Sickle Cell Crises (Vaso-occlusive, Acute Chest Syndrome, Stroke Prevention), Immune Thrombocytopenia (ITP), and Haemophilia A/B

Sickle Cell Disease Crises, Stroke Prevention, and Pediatric Bleeding Disorders

Hematological disorders in children encompass chronic genetic hemoglobinopathies requiring complex preventative and emergency protocols, as well as acute and inherited bleeding disorders. Mastering the pathophysiological mechanisms of sickling, the recognition of life-threatening sickle crises, and the structured evaluation of abnormal coagulation assays is critical for both acute hospital care and specialist certification on the European Board of Paediatrics (EBP-EAP) examination.

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Sickle Cell Disease: Genetics, Pathophysiology, and Preventative Protocols

Sickle Cell Disease (SCD) refers to a group of inherited hemoglobinopathies characterized by the presence of hemoglobin S (HbS). The most common and severe genotype is homozygous Sickle Cell Anemia (HbSS), followed by compound heterozygous states such as Sickle-Hemoglobin C disease (HbSC) and Sickle-Beta Thalassaemia (HbS/β⁰ or HbS/β⁺).

Molecular Genetics and Pathophysiology

• Molecular Defect: HbS arises from a single nucleotide point mutation in the beta-globin gene (HBB located on chromosome 11p15.5), where adenine is substituted by thymine (GAG → GTG). This encodes valine instead of glutamic acid at position 6 of the beta-globin polypeptide chain (β⁶Glu → Val).
• Polymerization Cascade: Under conditions of deoxygenation, acidosis, dehydration, or hypothermia, the exposed hydrophobic valine residue inserts into a hydrophobic pocket on an adjacent beta-globin subunit. Deoxy-HbS molecules polymerize into rigid, insoluble 14-strand helical crystalline fibers. These fibers align longitudinally, distorting the erythrocyte from a compliant biconcave disk into an inflexible, crescentic "sickle" cell.
• Microvascular Occlusion and Endothelial Injury: Sickled erythrocytes cannot deform through capillaries (diameter 3–5 µm). They adhere to activated endothelial cells via adhesion molecules (VLA-4, CD36), trapping leukocytes and platelets, obstructing microvascular beds, and causing ischemic tissue infarction, reperfusion oxidative damage, and chronic organ dysfunction.
• Hemolysis and Functional Asplenia: Sickled cells suffer membrane damage, reducing red cell survival to 10 to 20 days (compared to 120 days normally). By 6 months of age, as protective fetal hemoglobin (HbF) declines, repeated microvascular sickling within the splenic red pulp leads to progressive micro-infarctions and autosplenectomy. By 2 to 4 years of age, virtually all children with HbSS have complete functional asplenia, evidenced by Howell-Jolly bodies on the blood film.

Comprehensive Preventative Protocols

Because functional asplenia creates extreme vulnerability to encapsulated bacterial sepsis—particularly Streptococcus pneumoniae and Neisseria meningitidis—strict prophylactic protocols must be instituted:

1. Infection Prophylaxis:
   • Daily Oral Penicillin V: Initiated at 2 to 3 months of age (or immediately upon newborn screening confirmation) and continued until at least 5 years of age (or lifelong in many European centers). Dosing: 125 mg twice daily under 3 years; 250 mg twice daily from 3 years onwards.
   • Immunization Schedule: Follow current national schedules for functional asplenia, including risk-based pneumococcal, MenACWY/MenB and indicated Hib doses plus annual injectable influenza vaccination. Pneumococcal choices depend on age and previous doses and may include PCV20. If a PPSV23 strategy requires a repeat dose, the interval is at least 5 years after the first PPSV23, rather than fixed doses at ages 2 and 5.
2. Emergency Fever Protocol: Any temperature ≥ 38.5°C in a child with SCD constitutes a life-threatening medical emergency. Overwhelming pneumococcal bacteremia can progress from fever to septic shock and death within hours. Management requires immediate triage, blood cultures, full blood count, and administration of IV Ceftriaxone (50–80 mg/kg) within 60 minutes of arrival ("door-to-antibiotic time <1 hour"). Home antipyretics or prescribed oral antibiotics must never delay urgent hospital assessment and parenteral treatment; follow the child’s hematology emergency plan.
3. Transcranial Doppler (TCD) Screening and Primary Stroke Prevention:
   • Without screening, up to 10% of children with HbSS suffer an overt ischemic stroke before age 20. Large-vessel stenosis of the distal internal carotid and proximal middle cerebral arteries creates turbulent, high-velocity cerebral blood flow.
   • Annual TCD Ultrasonography is mandatory for all children with HbSS and HbS/β⁰ from age 2 to 16 years.
   • Risk Stratification by Non-Imaging TCD Time-Averaged Mean Maximum Velocity (TAMMV):
     • Normal: <170 cm/s (re-screen annually).
     • Conditional: 170 to 199 cm/s (re-screen in 3 months).
     • Abnormal (High Risk): ≥200 cm/s; confirm results of 200–220 cm/s promptly. A single >220 cm/s warrants urgent specialist treatment without awaiting another scan. Imaging TCD uses different treatment thresholds (two TAMX measurements ≥185 cm/s or one >205 cm/s).
   • Intervention for High Risk: Children meeting abnormal non-imaging TCD criteria (two measurements ≥200 cm/s or one >220 cm/s) should promptly enter a program of regular chronic blood transfusions (simple or automated red cell exchange) every 3 to 4 weeks to suppress endogenous erythropoiesis and maintain HbS <30%, reducing the 3-year risk of stroke by over 90%.
4. Disease-Modifying Pharmacotherapy: Hydroxycarbamide (Hydroxyurea):
   • Hydroxycarbamide is a ribonucleotide reductase inhibitor that stimulates γ-globin gene expression, markedly increasing fetal hemoglobin (HbF) levels. HbF does not participate in HbS polymer formation and physically prevents polymer chain elongation.
   • Additional effects: Lowers leukocyte and platelet counts (reducing microvascular cellular sludging), increases erythrocyte volume (MCV), and promotes endothelial nitric oxide release.
   • European guidelines recommend offering hydroxycarbamide to all children with HbSS from 9 to 12 months of age, regardless of symptoms, to prevent chronic organ damage, or at least for frequent vaso-occlusive crises (≥ 3 per year), history of acute chest syndrome, or severe symptomatic anemia.

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Acute Sickle Cell Crises: Diagnosis and Management Protocols

                                  [Acute Sickle Cell Crises]
                                             |
         +---------------------+-------------+-------------+---------------------+
         |                     |                           |                     |
  [Vaso-Occlusive Pain]  [Acute Chest Syndrome]   [Splenic Sequestration]  [Aplastic Crisis]
  - Dactylitis (infants) - New CXR infiltrate     - Rapid, painful         - Parvovirus B19
  - Bone / back pain     - Fever, tachypnea, O2 drop splenomegaly          - Reticulocytes <1%
  - Step-up analgesia    - O2, Ceftriaxone+Macrolide- Severe anemia+Shock  - Severe drop in Hb
  - IV hydration         - Simple or Exchange Tx  - Resuscitate + Tx       - Transfusion support

1. Vaso-occlusive Painful Crisis (VOC)

• Clinical Presentation: The most frequent acute manifestation. In infants aged 6 to 18 months, microvascular marrow infarction of metacarpals and metatarsals causes dactylitis ("hand-foot syndrome"), characterized by painful, symmetrical, warm swelling of the dorsal hands and feet. In older children, pain affects long bones (femur, humerus), spine, ribs, and pelvis.
• Management Protocol:
  • Prompt Analgesia: Administer effective analgesia within 30 minutes of arrival. Use the WHO analgesic ladder: mild pain with paracetamol and oral NSAIDs (ibuprofen); moderate to severe pain requires immediate parenteral opioids (intravenous morphine 0.1 mg/kg), followed by regular scheduled dosing or patient-controlled analgesia (PCA).
  • Hydration: Maintenance intravenous isotonic fluids (e.g., 0.9% NaCl with 5% Dextrose) if oral intake is reduced; avoid aggressive hyperhydration (>1.5 times maintenance) as it precipitates pulmonary edema and acute chest syndrome.
  • Supplemental Oxygen: Administer oxygen only if SpO2 <95% on room air. Hyperoxia suppresses erythropoiesis without relieving sickling in well-oxygenated patients.

2. Acute Chest Syndrome (ACS)

• Definition and Prognosis: Defined as a new pulmonary infiltrate on chest radiography combined with at least one new clinical sign or symptom: fever (≥ 38.5°C), chest pain, tachypnea, cough, wheezing, or an acute fall in SpO2 ≥ 2–3% from baseline. ACS is the leading cause of death and ICU admission in sickle cell disease.
• Pathophysiology: Triggered by a complex interplay of pulmonary infection (Chlamydia pneumoniae, Mycoplasma pneumoniae, viruses), pulmonary fat embolism from infarcted necrotic bone marrow, and microvascular pulmonary sickling causing local alveolar hypoxia and atelectasis (often worsened by hypoventilation from rib or sternal bone pain).
• Emergency Treatment Protocol:
  1. High-flow Oxygen: Titrate inspired oxygen to maintain SpO2 ≥ 95%.
  2. Empiric Antimicrobial Therapy: Immediate IV 3rd-generation cephalosporin (Ceftriaxone 50–80 mg/kg/day or Cefotaxime) PLUS a macrolide (Azithromycin or Clarithromycin) for atypical coverage.
  3. Careful Fluid Management: Limit IV fluids to 75% to 100% of maintenance. Overhydration leads to pulmonary capillary leak and rapid clinical deterioration.
  4. Incentive Spirometry and Pain Control: 10 deep breaths every 2 hours while awake to reverse atelectasis; adequate analgesia prevents chest wall splinting.
  5. Blood Transfusion Strategy:
     • Simple Transfusion (10 mL/kg packed RBCs): Consider early for hypoxia, increasing oxygen needs or symptomatic ACS with Hb >10 g/L below baseline; discuss with a sickle specialist. Keep post-transfusion Hb around 100–110 g/L or lower as appropriate to avoid hyperviscosity.
     • Emergency Red Cell Exchange (Erythrocytapheresis): Mandatory for severe or deteriorating ACS (worsening multilobar disease, severe tachypnea, PaO2 <60 mmHg, or falling SpO2 despite high-flow oxygen). It rapidly lowers HbS to <30% while avoiding hyperviscosity and volume overload.

3. Acute Splenic Sequestration Crisis

• Pathophysiology: Sudden, massive intrasplenic entrapment of red blood cells within the cords of Billroth, causing vascular pooling and acute organ enlargement.
• Clinical Presentation: Typically occurs in infants and young children with HbSS (between 6 months and 3 years, prior to complete autosplenectomy) or older children with HbSC. Presents with sudden lethargy, extreme pallor, and a rapidly enlarging, exquisitely tender, firm spleen extending into the pelvis, culminating in hypovolemic circulatory shock.
• Laboratory Profile: Severe, sudden fall in hemoglobin (often falling to 20–40 g/L), marked reticulocytosis (up to 15–30%, proving intact marrow response), and acute thrombocytopenia (platelets trapped alongside red cells in the spleen).
• Management: Emergency fluid resuscitation with isotonic crystalloid boluses to restore intravascular volume, followed by cautious blood transfusion (small aliquots of 5–10 mL/kg). As the spleen decompresses and pooled erythrocytes mobilize back into the central circulation, rapid "autotransfusion" can cause fluid overload and dangerous hyperviscosity if excessive blood was transfused.

4. Aplastic Crisis

• Pathophysiology: Caused by infection with Parvovirus B19, which selectively infects and arrests erythroid progenitor cells in the bone marrow, halting red cell production for 7 to 10 days.
• Clinical vs Sequestration: Insidious onset of severe pallor, extreme lethargy, and high-output cardiac flow murmurs. In contrast to splenic sequestration, the spleen does not enlarge, and platelet and white blood cell counts are relatively preserved.
• Laboratory Hallmarks: Precipitous fall in hemoglobin accompanied by severe reticulocytopenia (reticulocyte count <1%, often <0.1% or <10 × 10⁹/L).
• Management: Hospital admission, respiratory isolation (to prevent transmission to other hematology patients and pregnant healthcare workers), and packed red blood cell transfusions until spontaneous marrow recovery occurs (heralded by a surge in reticulocytes).

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Pediatric Bleeding Disorders

Abnormal bleeding in children is broadly divided into defects of primary hemostasis (platelets and von Willebrand factor, causing mucocutaneous bleeding: petechiae, purpura, epistaxis, menorrhagia) and defects of secondary hemostasis (coagulation factors, causing deep hematomas and hemarthroses).

Immune Thrombocytopenia (ITP)

• Pathogenesis: An acquired autoimmune disorder occurring most commonly in children aged 2 to 6 years, typically developing 1 to 3 weeks after an unremarkable viral illness. Anti-platelet autoantibodies (predominantly IgG directed against glycoprotein complexes GPIIb/IIIa or GPIb/IX) coat circulating platelets, leading to accelerated Fc-receptor-mediated clearance by splenic macrophages.
• Clinical Picture: Sudden onset of widespread petechiae, superficial ecchymoses, and purpura in an otherwise thriving, vigorous child. Physical examination reveals zero hepatosplenomegaly, zero bone tenderness, and zero pathological lymphadenopathy.
• Diagnostic Criteria: Isolated thrombocytopenia (platelet count often <20 × 10⁹/L) with a completely normal hemoglobin, normal red blood cell indices, normal total and differential white cell count, and normal blood film (showing normal or large platelets without malignant blasts). If constitutional symptoms, bone pain, organomegaly, or other cytopenias are present, leukemia must be excluded via bone marrow aspiration.
• "Dry" vs "Wet" Purpura and Management Strategy:
  • Dry Purpura (Cutaneous only): Children with bruising and petechiae without active mucosal bleeding require conservative observation and watchful waiting, regardless of how low the platelet count is. Advise avoidance of contact sports, intramuscular injections, and aspirin/NSAIDs. Over 75% to 80% of pediatric cases undergo complete, spontaneous remission within 6 to 12 months.
  • Mucosal Bleeding: Occasional mild, self-limited epistaxis may be observed. Significant or persistent epistaxis, extensive buccal blood blisters, gastrointestinal bleeding or gross hematuria warrants active treatment and specialist assessment; management depends on bleeding severity rather than the platelet count alone:
    • Intravenous Immunoglobulin (IVIG): Single infusion of 0.8 to 1.0 g/kg (saturates macrophage Fc receptors, rapidly elevating platelet count within 24–48 hours).
    • Short-course Corticosteroids: Oral Prednisolone (3–4 mg/kg/day for 4 days or 2 mg/kg/day for 14 days). Diagnostic caution: Routine marrow examination is unnecessary in typical isolated ITP, even before corticosteroids. Atypical examination, blood counts or smear findings require hematology assessment and investigation for leukemia before empiric steroids.
    • Platelet transfusions are ineffective for routine use because transfused platelets are rapidly cleared by circulating autoantibodies; transfusions are strictly reserved for life-threatening hemorrhage (e.g., suspected intracranial bleeding) alongside IVIG and IV methylprednisolone.

Haemophilia A and Haemophilia B

• Genetics and Coagulation Cascade: Both disorders are X-linked recessive coagulopathies affecting predominantly males (females are usually asymptomatic carriers). Haemophilia A involves deficiency of Factor VIII (representing 80–85% of hemophilia cases); Haemophilia B (Christmas disease) involves deficiency of Factor IX (15–20%). Factor VIII and IX form the "tenase complex" on the platelet surface, activating Factor X in the intrinsic pathway.
• Severity Classification:
  • Severe: <1% factor activity (<0.01 IU/mL). Manifests in infancy with spontaneous hemarthroses (bleeding into synovial joints: knees, elbows, ankles) and deep intramuscular hematomas. Spontaneous joint bleeds cause chronic inflammatory synovitis and destructive hemophilic arthropathy.
  • Moderate: 1% to 5% factor activity. Bleeding occurs with minor trauma or muscular strain.
  • Mild: >5% to 40% factor activity. Diagnosed late; bleeding occurs only with major surgery, tooth extraction, or severe trauma.
• Coagulation Profile: Characterized by an isolated prolonged activated partial thromboplastin time (aPTT) with a completely normal Prothrombin Time (PT), normal INR, normal thrombin time, normal fibrinogen, and normal platelet count. A 1:1 mixing study with normal plasma corrects the aPTT, confirming factor deficiency rather than an inhibitor.
• Treatment:
  • Prophylaxis: Individualize Factor VIII or IX dose and interval by product, pharmacokinetics and bleeding phenotype. Troughs >3–5% or higher are often targeted; the historical 1% target can permit breakthrough bleeding.
  • Non-Factor Prophylaxis (Emicizumab): A humanized bispecific monoclonal antibody that bridges activated Factor IX and Factor X, mimicking the cofactor activity of Factor VIII. Administered subcutaneously once weekly or every 2–4 weeks; approved for Haemophilia A with or without Factor VIII inhibitors.
  • Mild Haemophilia A Management: Desmopressin (DDAVP) (intravenous 0.3 mcg/kg or high-concentration nasal spray) stimulates release of endogenous Factor VIII and von Willebrand factor from endothelial Weibel-Palade bodies, temporarily tripling circulating levels. Desmopressin is completely ineffective in Haemophilia B and ineffective in severe Haemophilia A.

von Willebrand Disease (vWD)

• Etiology and Role of vWF: The most common inherited human bleeding disorder, transmitted primarily as an autosomal dominant trait (Types 1 and 2) or rarely autosomal recessive (severe Type 3). von Willebrand factor (vWF) serves two crucial hemostatic functions:
  1. Mediates platelet adhesion to exposed subendothelial collagen via glycoprotein Ib (GPIb) under high shear stress.
  2. Acts as the protective carrier chaperone protein for circulating Factor VIII, preventing its premature proteolytic degradation.
• Clinical Presentation: Characterized by mucocutaneous bleeding: recurrent spontaneous epistaxis (>10 minutes duration), excessive bruising without trauma, prolonged bleeding from dental extractions or minor cutaneous lacerations, and heavy menstrual bleeding (menorrhagia) in adolescent females.
• Laboratory Diagnosis: Complete blood count shows normal platelets (except mild thrombocytopenia in Type 2B). PT is normal; aPTT is normal or mildly prolonged (depending on how low Factor VIII levels are). Confirmatory diagnostics include quantitative vWF Antigen (vWF:Ag), qualitative vWF Activity (Ristocetin Cofactor assay, vWF:RCo), and Factor VIII coagulant activity.
• Management: Antifibrinolytic agents (oral or IV Tranexamic acid) for mucosal and dental bleeding; Desmopressin (DDAVP) for Type 1 vWD; and intermediate-purity plasma-derived factor concentrates containing both vWF and Factor VIII for major surgical procedures or severe bleeding episodes.