Is There an Increase in Sickle Cell Related Events Among the Adult Belgian Population?

Survival among children and adults with SCD in Belgium published in 2015 showed a low mortality rate (0.25/100 patient years (PY)) among the 469 patients included (5,110 PY) with no significantly increase above 18 years of age. Hydroxyurea (HU) had a positive impact on patients' survival rate when compared to those without disease-modifying treatments (DMT) or transplanted. This led us to compare the incidence of acute complications and hospitalizations among children and adults to see if significant change occurred.

Complications before and after the age of 18 for 84 patients with severe genotype followed after 18 years of age and not transplanted for whom the data during childhood were recorded in the registry are detailed in Figure 1 and Table I. Data on the pre- and post-18 years' events for the entire severe genotype cohort are detailed in Table II and include data from the patients under 18 years and data at the pediatric age of those more than 18 years. Data was censored from the time of the transplantation. Furthermore, data from adult patients before age 18 and beyond with severe genotype treated or not with HU are given in Table III.

There were 139 patients (1759 PY) more than 18 years who had at last follow up a median age of 21 years (range: 18-53 years). 42% (59/139) of the patients were treated with HU. The median number of vaso-occlusive crisis (VOC) was statistically lower during adulthood (0 vs 29.6/100 PY; p= 0.007) and the median number of acute chest syndrome (ACS) was not statistically higher in adult period compared to their follow-up during childhood. Overall, severe infections as well as hospitalizations number (19 vs 63/100 PY; p=0.0006) and days (119 vs 259/100 PY; p=0.021) are significantly higher in children. The comparison of adults under HU and without DMT showed a significant higher incidence of acute events in HU patients (Table III). This is true for follow-up in childhood and beyond the age of 18 (Table III). Patients without DMT obviously seems to have attenuated symptomatology (clinical phenotype not very severe despite their severe genotype) and were therefore not considered benefiting from DMT. In the latter, no ACS was recorded during their follow-up in adulthood (Table III).

The increase in CVO and ACS incidence with age is well described in the literature but not found in our results. Higher number of hospitalizations and infections before 18 years of age are observed. This can be explained by the fact that children consult more quickly and frequently and they are more readily monitored in hospital compared to adults. They also live in community, which increases the risk of seasonal viral infections.

Compared with data available in the literature, our adult patients, most of whom are treated with HU, have fewer acute complications. Nevertheless the comparison is jeopardized because of different methodological approaches. In addition, our cohort of adult patients is limited, has a relatively short follow-up and consists mainly of adults under 21 at the last follow-up. The shorter length of follow up and the underrepresentation of older adults who pay a heavier toll on SCD because of co-existence of chronic complications, may explain the reduced incidence of acute complications in our cohort.

In conclusion, the incidence of events observed in our adult population is lower than that described in the literature. This difference is probably related to a larger proportion of HU patients in our cohort. But these results should be confirmed by data from a larger number of adult patients with longer prospective follow-up.

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