Blood Transfusion Vs. Hydroxyurea for Stroke Prevention in Children With Sickle Cell Anemia: A Systematic Review and Meta-Analysis

Sickle cell anemia (SCA) is a hereditary condition that can lead to severe complications in children such as acute coronary syndrome, splenic sequestration, renal failure, and stroke. Blood transfusion and hydroxyurea (HU) therapy are used to prevent stroke in children with sickle cell disease (SCD). Preliminary data show considerable variation and inconsistency in the use of these two therapeutic interventions. Therefore, this systematic review was carried out to compare the effects of blood transfusion to HU therapy in preventing stroke for children with sickle cell disease. There was an extensive literature search in reliable and authentic databases like PubMed, Medline, Scopus, Cochrane, and Science Direct to obtain relevant articles. This study used the standards and guidelines from the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA). During the systematic review, data were obtained focusing on the following parameters: the size of the sample in the study, the age of the subjects involved in the study, the type of Intervention, and the outcome. After an initial search of 163 papers, 25 studies were included. The results of the research give the first evidence that HU is effective in the treatment of cerebrovascular problems in children with SCD. However, it is unclear under what circumstances HU may prevent a second stroke. It can be concluded that children with SCA can effectively avoid primary strokes through chronic blood transfusions and HU.


Introduction And Background
Sickle cell anemia (SCA) is a hereditary condition that increases the risk of cerebral vasculopathy, which can result in strokes, especially in children which exposed them to a high-risk category for developing a stroke. Stroke is among the most severe clinical consequences that can affect children with sickle cell disease (SCD) and is a major cause of mortality in this patient group [1][2][3]. A stroke occurs in around 5%-10% of children with SCD, most frequently in their first decade of life. The recurrence of stroke was observed in 46% of children with SCD at an average interval of nine months according to the transcranial Doppler (TCD), which detects patients at risk of strokes [4,5]. Patients who were observed as having high cerebral velocities (>200 cm/s) were considered abnormal and had a risk of 40% for stroke to occur within three years after detection [4][5][6][7]. However, people with normal cerebral velocities (<170 cm/s) had a 2% or less risk of stroke. Blood transfusions and hydroxyurea (HU) play the main role in the prevention of stroke in children with SCA. In individuals with SCD, transfusion treatment has been demonstrated to be useful in preventing both main and secondary strokes [8]. Specific requirements determine the choice for either using HU therapy or blood transfusion. According to international standards, individuals with an acute ischemic stroke should be treated with exchange transfusions with the aim of achieving hemoglobin (Hb) of 10 g/dL and HbS of <30% [8]. Also, treatment for SCD patients is mostly supportive with HU serving as the only widely prescribed medication that alters the etiology of the illness. By raising fetal hemoglobin (HbF), it enhances clinical outcomes and lowers the likelihood of "sickling" episodes [9]. HU therapy helps reduce the incidences of blood transfusion, acute chest syndrome, acute pain, and the mortality of SCD patients [10]. It is a useful alternative for regular blood transfusion for SCD [11].
Many pediatric patients with SCD and stroke have persistent physical and neuropsychological impairments [12][13][14]. Nine out of 10 (90%) untransfused juvenile patients with SCD and stroke experienced a recurrent stroke [15]. Transfusions can be given as a simple transfusion or as an exchange transfusion. The goals of transfusion in SCD are to enhance oxygen-carrying capacity while also decreasing the fraction of sickle hemoglobin (HbS) relative to hemoglobin A (HbA) to avoid or reverse vaso-occlusion consequences. In acute conditions, simple transfusion will enhance oxygen-carrying capacity. Exchange transfusion provides the benefit of both raising oxygen-carrying capacity and lowering HbS% [16,17]. In individuals on long-term transfusion, both repeated simple or exchange transfusions can maintain a low HbS%. Simple transfusion is the most frequently used type of transfusion in chronic transfusion programs in children with the need for iron chelation therapy after approximately one year of transfusion [16,17]. After terminating a short-term (1-2 years) transfusion regimen, a 70% recurrence rate was reported, and a 50% recurrence rate was observed after terminating a long-term (5-12 years) transfusion regimen [18,19]. Therefore, most juvenile hematologists recommend continuous chronic transfusions to minimize recurrent strokes, even with the long-term potential consequences of transfusions, which include the possibility of the transfer of infectious organisms, erythroid alloimmunization, and iron overload. Even though transfusion treatment is effective, some patients are unable to continue it. For instance, there is a report of two young individuals with SCD and stroke who were incapable of continuing their chronic transfusion treatment and had to stop receiving transfusions. Then, as a preventative measure against stroke recurrence, the patients received oral HU therapy [20]. A Phlebotomy regimen was adopted to minimize iron excess and promote endogenous erythropoiesis. Both patients reacted favorably to the HU treatment and neither patient experienced stroke recurrence during nearly three years of therapy. Based on this testimonial success, blood transfusions were terminated prospectively in a new and bigger sample of children with SCD and stroke in another report [4]. The findings show that some children with SCD and stroke might be able to stop receiving chronic transfusions and instead utilize daily oral HU treatment as stroke prevention. Although much of the morbidity related to chronic blood transfusion treatment has been decreased because of better testing of blood units for pathogenic pathogens, iron overload is still an important long-term issue [21]. Long-term cohort research showed that switching from continuous transfusion to HU with follow-up can be safe in some patients [22]. An advantage of HU is its potential to reduce the high white blood cell counts linked with early mortality, stroke, and acute chest syndrome in SCA [23]. Therefore, searching for effective alternatives and evidence regarding the clinical efficacy of HU in comparison to blood transfusion in pediatric patients with SCD is warranted to overcome such an issue. Preliminary findings demonstrate a significant variation and inconsistency in the use of these two therapy techniques. Limited studies have been conducted to address preventive options for stroke associated with SCD. Therefore, this study sought to compare the effects of blood transfusion to HU therapy in preventing stroke for children with SCD.

Study Design
This study used the standards and guidelines from the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA). The study was prepared using PRISMA extensions published in the Cochrane Handbook for Systematic Reviews of Interventions [24].

Search Strategy
Cochrane, PubMed, Medline, Scopus, and Science Direct were used as electronic databases to identify relevant articles for the study. The search strategy utilized keywords and their combinations, field tags, truncations, and Boolean operators "AND" and "OR." Search strings were used to ensure that an accurate scope of the materials required was covered effectively. The Mesh used in the study included "Anemia, Sickle Cell/prevention and control"[Mesh] OR "Anemia, Sickle Cell/therapy" [Mesh], "Stroke/blood" [Mesh] OR "Stroke/prevention and control" [Mesh] OR "Stroke/therapy" [Mesh], "Blood Transfusion/blood" [Mesh] OR "Blood Transfusion/therapy" [Mesh], "Hydroxyurea/administration and dosage" [Mesh] OR "Hydroxyurea/therapeutic use" [Mesh]. The studies identified were put together for search on their eligibility.

Eligibility Criteria
The researchers agreed upon the eligibility criteria and guidelines for the study to ensure the best articles were identified. Articles selected had to pass through the PICOS criteria (Population, Intervention, Comparison, and Outcomes). The population needed for the study had to be children. The intervention for the study was blood transfusion, the control was HU therapy, and the outcome was stroke prevention. Included articles had to have the participating children between one and 15 years old. Studies that were not related to children were excluded. Included articles had to have been published in English or translated to English. Unpublished articles were not selected as part of the research study. Articles that had written about other medicines and pharmaceutical compounds other than hydroxycarbamide or HU were excluded.

Quality Assessment
A quality appraisal for the selected studies was performed using AXIAL criteria. The quality assessment sections were divided into four. The first section was on the introduction and its validity on the aims and objectives of the study. Section 2 was on the validity of the methods for the included studies. Sections 3 and 4 covered the results and discussions, respectively. Answers for the assessment questions were categorized into yes, no, and not applicable, which were abbreviated to "Y," "N," and "NA," respectively.

Data Extraction
The researchers used predesigned Excel worksheets in the extraction of data. The researchers extracted the authors, the year of publication, interventions, outcomes, and observed results. An analysis of the studies identified and the results got was done to ensure that there was unity. Whenever cases of dispute arose in any matter, a third party was used in the resolution process.

Result Analysis
Two forms of analysis were adopted for this investigation. We used qualitative assessment (systematic review) and quantitative assessment (meta-analysis). Literal analysis was adopted to conduct a systematic review of the evidence provided by the included studies. Statistical analysis was conducted on STATA Version 17.0 (Stata V.17.0). The analysis determined the heterogeneity between the included studies using the T 2 , I 2 , and H 2 statistics. A level of heterogeneity between 25%<I 2 < 50% and below was considered more fitting. In the meta-analysis, the investigation adopted a random-effects model to find the odds ratio to determine the effect size of the outcomes. The P-values shall determine the test's significance; significance shall be attained if the P-value ≤0.05. The results of this meta-analysis were represented in various forest plots. Funnel plots were also generated to show the symmetry of the included studies and represent the studies' publication bias to give credence to the relationships created.

Study Selection
The study search conducted in the electronic databases identified 163 studies. These studies were identified using the search strings. Seven articles were excluded as they were duplicates remaining with 156 studies. A screening process was then conducted by sorting through the titles, abstracts, and the outcomes of interest. In this stage, 75 records were excluded remaining 81 articles. Finally, records were sought for retrieval, and only 36 were retrieved, excluding 45 records. Five studies were excluded because of having low-quality data and six were excluded for failing to meet the inclusion criteria. Twenty-five studies were identified and used for the systematic review and meta-analysis. Figure 1 represents a PRISMA flowchart diagram that shows the study selection process. The characteristics of the included studies are shown in Table 1.

Author Demography Intervention Outcome
Ware et al.

Quality Analysis
The study's quality analysis was conducted using AXIAL. The analysis was grouped into introduction, method, results, and discussions. The answers to the quality assessment were yes, no, and not applicable, which were shortened to "Y," "N," and "NA." The AXIAL assessment appraisal results of the included studies are shown in Table 2.

Methods
Was the study design appropriate for the stated aim of the study?
Was the target/reference population clearly defined? (Is it clear who the research was about?) Was the sample frame taken from an appropriate population base so that it closely represented the target/reference Was the selection process likely to select subjects/participants that were representative of the target/reference population Were measures undertaken to address and categorize non- Were the risk factor and outcome variables measured appropriate to the aims of the study?
Is it clear what was used to determine statistical significance and/or precision estimates? (e.g. P-values, confidence intervals) Were the methods (including statistical methods) sufficiently described to enable them to be repeated?

Results
Were the basic data adequately described?
Does the response rate raise concerns about non-response bias?
If appropriate, was information about non-responders described?
Were the results presented for all the analyses described in

Discussion
Were the authors' discussions and conclusions justified by the results? The AXIAL assessment appraisal results of the remaining included studies are presented in Table 3.

Introduction
Were clear aims and objectives of the study used?

Methods
Was the study design appropriate for the stated aim of the study?
Was the target/reference population clearly defined? (Is it clear who the research was about?) Was the sample frame taken from an appropriate population base so that it closely represented the target/reference Was the selection process likely to select subjects/participants that were representative of the target/reference population Were measures undertaken to address and categorize non- Were the risk factor and outcome variables measured appropriate to the aims of the study?
Is it clear what was used to determine statistical significance and/or precision estimates? (e.g. P-values, confidence intervals)

N Y
Were the methods (including statistical methods) sufficiently described to enable them to be repeated?

Results
Were the basic data adequately described?
Does the response rate raise concerns about non-response bias?
If appropriate, was information about non-responders described?
Were the results presented for all the analyses described in the methods?

Discussion
Were the authors' discussions and conclusions justified by the results?

Transfusion
Five studies reported the effects of transfusion on reducing the incidence of stroke. The overall test results reveal a random effects size and odds ratio of 14.92 [-1.32, 31.17] at 95% CI. This meta-analysis shows no significant impact of transfusion on children with SCA (P = 0.07). The studies used to analyze this were not heterogeneous (T 2 = 000; I 2 = 0.00%; H 2 = 1.00). Figure 2 is a forest plot summarizing this analysis.

FIGURE 2: Forest plot summarizing the analysis of the effects of transfusion on stroke incidences in children with sickle cell anemia.
Included studies are Aygun et al. [29], Brousse et al. [30], Mirre et al. [31], Moreira Franco et al. [32], and Wood et al. [33]. Figure 3 is a funnel plot reporting the publication bias between the previous five studies.

Hydroxyurea
Five of the 15 studies reporting on HU's effects on reducing stroke incidence provided data for a metaanalysis. The overall test results reveal a random effects size and odds ratio of 9.43 [-8.16, 27.01] at 95% CI. This meta-analysis shows no significant impact of HU on children with SCA (P = 0.29). The studies used to analyze this were not heterogeneous (T 2 = 82.77; I 2 = 9.37%; H 2 = 1.10). Figure 4 is a forest plot summarizing this analysis.

Both (Transfusion and HU)
Two of the five studies reporting on the effects of both transfusion and HU on reducing the incidence of stroke were used in this analysis. The overall test results reveal a random effects size and odds ratio of 5.17 [-25.72, 36.07] at 95% CI. This meta-analysis shows no significant impact of using both transfusion and HU on children with SCA (P = 0.74). The studies used to analyze this indicated a low level of heterogeneity (T 2 = 0.00; I 2 = 0.00%; H 2 = 1.00). Figure 6 is a forest plot summarizing this analysis.

Discussion
A stroke occurs in approximately 11% of children with SCA by the age of 20 years. The first episode occurs more frequently from 2-5 years old [39]. Approximately 50% of children with SCA and an initial stroke that do not receive chronic transfusions develop another stroke in the following three years. By comparison, only 10% of transfused patients have a reoccurring stroke [39]. Significant downsides of chronic transfusions are iron overload, which may lead to fatality or serious arrhythmias. It has also not been determined how much time they should be continued. Also, unidentified is the risk of reoccurring stroke after transfusions are terminated [39].
A preliminary information recommend HU therapy in some children with SCD as an alternative to blood transfusion for recurrent stroke prevention [4]. The increase in HbF criteria (%HbF and %F cells) achieved by HU is most likely important in avoiding in vivo sickling within the stenotic analytical vessels. Patients accomplished an average %HbF of approximately 20% and %F cells of approximately 80%, which could prevent intracellular sickling [4]. Additional feasible devices include reductions in total white blood cell matter and neutrophil matter with HU treatment. Moreover, improved rheological qualities of the erythrocytes brought about changes in erythrocyte morphology, adhesiveness, and cation material. Using the same HU dosage escalation schedule revealed that the Hb concentration, mean corpuscular volume (MCV), %HbF, and %F cells increase throughout the first six months of HU treatment [4]. However, they continued to increase between six months and a year of treatment. The phlebotomy program was designed to lower the iron concentration and promote erythropoiesis, thereby boosting the variety of HbF-containing reticulocytes and preventing stroke reoccurrence. The phlebotomy regimen was very well tolerated, and most patients had 10 mL/kg of blood removed every two weeks while preserving a Hb concentration above 8 g/dL. Phlebotomy has led to constant reductions in the total body iron stores, with considerable diminution in the lotion ferritin [4]. HU could also be considered for the avoidance of primary stroke. TCD can recognize children with SCD with a boosted risk of primary stroke, and transfusion treatment can prevent primary stroke in scientific setups.
Considering the risk of serious adverse effects associated with chronic transfusion and the constant improvement with the time of the transplant outcomes with gene-identical donors, domestic human leukocyte antigen (HLA) inputting was performed, and the transplant was discussed with parents of children with SCD [22]. The study reported previously that participants transplanted since 2000 had a 95% chance of cure, a percent confirmed in a French collection, with about a 3% risk of death. In the examined cohort, 24 of 92 participants were transplanted. Donor engraftment was effective, and chronic transfusion was done by all participants [22]. Moreover, quick normalization of velocities occurred in four participants that had irregular velocities before the transplant despite chronic transfusion. HU was efficiently re-introduced in six participants after the new normalization of chronic transfusion. The Kaplan-Meier (KM) estimate of the percentage of participants with irregular TCD imaging reversion on HU at one year was 19.2% (95% CI, 7-31.4), which is below the 45% possibility of occasions observed in the transfusion [22]. HU treatment was originally promising for additional stroke prevention, and the Stroke with Transfusions Changing to Hydroxyurea (SWiTCH) trial randomized participants with a stroke background to either pursuing chronic transfusion with iron chelation or HU with phlebotomy. It was concluded that transfusion/chelation remains the best way to manage children with SCA and stroke [29]. The first stroke was reported to occur when seven years old. Stroke was more common in serious genotypes, and ischemic stroke was more common compared to hemorrhagic stroke at this age [40]. HU treatment results in beneficial hematological and biochemical impacts that resemble those observed in participants in chronic transfusion programs and which are most likely to add to improved blood flow in vulnerable analytical vessels [40]. These impacts include raised hematocrit, lowered sickling, red cell adhesion, improved red cell rheology, and lowered hemolysis. Although the system of activity of chronic blood transfusions in minimizing the risk of stroke in children with SCD is not well comprehended, the resemblance of the observed impacts recommends HU as a feasible replacement for chronic blood transfusions. However, there are a couple of records where HU was used as the single treatment to avoid stroke reoccurrence. Most records on the use of HU for additional avoidance have remained in patients who formerly began on transfusion programs but were reluctant to proceed with chronic transfusion, with fairly excellent outcomes reported.
It was shown that 90% of the participants revealed a surge in Hb degree after HU therapy, and 75% of those that had the available outcome of HbF degree revealed a substantial increase in HbF after HU therapy consistent with a reputable effect of HU in children and adult with SCD [45]. It was well established that a high HbF concentration minimizes the intensity of SCD by preventing the development of HbS polymers. Also, the MCV was raised after HU therapy in 80% of the participants [45]. MCV values increased in lockstep with HbF concentrations, making MCV a good affordable substitute for HbF during therapy. When MCV increases, it improves sickle erythrocyte hydration, which increases sickle cell deformability which further adds to the effect of HU in minimizing the regularity of painful crisis. It was shown that a regular transfusion program designed to maintain HbS values below 30% is practical and safe in children with SCD at high risk for stroke [31].
Children registered in TCD With Transfusions Changing to Hydroxyurea (TWiTCH) trial had elevated iron build-up in both the liver and spleen; most had iron deposition in the kidney and some also had pancreatic deposition. If the intensity of parenchymal iron deposition was unexpected, the heterogeneity is perhaps not because the liver, pancreatic, kidney, and spleen have various systems and amounts of iron uptake. Considerable iron amount within the organs is a concern of a large cohort of children with SCA who are receiving chronic transfusions for proven cerebrovascular conditions. TWiTCH trial outcomes document the effectiveness of HU treatment for a cohort of children with SCA at high risk for primary stroke [46]. Particularly, children on the alternative arm that received an overlap duration with transfusions until attaining a steady HU maximum tolerated dose (MTD) efficiently maintained their average TCD rate throughout the study therapy duration. HU has numerous beneficial effects that should benefit children with SCA and irregular TCD velocities. One of the most important benefits is HbF induction, which decreases the intracellular % of HbS, prevents sickle polymer development, and prolongs erythrocyte survival [46]. TWiTCH individuals had a durable therapy reaction to HU, reaching an average of 27% HbF at MTD. However, since swelling and endothelial vasculopathy are features of the cerebrovascular condition, the lowered leukocyte and reticulocyte matters from HU should also be useful. Managing transfusionacquired iron overload is also difficult for children on chronic transfusion treatment. After getting to a constant HU MTD and discontinuing transfusions, individuals in the alternative arm received monthly healing phlebotomy to lower their iron worry. Chronic transfusions are suggested for children with SCA and irregular TCD velocities (≥200 cm/s) to provide help prevent the event of the main stroke [28]. Irregular TCD velocities (>200 cm/s) are associated with a high risk for stroke and require transfusion treatment to lower the risk of primary stroke. The objective is usually to maintain the HbS concentration <30%; however, this objective is often difficult to attain in clinical practice. HU treatment can replace chronic transfusions to sustain TCD velocities and assist in the prevention of primary stroke in high-risk children with SCA, abnormal TCD velocities, and at least one year of transfusions who do not have MRA-defined severe vasculopathy [46]. In addition, continuous HU administration since childhood seems safe and effective for SCA [47]. Initial moderate fixed-dose HU therapy is equivalent to initial regular blood transfusion therapy and preferable to no therapy [48].

Conclusions
Ninety percent of children with SCA and irregular TCD velocities can be efficiently protected against the first stroke using chronic transfusion treatment to maintain HbS concentrations lesser than 30%. Prevention of cerebrovascular conditions in children with SCA, preferably through very early treatment with diseasemodifying treatment, is the supreme objective. Although hereditary versions may influence stroke sensitivity, most stroke occasions in children with SCA remain inexplicable. HU can currently be considered a substitute treatment for preserving TCD velocities and possibly preventing primary stroke in selected highrisk children with SCA.

Conflicts of interest:
In compliance with the ICMJE uniform disclosure form, all authors declare the following: Payment/services info: All authors have declared that no financial support was received from any organization for the submitted work. Financial relationships: All authors have declared that they have no financial relationships at present or within the previous three years with any organizations that might have an interest in the submitted work. Other relationships: All authors have declared that there are no other relationships or activities that could appear to have influenced the submitted work.