease, and laboratory data were collected. Medians were used to describe continuous variables, while percentages or ratios for categorical variables. Logistic regression was used to examine factors influencing the main clinical complications.

Results
104 patients were included in the analysis, comprising of 38.5% males and 61.5% females. Median length of follow-up was 3.5 years (1 - 19) and median age at last recorded visit was 35 years (18 - 68). Median baseline hemoglobin was 119 g/L (82 - 153 g/L), hematocrit 0.340 (0.250 - 0.440), and fetal hemoglobin (HbF) fraction 1% (0 - 7.7%). Most frequent complications encountered were retinopathy (55.8%) and symptomatic avascular necrosis (27.9%), followed by painful vaso-occlusive crises requiring emergency room visit (23.1%). Presence of retinopathy was associated with higher baseline hemoglobin (OR 2.72 for every 10 g/L of hemoglobin, p = 0.037) and older age (OR 2.72 for every decade, p < 0.001). Acute chest syndrome (7.7%), priapism (7.5% of men), and renal involvement (8.2%), were less common than reported in the literature, while the rates of venous thromboembolism (8.7%), symptomatic infarctive or hemorrhagic stroke (2.9%) were slightly more common. Median right ventricular systolic pressure on 2D-transthoracic echocardiogram was 29 mmHg (17 – 43 mmHg). No patient underwent a right heart catheterization. Two patients died from septic shock, both at the age of 29. Disease-modifying therapy most often consisted of hydroxyurea (28.8%), followed by exchange transfusion (6.7%) and phlebotomies (5.8%). Hydroxyurea significantly increased the median HbF fraction (from 1% to 2.75%, p = 0.004 by related-samples Wilcoxon signed rank test).

Conclusion
In this large North American cohort of adult patients, we have again shown that HbSC disease is not as benign as traditionally thought. As such, patients with HbSC disease warrant similar follow-up to that provided to SCA. Retinopathy, avascular necrosis and painful vaso-occlusive crises were the most common complications in our study, albeit lower than in other reported cohorts. The frequent use of hydroxyurea in this cohort is quite unique compared to other sickle cell comprehensive care centers reported in the literature. However, therapeutic phlebotomy is less often used compared to the European experience. We also observed a lower frequency of retinopathy, avascular necrosis, painful vaso-occlusive crises, priapism and acute chest syndrome. Whether this observation is due to hydroxyurea use or to other genetic or environmental factors remains to be determined. Further studies are also required to develop a more evidence-based therapeutic strategy for this genotype of Sickle Cell Disease.

Disclosures:
No relevant conflicts of interest to declare.