Sickle Cell Disease

Definition & Epidemiology

• Autosomal codominant genetic disorder.
• Homozygous state (HbSS) produces Sickle Cell Anemia.
• Gene frequency in India: 4.3%; high prevalence in tribal populations of Orissa, Maharashtra, Madhya Pradesh, Jharkhand, and Gujarat.
• United States prevalence: 1 in 365 African-American births.
• Sickle cell trait (HbAS) provides selective survival advantage against Plasmodium falciparum malaria (balanced polymorphism).

Genetics & Molecular Pathophysiology

• Point mutation (adenine replaced by thymine) at 6th codon of $\beta$-globin gene on chromosome 11.
• Amino acid substitution: Valine replaces glutamic acid (or glutamine).
• Hemoglobin S (HbS) tetramers undergo conformational change in deoxygenated (T) state.
• Hydrophobic interactions between deoxygenated HbS molecules cause aggregation into large, rigid polymers.
• Red blood cells (RBCs) distort into characteristic “sickled” shape.
• Downstream effects: Hemolysis-associated nitric oxide depletion, chronic inflammation, oxidative stress, impaired fibrinolysis, platelet/leukocyte activation, and endothelial dysfunction.
• Fetal Hemoglobin (HbF) inhibits HbS polymerization, ameliorating clinical severity.
• Co-inheritance of $\alpha$-thalassemia trait (35% frequency in African-Americans) reduces baseline anemia and stroke risk, but does not alter vaso-occlusive pain frequency.

Steady-State Clinical & Hematological Profile

• Chronic hemolytic anemia: Baseline hemoglobin 6-11 g/dL.
• Reticulocytosis: 5-30% reflecting brisk erythropoiesis.
• White blood cell count: Elevated baseline neutrophilia.
• Platelet count: Thrombocytosis common.
• Hemolysis markers: Elevated indirect bilirubin, elevated lactate dehydrogenase (LDH), decreased haptoglobin.
• Functional asplenia develops early; completely absent splenic function by 5 years of age.
• Peripheral blood smear: Sickled RBCs, target cells, nucleated RBCs, polychromasia, and Howell-Jolly bodies (indicating hyposplenism).

Diagnosis & Evaluation

Newborn Screening

• Mandatory state screening utilizes isoelectric focusing (IEF) or high-performance liquid chromatography (HPLC) from dried blood spots.
• Hemoglobins reported in order of quantitative predominance.

Newborn Screen Pattern	Probable Diagnosis	Confirmatory Requirement
FS	HbSS, HbS-$\beta 0$-thalassemia, HbS-HPFH	Repeat hemoglobin analysis >6 months; parental $\beta$-globin gene testing.
FSC	HbSC disease	Confirmatory testing at initial clinic visit.
FSA	HbS-$\beta +$-thalassemia	Confirm >50% HbS, elevated HbA2 (>3.5%) post-newborn period.
FAS	Sickle cell trait (HbAS)	Exclude prior RBC transfusion.
AFS	Invalid (Transfused)	Infant received RBC transfusion prior to sampling. Diagnosis indeterminate.

Confirmatory Testing

• Hemoglobin Electrophoresis/HPLC: HbSS profile shows 80-90% HbS, 2-20% HbF, absent HbA.
• DNA-based mutation analysis confirms specific genotype.
• Solubility test (sodium metabisulfite) induces in vitro sickling; positive in all sickle syndromes.

Differential Diagnosis of Sickle Cell Syndromes

Genotype	Clinical Severity	Steady-State Hb (g/dL)	MCV (fL)	Reticulocytes (%)	Hb Electrophoresis
HbSS	Severe	6-11	Normal (>80)	5-30	80-90% S; 2-20% F
HbSC	Mild/Moderate	10-15	Normal/Low	2-6	50-55% S; 45-50% C
HbS-$\beta 0$-Thal	Moderate/Severe	6-10	Low (<70)	3-20	50-85% S; >3.5% A2
HbS-$\beta +$-Thal	Mild/Moderate	9-12	Low (<75)	2-6	50-80% S; 10-30% A
HbS-HPFH	Asymptomatic	12-14	Normal (>80)	1-3	60-80% S; 15-35% F
HbAS (Trait)	Asymptomatic	Normal	Normal	Normal	55-60% A; 35-45% S

Comprehensive Preventive Care & Health Maintenance

Infection Prophylaxis

• Functional asplenia imparts 300-600 fold increased risk of overwhelming sepsis (S. pneumoniae, H. influenzae type b, N. meningitidis, Salmonella spp.).
• Penicillin Prophylaxis: Initiate oral penicillin VK by 3-4 months of age.
  • Dose: 125 mg twice daily (<3 years); 250 mg twice daily ($\ge$ 3 years).
  • Duration: Continue until at least 5 years of age.
  • Alternative: Erythromycin ethylsuccinate for penicillin allergy.
• Immunizations:
  • Routine childhood schedules.
  • Conjugate 13-valent pneumococcal vaccine (PCV-13).
  • 23-valent pneumococcal polysaccharide vaccine (PPV-23) administered at age 2, booster 5 years later.
  • Meningococcal vaccine.
  • Annual influenza vaccine.

Routine Screening Protocols

Assessment	Starting Age	Frequency/Indications
Complete Blood Count & Reticulocytes	Diagnosis	Every 1-3 months. Monthly if on Hydroxyurea.
Transcranial Doppler (TCD)	2 years	Annually until 16 years. Detects stroke risk (Time-Averaged Mean Maximum velocity $\ge$ 200 cm/s indicates high risk).
Ophthalmology Exam	8-10 years	Annually. Screens for proliferative retinopathy (“sea fans”).
Renal Function / Urinalysis	1-10 years	Annually. Screens for microalbuminuria/proteinuria and concentrating defects (hyposthenuria).
Echocardiography	10 years	Every 3 years. Evaluates pulmonary hypertension (elevated tricuspid regurgitant velocity).
Pulmonary Function	5 years	Every 3 years. Screens for lower airway disease/asthma.
Brain MRI/MRA	5-6 years	Baseline screening for silent cerebral infarcts. Indicated for cognitive difficulties or conditional TCD.

Disease-Modifying Pharmacotherapy

Hydroxyurea (HU)

• Mechanism: Chemotherapeutic agent. Primary HbF modulatory therapy; inhibits polymerization. Additional benefits: increases red cell hydration, decreases adhesion molecule expression, lowers WBC/platelet/reticulocyte counts, increases nitric oxide production.
• Efficacy: Reduces frequency of vaso-occlusive pain, acute chest syndrome (ACS), dactylitis, and hospitalizations.
• Indications: Recommended for all children with sickle cell anemia starting at 9 months of age, regardless of clinical symptoms.
• Dosing: Start 10-20 mg/kg/day. Escalate by 5 mg/kg/day increments up to maximum 35 mg/kg/day.
• Monitoring: Strict CBC monitoring required every 4-8 weeks to assess for transient myelosuppression.

Newer FDA-Approved Agents

• L-Glutamine: Oral amino acid therapy. Reduces oxidative stress. Indicated for ages $\ge$ 5 years; reduces hospitalizations and pain crises.
• Voxelotor: Small molecule oral inhibitor of HbS polymerization. Increases hemoglobin affinity for oxygen. Indicated for ages $\ge$ 4 years. Demonstrates reduced hemolysis and mean hemoglobin increase of 1.1 g/dL.
• Crizanlizumab: Humanized monoclonal anti-P-selectin antibody. Administered intravenously every 4 weeks. Indicated for ages $\ge$ 16 years. Reduces annual vaso-occlusive event rates by 50%.

Prophylactic Blood Transfusion Therapy

• Indications: Primary and secondary stroke prevention, recurrent ACS, pulmonary hypertension.
• Methods: Automated erythrocytapheresis (preferred to limit iron burden), manual exchange, or simple transfusion.
• Alloimmunization Prevention: Perform extended RBC phenotyping/genotyping at diagnosis. All transfused units must be leukoreduced and prophylactically matched for C, E, and K antigens.
• Iron Overload Management: Monitor serum ferritin and liver/cardiac T2* MRI. Initiate oral chelation (deferasirox, deferiprone) or subcutaneous deferoxamine for chronic iron toxicity.

Curative Therapies

• Hematopoietic Stem Cell Transplant (HSCT): Only established cure. Highest success utilizes HLA-matched sibling donors. Prevents end-organ damage and eliminates disease phenotype.
• Gene Therapy: Lyfgenia (lovotibeglogene autotemcel). Utilizes lentiviral vector carrying modified beta-A-T87Q globin gene to increase non-sickling hemoglobin levels. Evaluated for severe disease phenotypes.