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Haematology · Clinical Topics
Sickle Cell Disease
Sickle Cell Disease (SCD) is an autosomal recessive haemoglobinopathy caused by a point mutation in the β-globin gene (HbS). Under deoxygenated conditions, HbS polymerises, causing red cells to become sickle-shaped, leading to chronic haemolysis and vaso-occlusive crises.
📌 Learning Objectives
- Describe the pathophysiology of sickle cell disease, including the genetic mutation and its consequences at a cellular level.
- Explain the clinical manifestations and complications of sickle cell disease, differentiating between acute and chronic issues.
- Identify the diagnostic methods used for sickle cell disease, including newborn screening.
- Outline the principles of management for sickle cell disease, focusing on acute crisis management and long-term preventative strategies.
- Discuss the rationale behind common treatments like hydroxycarbamide and penicillin prophylaxis.
📋 Overview
SCD most commonly affects individuals of African, Caribbean, Middle Eastern, or Mediterranean descent. The most common form is homozygous HbSS, though HbSC and HbS/β-thalassaemia also occur. The hallmark of the disease is the vaso-occlusive crisis (VOC), where sickled cells obstruct microvasculature, causing ischaemia and severe pain. Chronic complications include splenic infarcts (leading to hyposplenism and susceptibility to encapsulated bacteria like S. pneumoniae), chronic kidney disease, and pulmonary hypertension. Acute Chest Syndrome (ACS) is a life-threatening complication characterized by new pulmonary infiltrates, fever, and respiratory distress. Management involves lifelong penicillin prophylaxis, hydroxycarbamide (to increase HbF), and aggressive analgesia during crises. All newborns in the UK are screened via the heel prick test (Guthrie test).
🔬 Basic Science
The molecular basis is a single nucleotide substitution (GAG to GTG) in the sixth codon of the β-globin gene on chromosome 11, resulting in the substitution of glutamic acid with valine. This creates HbS. Unlike normal HbA, HbS polymerises into long insoluble chains when deoxygenated. These polymers distort the erythrocyte into a sickle shape. Initially, this is reversible with reoxygenation, but repeated sickling damages the cell membrane, making it permanently 'irreversibly sickled.' These cells are rigid, have a shortened lifespan (haemolysis), and express adhesion molecules that cause them to stick to the endothelium, leading to vaso-occlusion. Sickling is exacerbated by hypoxia, acidity, dehydration, and cold.
🏥 Clinical Relevance
Acute presentations: 1) Vaso-occlusive pain (bones, joints, abdomen). 2) Sequestration crisis (sudden pooling of blood in the spleen/liver, leading to shock). 3) Aplastic crisis (associated with Parvovirus B19, sudden drop in Hb and reticulocytes). 4) Acute Chest Syndrome. Chronic complications: Osteonecrosis (femoral/humeral heads), dactylitis (in infants), priapism, stroke (monitored by Transcranial Doppler in children), leg ulcers, and chronic renal failure. Patients are at high risk of infections from encapsulated organisms (H. influenzae, S. pneumoniae, N. meningitidis) and Salmonella osteomyelitis.
🧪 Investigations
Bedside: Pulse oximetry (vital in ACS). Bloods: FBC (Hb 60-90g/L), Reticulocyte count (elevated), Blood film (sickle cells, target cells, Howell-Jolly bodies reflecting hyposplenism). Diagnostic: Haemoglobin electrophoresis or High-Performance Liquid Chromatography (HPLC) showing HbS and absence of HbA. Imaging: Chest X-ray (if ACS suspected), Transcranial Doppler (stroke risk screening in children).
💊 Management
Long-term: Prophylactic Phenoxymethylpenicillin (lifelong) and immunization (Pneumococcal, Haemophilus, Meningococcal). Hydroxycarbamide (hydroxyurea) to increase HbF. Folic acid supplementation. Acute Crisis: 'ABC' approach. High-flow oxygen (if hypoxic), IV fluids (hydration), and rapid, potent analgesia (typically Opiates - NICE recommends within 30 mins). Acute Chest Syndrome: Antibiotics, oxygen, fluids, and urgent exchange transfusion. Aplastic Crisis: Urgent Red Cell Transfusion. Transfusion: Simple or exchange transfusion indicated for stroke or ACS; avoid over-transfusion due to iron overload and stroke risks.
Revision Resources – expand the sections below for high-yield notes, exam pearls, key facts and further reading.
MLA High-Yield Notes & Quick Revision ⌄
Exam pearl: Howell-Jolly bodies on a film indicate hyposplenism. Remember Salmonella is a common cause of osteomyelitis in SCD patients. Red flag: Fever in an SCD patient is a medical emergency requiring broad-spectrum antibiotics.
Anaemia
Pain (acute and chronic)
Infection (e.g., sepsis, pneumonia)
Acute abdomen (mimics appendicitis due to splenic/mesenteric ischaemia)
Stroke
Renal impairment
Pulmonary hypertension
- Autosomal recessive haemoglobinopathy (HbS).
- Point mutation (Glu6Val) in β-globin gene.
- HbS polymerises under deoxygenated conditions, causing sickling.
- Leads to chronic haemolysis and vaso-occlusion.
- Vaso-occlusive crises (VOCs) are hallmark, causing severe pain.
- Common complications: hyposplenism, ACS, chronic kidney disease.
Exam Pearls ⌄
⭐ High Yield
Sickle Cell Disease is an autosomal recessive haemoglobinopathy caused by a point mutation (Glu6Val) in the β-globin gene.
HbS polymerises under deoxygenated conditions, leading to red cell sickling, chronic haemolysis, and vaso-occlusion.
Vaso-occlusive crises (VOCs) are the hallmark, causing severe pain due to microvascular obstruction and ischaemia.
Splenic dysfunction (hyposplenism) is common, increasing susceptibility to encapsulated bacteria (e.g., Streptococcus pneumoniae).
Acute Chest Syndrome (ACS) is a life-threatening complication requiring prompt recognition and treatment.
Hydroxycarbamide (hydroxyurea) increases fetal haemoglobin (HbF) production, which reduces sickling.
Newborn screening via the heel prick test is standard in the UK for early diagnosis.
Lifelong penicillin prophylaxis is crucial to prevent infections in young children with SCD.
💡 Clinical Pearl
Splenic Infarct: Repeated sickling in the spleen leads to auto-infarction, causing functional asplenia and increased infection risk.
Acute Chest Syndrome: A severe pulmonary complication of SCD, presenting with fever, new infiltrates, and respiratory symptoms, often triggered by infection or fat embolism.
Aplastic Crisis: Often triggered by Parvovirus B19 infection, leading to a sudden drop in haemoglobin due to temporary cessation of red cell production.
Priapism: Prolonged painful erection due to vaso-occlusion in the penile vasculature, requiring urgent medical attention.
⚠️ Exam Tip — Common Mistakes
Confusing sickle cell trait (heterozygous) with sickle cell disease (homozygous or compound heterozygous) and their respective clinical implications.
Underestimating the severity of pain during a vaso-occlusive crisis and the need for aggressive analgesia.
Forgetting the increased risk of infection in hyposplenic patients and the importance of vaccinations and penicillin prophylaxis.
Not recognising Acute Chest Syndrome as a medical emergency distinct from pneumonia.
Misinterpreting the role of hydroxycarbamide as a cure rather than a disease-modifying therapy.
Key Facts ⌄
Caused by a mutation in the β-globin gene (Glu6Val substitution).
HbS polymerises when deoxygenated/dehydrated, causing sickling.
Vaso-occlusive crisis (VOC) is the most common acute presentation.
Functional hyposplenism occurs by age 5 due to repeated infarction.
Acute Chest Syndrome is a leading cause of mortality.
Hydroxycarbamide is used to increase fetal haemoglobin (HbF).
Autosomal recessive inheritance; carriers (HbAS) usually asymptomatic.
Related Topics ⌄
References ⌄
- NICE CKS - Sickle cell disease
- NICE Guideline CG143 - Sickle cell disease: managing acute painful episodes
- Kumar & Clark's Clinical Medicine
Further Resources
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