sickle cell anemia

sickle cell anemia
Name	Sickle cell anemia
Synonyms	Sickle cell disease, HbSS disease
Field	Hematology, Medical genetics
Symptoms	Anemia, episodes of pain, swelling, infections
Complications	Stroke, Acute chest syndrome, Priapism, Leg ulcers, Gallstones
Onset	5–6 months of age
Duration	Lifelong
Causes	Inherited Genetic disorder
Risks	Having both parents carry the Sickle cell trait
Diagnosis	Blood test, Genetic testing
Differential	Iron-deficiency anemia, Thalassemia, Hemolytic anemia
Prevention	Genetic counseling, Prenatal testing
Treatment	Hydroxyurea, Blood transfusion, Pain management, Bone marrow transplant
Medication	Hydroxyurea, L-glutamine, Crizanlizumab, Voxelotor
Prognosis	Variable; life expectancy into the 40s–60s in high-income countries
Frequency	~100,000 in the United States, millions globally

sickle cell anemia. It is an inherited blood disorder caused by a specific mutation in the HBB gene, which provides instructions for making part of Hemoglobin. This mutation leads to the production of abnormal hemoglobin molecules, called Hemoglobin S, that can distort red blood cells into a rigid, sickle shape under certain conditions. These sickled cells are fragile, prone to hemolysis, and can obstruct blood flow, leading to a wide array of clinical complications.

Introduction

The molecular basis of the condition was first elucidated by Linus Pauling and colleagues in 1949, who identified it as a "molecular disease." The condition is inherited in an Autosomal recessive pattern, meaning an individual must inherit two copies of the mutated gene, one from each parent. Carriers of a single copy, a condition known as Sickle cell trait, are typically asymptomatic but can pass the gene to their offspring. The persistence of the sickle cell gene in populations is linked to Malaria endemicity, as the trait provides a Heterozygote advantage against severe forms of the parasitic infection caused by Plasmodium falciparum.

Pathophysiology

The primary defect is a single Nucleotide substitution in the HBB gene on Chromosome 11, resulting in the replacement of Glutamic acid with Valine at the sixth position of the Beta-globin chain. This produces Hemoglobin S, which polymerizes into long fibers under conditions of low oxygen tension, such as in the Microvasculature. This polymerization causes the characteristic sickling of Red blood cells, which become rigid and adhesive. These abnormal cells cause Vaso-occlusion, blocking blood flow and leading to Ischemia and Infarction in tissues, and undergo premature destruction (Hemolysis) in the Spleen, leading to chronic Anemia.

Symptoms

Clinical manifestations are highly variable but often include chronic Hemolytic anemia, resulting in Fatigue, Pallor, and Jaundice. A hallmark is the acute, painful Vaso-occlusive crisis, often requiring hospitalization. Other acute complications include Acute chest syndrome, a leading cause of death, and Splenic sequestration. Chronic organ damage affects multiple systems, leading to complications such as Stroke, Pulmonary hypertension, Avascular necrosis of the Femoral head, Leg ulcers, and Retinopathy. Increased susceptibility to infections, particularly from encapsulated bacteria like Streptococcus pneumoniae, is due to functional Asplenia.

Diagnosis

Diagnosis is typically confirmed through Blood tests, most commonly Hemoglobin electrophoresis, which identifies the presence of Hemoglobin S. High-performance liquid chromatography and Isoelectric focusing are also used. Newborn screening programs, such as those mandated in all 50 U.S. states, allow for early identification. Genetic testing can identify specific mutations and is used in Prenatal diagnosis via procedures like Chorionic villus sampling or Amniocentesis. Differential diagnosis includes other Hemoglobinopathies like Hemoglobin C disease and Beta-thalassemia.

Management

Management is multidisciplinary and focuses on preventing complications and treating symptoms. The disease-modifying drug Hydroxyurea reduces the frequency of crises and the need for Blood transfusions by increasing fetal hemoglobin production. Recently, newer agents like Crizanlizumab, Voxelotor, and L-glutamine have been approved. Acute pain crises are managed with aggressive Analgesia, often including Opioids. Chronic transfusion therapy is used for primary stroke prevention. The only curative treatment is Hematopoietic stem cell transplantation, though its use is limited by donor availability and risks. Supportive care includes Penicillin prophylaxis in children and vaccinations against Streptococcus pneumoniae and Haemophilus influenzae.

Epidemiology

The disorder has a high prevalence in regions historically endemic for Malaria, including sub-Saharan Africa, parts of the Mediterranean Basin, the Middle East, and India. In the United States, it affects approximately 100,000 individuals, primarily among African Americans, with about 1 in 365 Black or African American births and 1 in 16,300 Hispanic American births. Significant populations also exist in the Caribbean, Central America, and South America. Global burden estimates are in the millions, with the highest number of affected births occurring in Nigeria, the Democratic Republic of the Congo, and India.

Category:Genetic disorders Category:Hematology Category:Autosomal recessive disorders