Sickle Cell Anemia Notes

Learning Outcomes

• Knowledge of the mutation that causes sickle cell anemia.

• Knowledge of the pathophysiological features of sickle cell anemia, including the mechanisms leading to altered blood flow and chronic complications.

• Understanding of the altered oxygen dissociation characteristics of sickle hemoglobin and how this affects systemic oxygen delivery.

• Understanding of qualitative and quantitative laboratory methods to detect sickle hemoglobin, including their clinical relevance and limitations.

Overview

• Sickle cell anemia is a genetic disorder characterized by the production of abnormal hemoglobin (HbS) which causes red blood cells (RBCs) to assume a rigid, sickle shape under low oxygen conditions. This leads to impaired blood flow through small blood vessels (capillaries), reduced oxygen delivery to tissues, and a range of clinical complications associated with this condition.

• The abnormal red cells are identified and eliminated by the reticuloendothelial system, specifically through the action of macrophages, resulting in anemia due to the loss of functioning RBCs.

• Hemolytic anemia primarily occurs through extravascular hemolysis, where sickled cells are recognized and removed by the spleen and liver.

Hemoglobinopathy

• Hemoglobinopathies arise from genetic mutations affecting the globin chain of hemoglobin, leading to the synthesis of a structurally abnormal form or an altered quantity of hemoglobin. Such conditions can manifest as severe anemia and involve alterations in oxygen transport.

• Sickle cell anemia and thalassemia are key examples of hemoglobinopathies, underscoring the vast impact of genetic mutations on hemoglobin function.

• Most hemoglobinopathies, including sickle cell anemia, are inherited in an autosomal recessive pattern, making family planning an important public health consideration.

Sickle Cell Anemia

• The specific genetic mutation in sickle cell anemia occurs within the beta globin chain of hemoglobin, caused by a single nucleotide substitution that changes glutamic acid to valine at codon 6 of the HBB gene on chromosome 11. This single amino acid switch fundamentally alters the properties of the hemoglobin molecule.

• Unlike thalassemia, where there is insufficient globin protein production, sickle cell anemia results from a qualitative defect in hemoglobin, leading to the sickling of cells under hypoxic conditions.

Epidemiology

• Sickle cell anemia is among the most common genetic disorders, with about 275,000 new cases diagnosed each year globally. It is the most prevalent hemoglobinopathy and one of the most widespread monogenic diseases, particularly affecting populations of African descent, Mediterranean regions, and parts of India.

• In England, approximately 350 babies are diagnosed with sickle cell anemia annually, with an additional 9,500 carriers identified, highlighting the importance of screening and education.

Sickle Cell Anemia Genotype

• The classification of sickle cell anemia genotype is determined by the specific configuration and number of mutated genes associated with hemoglobin production.

• Individuals with two copies of the mutated gene (HbSS) express the severe form of the disease, while those with one mutated gene (HbAS) are classified as carriers, typically exhibiting mild or no symptoms but having potential impacts on offspring.

HbS Point Mutation

• The substitution of valine for glutamic acid at the sixth position of the beta globin chain compromises the solubility of hemoglobin during deoxygenation, compelling the hemoglobin molecules to polymerize into long, rigid fibers that distort the red blood cells. These deformed cells are less efficient in oxygen transportation and more susceptible to hemolysis.

Right Shift

• In sickle cell patients, HbS has a tendency to release oxygen ( extit{O2}) more readily than normal hemoglobin (HbA), which can impact oxygen delivery to tissues during hypoxic events. A right shift in the oxygen dissociation curve indicates decreased affinity for oxygen due to factors such as elevated carbon dioxide ( extit{CO2}), decreased pH, and increased temperature. Conversely, conditions promoting a left shift (increased oxygen affinity) include lower extit{CO2} levels, higher pH, and lower temperatures.

Red Cell ‘Sickling’

• The process of hemoglobin polymerization is primarily triggered by hypoxic conditions, dehydration, and febrile episodes. Initially, the sickling of red blood cells can be reversed with re-oxygenation; however, repeated sickling episodes may cause permanent damage to the red blood cell membrane, leading to irreversible sickling and further complications.

Hb Solubility

• HbA is soluble under normal conditions. HbS, however, remains soluble only when fully oxygenated; once deoxygenation occurs, it induces polymerization, leading to rigid polymer structures that distort the original biconcave shape of the red blood cells. This distortion causes obstruction in the microcirculation and contributes to vaso-occlusive crises.

Vaso-occlusion

• The sickled red blood cells impede blood flow through capillaries, significantly impairing tissue perfusion and leading to ischemic damage and pain. The reduced deformability of sickled cells, along with their increased adhesion to vascular endothelium, exacerbates this condition, creating a vicious cycle of hypoxia and pain.

Red Cell Destruction

• The life span of sickled cells is significantly reduced due to their susceptibility to destruction by the spleen and macrophages, leading to chronic hemolytic anemia. Extravascular hemolysis is the dominant mechanism of red cell destruction, causing increased bilirubin levels, which may result in jaundice and other complications.

Sickle Crisis

• Sickle cell crises typically occur around 9 months of age when fetal hemoglobin (HbF) levels drop. These crises are characterized by severe, debilitating pain resulting from vaso-occlusion-induced tissue hypoxia. Locations commonly affected include bones, lungs (leading to acute chest syndrome), spleen (causing splenic sequestration), brain (risking stroke), and extremities (causing dactylitis).

Treatment Options

• Transfusion: Transfusions serve as an effective measure to relieve anemia by increasing the number of functional HbA red cells in circulation. This intervention not only alleviates symptoms of anemia but also improves overall oxygen transport capacity while mitigating the complications associated with sickle hemoglobin via dilution.

• Prophylaxis: Prophylactic measures, including pneumococcal immunization and the use of antibiotics, are vital for preventing severe infections, particularly in young children and during sickle crises. Penicillin plays a critical role in protecting against opportunistic infections, including those affecting the respiratory system. Additionally, routine trans-cranial Doppler screening aids in identifying children at risk for stroke, allowing for early interventions.

• Hydroxyurea: This cytotoxic agent is used to stimulate the production of fetal hemoglobin (HbF), which ameliorates sickle cell-related complications and promotes reduced frequency of painful crises owing to its anti-sickling properties.

• Bone Marrow Transplant: This procedure is considered a curative option under specific conditions, particularly for patients with severe disease and a compatible donor, though risks and challenges remain significant.

HbF - Left Shift

• The presence of HbF is associated with an increased affinity for oxygen, which can counteract some of the adverse effects of HbS, improving overall oxygen delivery to tissues.

Laboratory Methods - Full Blood Count

• RBC: Reduced RBC count is indicative of hemolytic anemia due to the shortened lifespan of sickled cells.

• Hb: Hemoglobin levels are generally low, confirming anemia.

• MCV: Microcytic anemia suggested by low mean corpuscular volume (MCV).

• MCH: Normal mean corpuscular hemoglobin (MCH) reflects a normochromic microcytic anemia.

• MCHC: Normal mean corpuscular hemoglobin concentration (MCHC) is associated with the MCH findings.

• HCT: Hematocrit is low, reflecting the decreased RBC count.

• RDW: A high red cell distribution width (RDW) indicates a mixed population of red cells, including both sickled and normal cells.

Peripheral Blood Film

• Microscopic examination may reveal sickle cells, Howell-Jolly bodies (indicative of asplenia or dysfunction of the spleen), and reticulocytes, suggesting a compensatory response to hemolysis.

Investigations of Sickle Hb

• Solubility test (‘Sickledex test’): This qualitative test involves mixing patient blood with saponin and sodium dithionite to determine the presence of HbS, with results classified as negative (NEG) or positive (POS).

Alkaline Electrophoresis (pH 8.4 – 8.7)

• Alkaline electrophoresis utilizes cellulose acetate to separate hemoglobin types based on their charge; HbS can be identified and differentiated from HbA, crucial for clinical diagnosis.

Quantitative Analysis of Hb - High-Performance Liquid Chromatography (HPLC)

• HPLC is a sophisticated technique where hemoglobins are separated based on their charge and analyzed for concentration via spectrophotometry, enabling the quantification and proportion of HbS, HbA, and other variants, thereby aiding in precise diagnosis and management.

Sickle Genotypes

• HbSS: Classic sickle cell anemia characterized by severe complications.

• HbAS: Sickle trait with benign outcomes typically, yet significant implications for offspring.

• HbSC: Phenotype of moderate severity resulting from different mutations in beta globin genes; clinical manifestation varies greatly.

• HbSD: Similar to HbSC, with variations in clinical severity due to different substitutions.

• HbS beta thalassemia: A compound phenotype exhibiting features of both sickle cell anemia and thalassemia, which complicates clinical presentation and management strategies.

Recommended Reading

• Fundamentals of Biomedical Science: Haematology (2nd edition) by Gary Moore, Gavin Knight & Andrew Blann (2016; Oxford University Press), Chapter 6.3 - Haemoglobinopathy.