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\begin{document}

\title{Splenic Complications in Pediatric Sickle Cell Disease: A Retrospective
Cohort Review}



\author[1]{Alex George}%
\author[2]{Shannon E. Conneely}%
\author[3]{Ross Mangum}%
\author[2]{Titilope Fasipe}%
\author[2]{Philip Lupo}%
\author[2]{Michael Scheurer}%
\affil[1]{Wake Forest University School of Medicine}%
\affil[2]{Baylor College of Medicine Department of Pediatrics}%
\affil[3]{Phoenix Children's Hospital Center for Cancer and Blood Disorders}%


\vspace{-1em}



  
  \date{May 24, 2024}


\begingroup
\let\center\flushleft
\let\endcenter\endflushleft
\maketitle
\endgroup





\selectlanguage{english}
\begin{abstract}
\textbf{Objective:} To delineate the natural history of splenic
complications other than the loss of splenic function in children with
sickle cell disease (SCD), we performed a retrospective chart review of
patients with SCD treated at the Texas Children's Hospital.
\textbf{Methods: We} determined the dates of diagnoses of splenic
complications, the number of ASSC events, and hydroxyurea treatment in
patients with SCD. We also examined the association of hydroxyurea
therapy with the onset and severity of ASSC. \textbf{Results:} The
cumulative prevalence of splenic complications was 24.7\% for
splenomegaly, 24.2\% for ASSC, 9.6\% for hypersplenism, and 5.6\% for
splenectomy. The cumulative prevalence of all splenic complications was
highest in patients with hemoglobin S\selectlanguage{greek}β \selectlanguage{english}\textsuperscript{0} (69.2\%),
intermediate in hemoglobin SS (33.3\%), low in hemoglobin SC (9.0\%),
and non-existent in hemoglobin S\selectlanguage{greek}β \selectlanguage{english}\textsuperscript{+}. The overall
event-rate of ASSC was 8.3 per hundred patient-years. The event-rate was
28.4 in the hemoglobin S\selectlanguage{greek}β \selectlanguage{english}\textsuperscript{0}, 10.9 in hemoglobin SS,
and 3.5 in hemoglobin SC Patients with hemoglobin SS and hemoglobin S\selectlanguage{greek}β
\selectlanguage{english}\textsuperscript{0} on hydroxyurea therapy had a significantly higher
occurrence of ASSC than those who were not, with event-rates of 14.2 and
3.1, respectively. The event-rate was also higher for children who
started hydroxyurea before age 2 years than for those who started after
this age (19.8 and 9.2 respectively). \textbf{Conclusions:} The
prevalence and severity of splenic problems vary widely between
different sickle cell genotypes, with hemoglobin S\selectlanguage{greek}β \selectlanguage{english}\textsuperscript{0}
having the most severe complications. Hydroxyurea therapy is strongly
associated with incidence of ASSC, particularly when initiated before
two years of age.%
\end{abstract}%



\sloppy


\textbf{Splenic Complications in Pediatric Sickle Cell Disease:}

\textbf{A Retrospective Cohort Review}

Alex George, M.D., Ph.D.\textsuperscript{1}, Shannon E. Conneely,
M.D.\textsuperscript{2}, Ross Mangum, M.D.\textsuperscript{3}, Titilope
Fasipe, M.D, Ph.D.\textsuperscript{2}, Philip J. Lupo, Ph.D.,
M.P.H.\textsuperscript{2} and Michael E. Scheurer, Ph.D.,
M.P.H.\textsuperscript{2}

\textsuperscript{1}Department of Pediatrics, Section of
Hematology/Oncology, Wake Forest University School of Medicine and
Brenner Children's Hospital, Winston-Salem, N.C.

\textsuperscript{2}Department of Pediatrics, Division of
Hematology/Oncology, Baylor College of Medicine/Texas Children's
Hospital, Houston, TX;

\textsuperscript{3}Center for Cancer and Blood Disorders, Phoenix
Children's Hospital, Phoenix, AZ

Corresponding Author: Alex George, M.D., Ph.D.

Medical Center Boulevard, Meads Hall 3

Winston-Salem, N.C. 27157

algeorge@wakehealth.edu

Office: 336-716-0279

Fax: 336-716-3010

Abstract: 249 words

Main Text: 2632 words

Figures: 3

Tables: 3

Supplemental Figures and Tables: 2

Running Title: Splenic Dysfunction in Pediatric Sickle Cell Disease

Keywords: Sickle cell disease; Splenomegaly; Acute splenic sequestration
crises; Hypersplenism; Splenectomy; Hydroxyurea\selectlanguage{english}
\begin{longtable}[]{@{}ll@{}}
\toprule
\begin{minipage}[b]{0.47\columnwidth}\raggedright\strut
\textbf{Abbreviations}\strut
\end{minipage} & \begin{minipage}[b]{0.47\columnwidth}\raggedright\strut
\textbf{Expanded Form}\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.47\columnwidth}\raggedright\strut
SCD\strut
\end{minipage} & \begin{minipage}[t]{0.47\columnwidth}\raggedright\strut
Sickle cell disease\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.47\columnwidth}\raggedright\strut
Hb SS\strut
\end{minipage} & \begin{minipage}[t]{0.47\columnwidth}\raggedright\strut
Homozygous sickle cell disease\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.47\columnwidth}\raggedright\strut
Hb SC\strut
\end{minipage} & \begin{minipage}[t]{0.47\columnwidth}\raggedright\strut
Hemoglobin SC disease\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.47\columnwidth}\raggedright\strut
Hb S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0}\strut
\end{minipage} & \begin{minipage}[t]{0.47\columnwidth}\raggedright\strut
Hemoglobin S \selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0} Thalassemia\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.47\columnwidth}\raggedright\strut
ASSC\strut
\end{minipage} & \begin{minipage}[t]{0.47\columnwidth}\raggedright\strut
Acute splenic sequestration crisis\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.47\columnwidth}\raggedright\strut
RBC\strut
\end{minipage} & \begin{minipage}[t]{0.47\columnwidth}\raggedright\strut
Red blood cell\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.47\columnwidth}\raggedright\strut
EMR\strut
\end{minipage} & \begin{minipage}[t]{0.47\columnwidth}\raggedright\strut
Electronic medical record\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.48\columnwidth}\raggedright\strut
ICD10\strut
\end{minipage} & \begin{minipage}[t]{0.48\columnwidth}\raggedright\strut
International statistical classification of diseases and related health
problems, version 10\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.47\columnwidth}\raggedright\strut
ER\strut
\end{minipage} & \begin{minipage}[t]{0.47\columnwidth}\raggedright\strut
Event rate\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.47\columnwidth}\raggedright\strut
IRR\strut
\end{minipage} & \begin{minipage}[t]{0.47\columnwidth}\raggedright\strut
Incidence rate ratio\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}

This project was first presented at the 2021 American Society of
Hematology Annual Meeting with the title: ``Splenic Complications in
Pediatric Sickle Cell Disease:

A Retrospective Cohort Review.'' An abstract of the presentation was
published in a special supplement of the journal \emph{Blood.}

Conflicts of Interest:

Dr. Alex George: research funding from Novo Nordisk; membership in a
study adjudication committee and a data safety monitoring board for
Pfizer; royalties from Walter-Kluwers.

Dr. Shannon Conneely: no conflicts to declare.

Dr. Ross Mangum: no conflicts to disclose.

Dr. Titilope Fasipe: consulting services for Pfizer, Novartis, and Novo
Nordisk.

Dr. Philip Lupo: no conflicts to disclose.

Dr. Michael Scheurer: no conflicts to disclose.

ABSTRACT

\textbf{Objective:} To delineate the natural history of splenic
complications other than the loss of splenic function in children with
sickle cell disease (SCD), we performed a retrospective chart review of
patients with SCD treated at the Texas Children's Hospital.

\textbf{Methods: We} determined the dates of diagnoses of splenic
complications, the number of ASSC events, and hydroxyurea treatment in
patients with SCD. We also examined the association of hydroxyurea
therapy with the onset and severity of ASSC.

\textbf{Results:} The cumulative prevalence of splenic complications was
24.7\% for splenomegaly, 24.2\% for ASSC, 9.6\% for hypersplenism, and
5.6\% for splenectomy. The cumulative prevalence of all splenic
complications was highest in patients with hemoglobin
S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0} (69.2\%), intermediate in hemoglobin SS (33.3\%),
low in hemoglobin SC (9.0\%), and non-existent in hemoglobin
S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{+}. The overall event-rate of ASSC was 8.3 per
hundred patient-years. The event-rate was 28.4 in the hemoglobin
S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0}, 10.9 in hemoglobin SS, and 3.5 in hemoglobin SC
Patients with hemoglobin SS and hemoglobin S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0} on
hydroxyurea therapy had a significantly higher occurrence of ASSC than
those who were not, with event-rates of 14.2 and 3.1, respectively. The
event-rate was also higher for children who started hydroxyurea before
age 2 years than for those who started after this age (19.8 and 9.2
respectively).

\textbf{Conclusions:} The prevalence and severity of splenic problems
vary widely between different sickle cell genotypes, with hemoglobin
S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0} having the most severe complications. Hydroxyurea
therapy is strongly associated with incidence of ASSC, particularly when
initiated before two years of age.

INTRODUCTION

The spleen is among the earliest organs affected in people with sickle
cell disease (SCD) \textsuperscript{1,2}. Splenic dysfunction, as
measured by the appearance of Howell-Jolly bodies and pitted red blood
cells (RBCs) in circulation and decreased radiotracer update, can be
detected as early as six months of age and can be seen in up to 86\% of
children by age one year\textsuperscript{3}. A significant proportion of
children with homozygous sickle cell disease (Hb SS) and sickle
beta-zero thalassemia (Hb S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0}) undergo complete
infarction of the spleen early in life. This progressive loss of splenic
function is associated with impaired ability to clear encapsulated
bacteria from the blood and an increased risk of bacteremia. Loss of
splenic function occurs later in life in people with hemoglobin SC
disease (Hb SC) and may not occur at all in those with sickle beta-plus
thalassemia disease (Hb S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{+})\textsuperscript{4,5}.

In addition to impaired filtration and bacterial clearance, splenic
dysfunction can manifest as splenomegaly, acute splenic sequestration
crises (ASSC) characterized by acute trapping of blood in an enlarged
spleen, and hypersplenism resulting in chronic
cytopenias\textsuperscript{2}. ASSC in particular can be a
life-threatening complication due to the risk of sudden hypovolemic
shock in affected people. Previous studies have reported a prevalence of
7-30\% for ASSC in children with SCD\textsuperscript{2}. A retrospective
cohort study of ASSC in 190 patients with Hb SS and Hb
S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0} revealed a prevalence of 12.6\% and an event rate
of 6 per 100 patient-years, with a median age of onset of 1.4
years\textsuperscript{6}. A prospective cohort study of 153 children
with Hb SS disease in Jamaica reported a prevalence of splenomegaly of
65\% by age one year\textsuperscript{7}. Splenomegaly has been reported
in up to 34\% of children with Hb SC disease, with ASSC and
hypersplenism occurring in 12\% and 10\%
respectively\textsuperscript{8}. One study of children with Hb
S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{+} found no evidence of splenic dysfunction by age 18
years\textsuperscript{5}. Another study of children from Tunisia,
however, found a higher prevalence of ASSC in both Hb
S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0} and Hb S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{+} genotypes than in
patients with Hb SS disease\textsuperscript{9}. Of note, the pattern of
splenic disease in sub-Saharan African countries appears to differ
significantly from other regions. One meta-analysis of published studies
describes a prevalence of splenomegaly of 10-73\% but a prevalence of
less than 10\% for ASSC and 5\% for hypersplenism\textsuperscript{10}.
The greater prevalence of splenomegaly in this region is hypothesized to
be linked to malaria exposure, though the impact of this interaction on
other aspects of splenic disease in SCD has not been
described\textsuperscript{11}.

In this study, we describe a retrospective chart review of splenic
complications, including splenomegaly, ASSC, and hypersplenism, in a
large cohort of pediatric patients with SCD at a single children's
hospital. We identified the prevalence of these complications, as well
as that of splenectomy, by age across different SCD genotypes. We also
analyzed the event rate of splenic sequestration across genotypes.
Finally, we examined the correlations between hydroxyurea therapy and
the incidence and severity of splenic sequestration in patients with Hb
SS and Hb S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0}.

METHODS

This project was approved by the Institutional Review Board at Baylor
College of Medicine prior to initiation of chart review. We searched our
electronic medical records (EMR) system using ICD10 codes to identify
all children with sickle cell disease (SCD) born between January 1,
2011, and December 31, 2018, who were seen at least once at our
institution. The ICD10 codes used were D57.0 for all SCD diagnoses,
D57.1 for Hb SS, D57.2 for Hb SC, D57.4 for Hb S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{+} and
Hb S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0}, and D57.8 for other genotypes. The D57.3 code
was used to exclude children with sickle trait. We also identified
extended cohorts from January 1, 2000, to December 31, 2018, for the Hb
SC and Hb S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{+} genotypes at Texas Children's Hospital.

Within our patient population, we used ICD10 codes to identify patients
with splenic dysfunction (splenomegaly, ASSC, and hypersplenism), and
splenectomy. The ICD10 codes used were: R16.1, D73.1, and D73.2 for
splenomegaly; D73.89, D57.02, D57.212, D57.412, D57.432, D57.452, and
D57.812 for splenic sequestration; D73.1 for hypersplenism; and Z90.81
for splenectomy. These diagnoses were verified by direct chart review
using previously defined diagnostic criteria\textsuperscript{12}.
Splenomegaly was defined as a spleen palpable by [?]2 cm below the costal
margin; ASSC was defined as an acute decrease in hemoglobin of
\textgreater{}20\% from baseline with concomitant acute splenic
enlargement and a rise in reticulocyte count; and hypersplenism was
defined as a combination of splenomegaly with one or more cytopenias
with no evidence of other etiologies. We also reviewed a random
selection of charts that did not have an ICD10 code for splenic
dysfunction to confirm the accuracy of diagnostic designations by ICD10
coding. For children with a confirmed diagnosis of splenic dysfunction,
we identified the date of diagnosis or surgical procedure and number of
ASSC events. Among patients with Hb SS and Hb S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0}, we
also identified whether hydroxyurea therapy had been initiated and the
date of initiation. Patients who had inadequate information about
diagnoses or dates of onset of complications were excluded from the
study cohort.

Continuous variables are presented as medians or means with standard
deviation and compared between groups using the paired or unpaired
Student's t test as appropriate. The event rate (ER) of ASSC within each
cohort was calculated as events per one hundred patient-years and
compared by generating incidence rate ratios (IRR) with 95\% confidence
intervals.

For survival analysis of each diagnosis, the age of diagnosis was
determined using the date of diagnosis as documented in the patient's
chart. Patients who did not have a particular diagnosis were censored at
the date of the last documented visit. The times to diagnoses were
compared between sub-cohorts using the log-rank test. All statistical
analyses were performed using GraphPad Prism software version 10.0.3
(GraphPad Software LLC) and MedCalc software version 22.014 (MedCalc
Software Ltd).

RESULTS

We identified 688 patients with a diagnosis of SCD who were seen in any
setting at our institution between January 1, 2011, and December 31,
2018. On further review of this group, we identified four misdiagnoses,
nine patients with sickle trait, and sixty-eight duplicates who had
ICD10 codes for both homozygous (SS) SCD and a variant (SC,
S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{+}, S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0}, or S-Other). An additional
thirteen patients had insufficient information in their charts for the
planned analysis and were excluded. After accounting for these
anomalies, we derived a final cohort of 594 patients for analysis. Of
these, 361 (60.7\%) had Hb SS, 160 (26.9\%) had Hb SC, 42 (7.1\%) had Hb
S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{+}, 16 (2.7\%) had Hb S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0}, and 15
(2.5\%) had other SCD genotypes (Supplemental Figure 1).

The cumulative prevalence of splenomegaly, ASSC, hypersplenism, and
splenectomy in our overall cohort by age 8 years was 24.7\%, 24.2\%,
9.6\%, and 5.6\%, respectively (Figure 1 and Supplemental Table 1).
Stratification of splenic dysfunction by genotype revealed that all
types of dysfunction and splenectomy were highest in patients with Hb
S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0}, intermediate in Hb SS, low in Hb SC, and
non-existent in Hb S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{+} and Hb S-Other, with
statistically significantly differences between the SCD genotypes
(Figure 2 and Supplemental Table 1).

Since ASSC is the most severe complication of splenic dysfunction and is
often recurrent, we assessed its event rate in our patient cohorts. The
overall ER of ASSC for the entire patient cohort was 8.3 per 100 patient
years. The ER for patients with Hb S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0} was 28.4
compared to 10.9 for Hb SS and 3.5 for Hb SC. The IRR for patients with
Hb S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0} and Hb SC relative to Hb SS were 2.6 and 0.3
respectively, indicating statistically significant differences in the
incidence of ASSC between the groups (Table 1).

To determine if splenic dysfunction worsened with age in patients with
Hb SC and Hb S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{+} genotypes, we extended our analysis
of patients with these genotypes to include those born between January
1, 2000, and December 31, 2010. We identified an additional 149 patients
with Hb SC and 32 with Hb S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{+} for whom detailed
medical records were available, for totals of 309 and 74 patients
respectively. The prevalence of splenic dysfunction remained very low in
the Hb S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{+} group, with only one patient developing
ASSC and requiring splenectomy. In the Hb SC group, by contrast, the
prevalence of splenomegaly, ASSC, and hypersplenism continued to
increase with age, more than doubling by the end of this extended study
interval (Figure 3 and Supplemental Table 1).

Finally, we assessed the association between hydroxyurea therapy and the
incidence and prevalence of ASSC in patients with Hb SS and Hb
S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0}. Log-rank survival analysis of patients on
hydroxyurea therapy compared to those who were not revealed a
significantly higher prevalence of ASSC in the former group (Figure 4A).
The ER of ASSC was 3.1 in patients who had no prescriptions for
hydroxyurea and 14.2 in those prescribed hydroxyurea therapy, with an
IRR of 4.6 (Table 2; p \textless{} 0.001). We also assessed whether the
timing of hydroxyurea initiation correlated with prevalence and severity
of ASSC. Among patients on hydroxyurea experiencing ASSC, the mean age
of hydroxyurea initiation was 1.2 \selectlanguage{ngerman}± 0.4 years and the mean age of first
ASSC was 1.6 ± 1.2 years; the median ages of HU initiation and ASSC
onset were 1.13 years and 1.24 years respectively. Patients initiated on
hydroxyurea before or at age 2 years had a significantly higher
prevalence of ASSC than those initiated after age 2 years (Figure 4B).
The ER was of ASSC in the early hydroxyurea group was 19.8 compared to
9.2 in the late hydroxyurea group, with an IRR of 2.1 (Table 3; p
\textless{} 0.001).

DISCUSSION

In this study, we present a retrospective analysis of splenic
dysfunction in a cohort of 594 pediatric patients with SCD who were
cared for at a single children's hospital between January 1, 2011, and
December 31, 2018. As in previous reports, we identified a high
prevalence of splenomegaly, ASSC, and hypersplenism in children with
SCD. By age 2 years, the cumulative prevalence of splenomegaly and ASSC
in the entire cohort were 13.5\% and 11.2\% respectively, plateauing at
24.7\% and 24.2\% respectively by age 6 years. The event rate for ASSC
was 8.3 per 100 patient-years. Strikingly, there were significant
differences in the prevalence of splenic dysfunction and the event rates
of ASSC between genotypes. Children with Hb S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0} had
the highest prevalence of all splenic complications and event rate for
ASSC, followed by those with Hb SS and Hb SC. In the context of other
studies that have reported a higher baseline hemoglobin level, a lower
rate of acute chest syndrome, and similar cerebral hemodynamics in Hb
S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0} patients compared to Hb SS patients, our results
demonstrating a more severe course of splenic disease in patients with
Hb S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0} suggest that Hb SS and Hb
S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0}are phenotypically distinct
entities\textsuperscript{13,14}. Additionally, while children with Hb SS
and Hb S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0}appeared to have a plateau of disease
prevalence by age 6 years, children with Hb SC disease continued to
develop splenic disease well into the second decade of life. Conversely,
children with Hb S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{+} had virtually no evidence of
splenic disease, even on extended analysis into late adolescence. Our
data on Hb SC and Hb S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{+} patients correspond with
those from earlier studies, which indicated that Hb SC patients have
progressive splenic disease throughout life while patients with Hb
S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{+}mostly remain disease-free\textsuperscript{4,5}.

Hydroxyurea therapy has revolutionized care for people with SCD,
dramatically reducing the rates of pain crises and acute chest syndrome
and potentially reducing chronic organ damage\textsuperscript{15}. The
effectiveness of hydroxyurea therapy in preserving splenic function is
less clear. A major study of hydroxyurea therapy in very young children
examined its potential role in preserving splenic function but found no
clear evidence of such benefit\textsuperscript{16}. Children enrolled in
this study had other significant benefits, including fewer pain crises,
episodes of acute chest syndrome, RBC transfusion burden, and overall
hospitalizations. Another study that examined splenic function in
children escalated to their maximum tolerated dose of hydroxyurea did
reveal some benefit in preserving splenic function\textsuperscript{17}.
Surprisingly, we noted a significantly higher prevalence and event rate
of ASSC in children started on hydroxyurea before age 2 years compared
to those who started therapy after age 2 years. The difference was even
more striking when children started on hydroxyurea were compared to
those who were never started on this medication. While the retrospective
nature of our analysis and inherent differences between the sub-cohorts
clearly limit conclusions about causality, our results indicate a strong
correlation between the early initiation of hydroxyurea therapy and the
burden of splenic dysfunction in SCD. In general, hydroxyurea therapy
was initiated before the onset of ASSC among children who started
hydroxyurea before age 2 years, supporting the possibility of a causal
role for hydroxyurea (Table 4). Interestingly, there are other studies
that suggest a link between hydroxyurea therapy and splenic disease. In
a retrospective study of 28,580 patients in the Pediatrics Health
Information System database, the prevalence of splenectomy was 7.2\%
among patients on hydroxyurea therapy compared to 3.2\% in those not on
therapy. The age of splenectomy was also lower in the treated
group\textsuperscript{18}. In a cohort of 23 patients with Hb SS and Hb
S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0} treated with hydroxyurea, the onset of ASSC was
noted at substantially older ages than has been previously reported; the
authors hypothesize that preserved splenic function on hydroxyurea
therapy may be responsible for this delayed onset of
ASSC\textsuperscript{19}. Splenomegaly and hypersplenism have been
associated with elevated Hb F levels in Indian and Saudi Arabian
patients with Hb SS disease\textsuperscript{20,21,22}. A possible
mechanism for our association between hydroxyurea therapy and ASSC,
therefore, is that hemoglobin F induction by early initiation of
hydroxyurea therapy may counteract splenic infarction sufficiently to
preserve some splenic activity but paradoxically results in disordered
splenic filtration with progressive splenomegaly, ASSC, and
hypersplenism. It is also possible, on the other hand, that the
correlation we describe is due to increased hydroxyurea use in a
subpopulation of patients with more severe disease phenotype, including
splenic dysfunction.

There are some limitations to our study. The most prominent of these is
the retrospective nature of our analysis, which of necessity introduces
differences in baseline characteristics and treatment decisions between
groups as well as in the duration of follow-up. In comparing our group
of patients started on hydroxyurea by age 2 years and those started
after age 2 years, for example, the former group is predominantly from
the latter part of our study window while the latter is from the earlier
part. Additionally, clinical practices may have changed during the
period of our study, making comparisons over time prone to error. The
small sample size of our Hb S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0} patient cohort also
limits our analysis of this SCD subtype. Finally, while we reviewed our
patient charts extensively to minimize the likelihood of inaccurate or
missed diagnoses, it is possible that there are errors in our
stratification of patients into groups by genotype, types of splenic
dysfunction, and treatment status.

In summary, this study presents the largest retrospective cohort
analysis of pediatric splenic disease excluding loss of splenic
function. Our results indicate that splenomegaly and ASSC affect
approximately 25\% of all children with SCD , while chronic
hypersplenism has a prevalence of about 10\%. Additionally, children
with Hb S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0} have a significantly higher prevalence of
splenic dysfunction and incidence of ASSC than those with SS disease.
Children with Hb SC disease have a later onset of splenic dysfunction
but remain at risk of complications through the second decade of life,
while those with S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{+} disease have very low rates of
splenic complications. Finally, we identify a strong correlation between
the early initiation of hydroxyurea therapy and ASSC, though the
causative nature of this association remains unproven. It will be
important to prospectively evaluate the prevalence and severity of
splenic disease in children treated with hydroxyurea escalated to
maximum tolerated dose.

ACKNOWLEDGEMENTS

We acknowledge the invaluable assistance of Debasis Dash in extracting
diagnostic data on our study cohort from our electronic medical record
system. We also acknowledge the children living with sickle cell disease
who receive care at Texas Children's Hospital and their families for
their continuing support of our research activities.

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FIGURE LEGENDS

FIGURE 1: Prevalence of splenic complications by age: The cumulative
prevalence of splenomegaly, acute splenic sequestration crises (ASSC),
hypersplenism, and splenectomy in the entire cohort of 594 patients are
presented with the number at risk by age. Numeric values for cumulative
prevalence are presented in Supplemental Table 1.

FIGURE 2: Prevalence of splenic complications stratified by genotype.
The cumulative prevalence of (A) splenomegaly, (B) acute splenic
sequestration crises (ASSC), (C) hypersplenism, and (D) splenectomy are
presented for the Hb SS, Hb SC, and Hb S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0} genotypes
with the number at risk by age for each genotype. Numeric prevalence
values for each panel are presented in Supplemental Table 1.

FIGURE 3 Prevalence of splenic complications in Hb SC disease: extended
cohort analysis. The cumulative prevalence of splenomegaly, acute
splenic sequestration crises (ASSC), hypersplenism, and splenectomy in
the extended Hb SC cohort (0-18 years) of 309 patients, with the number
at risk by age. Numeric values for cumulative prevalence are presented
in Supplemental Table 1.

FIGURE 4 Prevalence of acute splenic sequestration crises stratified by
hydroxyurea exposure. The cumulative prevalence of acute splenic
sequestration crises (ASSC) for Hb SS and Hb
S\selectlanguage{greek}β\selectlanguage{english}\textsuperscript{0}patients is presented with the number at risk by
age. (A) Cumulative prevalence in patients treated with hydroxyurea (HU)
compared to those not treated (No HU). (B) Cumulative prevalence in
patients with hydroxyurea therapy initiated by age 2 years (HU [?]2)
compared to those with therapy initiated after age 2 years (HU
\textgreater{}2).\selectlanguage{english}
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\textbf{Hosted file}

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