Sickle cell disease

Contents

Overview

Sickle cell disease (SCD) is caused by a point mutation in the beta-globin gene, producing haemoglobin S (HbS). Under hypoxic, dehydrated, or acidotic conditions, HbS polymerises and deforms erythrocytes into the characteristic sickle shape. Sickled cells obstruct microvasculature, causing vaso-occlusion, ischaemia, haemolysis, and multi-organ damage. ICU admission is required for the most severe acute complications: acute chest syndrome, stroke, multi-organ failure, and sepsis.

Pathophysiology

The HbS mutation substitutes valine for glutamic acid at position 6 of the beta-globin chain. Deoxygenated HbS polymerises, distorting the erythrocyte membrane and reducing cell deformability. Triggers include hypoxia, acidosis, dehydration, cold, and infection — all common in critical illness.

Consequences of sickling:

Vaso-occlusion: Rigid sickled cells obstruct small vessels, causing ischaemia and infarction in any organ. The bone marrow of long bones, lungs, spleen, and brain are particularly affected.

Haemolysis: Sickled cells have a shortened survival (10–20 days vs the normal 120 days). Chronic haemolytic anaemia results. Intravascular haemolysis releases free haemoglobin, which scavenges nitric oxide, impairing vasodilation and contributing to pulmonary hypertension.

Endothelial activation: Sickled cells, free haemoglobin, and inflammatory mediators activate endothelium, promoting adhesion molecule expression and a prothrombotic state.

Chronic end-organ damage: Repeated vaso-occlusion causes organ fibrosis and dysfunction over time — asplenia (functional from repeated infarction), chronic kidney disease, pulmonary hypertension, avascular necrosis, retinopathy, stroke.

Genotype correlates with severity: HbSS (homozygous) is most severe; HbSC disease and HbS/beta-thalassaemia are milder.

Acute Presentations Requiring ICU

Acute chest syndrome (ACS): The most common cause of ICU admission and the leading cause of death in SCD (see below).

Stroke: Ischaemic stroke from intracranial vessel occlusion or haemorrhagic stroke from moyamoya-like vasculopathy. Children with SCD have a 10% lifetime risk of stroke. Transcranial Doppler screening and regular exchange transfusion reduce this risk.

Severe vaso-occlusive crisis (VOC): Bone infarction with extreme pain, usually managed with analgesia on the ward but may require ICU for opioid dosing, severe pain, or complications.

Aplastic crisis: Parvovirus B19 infects erythroid precursors, causing transient red cell aplasia. Haemoglobin falls precipitously in a patient with already shortened red cell survival. Managed with transfusion.

Splenic sequestration: Predominantly in young children (before functional asplenia develops). Rapid splenic pooling of sickled cells causes acute splenomegaly, hypovolaemia, and haemoglobin fall. May require emergency transfusion and splenectomy.

Sepsis: Functional asplenia severely impairs immunity to encapsulated organisms (Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis). Overwhelming post-splenectomy infection (OPSI) can develop rapidly. Patients should be on lifelong penicillin prophylaxis and up to date with vaccinations.

Multi-organ failure: Severe VOC or ACS may precipitate hepatic, renal, and cardiac involvement. Exchange transfusion is considered in multi-organ failure.

Acute Chest Syndrome

Acute chest syndrome (ACS) is defined as a new pulmonary infiltrate on chest imaging plus at least one of: fever, respiratory symptoms, or a fall in oxygen saturation. It is the most feared acute complication and carries significant mortality.

Causes

ACS has multiple triggers, often coexisting:

  • Pulmonary vaso-occlusion and infarction (fat embolism from bone marrow infarction is a recognised precipitant)
  • Infection: Chlamydia pneumoniae, Mycoplasma pneumoniae, viruses, and typical bacteria
  • Hypoventilation (post-operative splinting, opioid-induced)
  • Aspiration

Management

Oxygen: Maintain SpO2 ≥95%. Hypoxia worsens sickling.

Analgesia: Adequate analgesia is essential to allow deep breathing and prevent atypical pneumonia from splinting. Use multimodal analgesia; opioids should be titrated carefully — excessive opioids cause respiratory depression and hypoventilation, worsening ACS.

Incentive spirometry: Proven to reduce the incidence of ACS post-operatively in SCD.

Hydration: IV fluids to avoid dehydration, but avoid fluid overload — the lung in ACS is already compromised.

Antibiotics: Empirical cover for atypical organisms (macrolide or fluoroquinolone) plus cover for typical bacterial pneumonia (beta-lactam). Continue for 5–7 days.

Transfusion:

  • Simple transfusion if Hb significantly below the patient's baseline
  • Exchange transfusion (red cell exchange) if ACS is severe, deteriorating rapidly, or the patient is on a chronic transfusion programme. The target is to reduce HbS to below 30% of total haemoglobin.

Bronchodilators: Bronchospasm occurs in some patients; salbutamol nebulisers are appropriate.

Ventilation: CPAP or HFNO for moderate hypoxaemia. Intubation and invasive ventilation for respiratory failure; lung-protective strategy.

Hydroxyurea: Long-term preventive treatment that increases HbF production, reducing polymerisation. Reduces frequency of ACS and VOC in patients with recurrent complications.

Management Principles

The four pillars of managing acute SCD complications:

  1. Identify and reverse triggers: treat infection, correct dehydration, maintain normothermia, ensure adequate oxygenation
  2. Analgesia: adequate pain control is mandatory. Strong opioids are often required. Patient-controlled analgesia is useful. Multimodal regimens reduce opioid requirements.
  3. Hydration: IV fluids to correct and prevent dehydration. Avoid excess.
  4. Transfusion: simple or exchange, depending on severity and indication

Transfusion in Sickle Cell Disease

Simple transfusion: Increases total Hb and oxygen-carrying capacity. Risk of hyperviscosity if Hb is raised too high (target Hb 90–100 g/L, not higher).

Exchange transfusion: Replaces HbS-containing blood with donor HbA blood, reducing HbS percentage without raising total Hb. Preferred for acute stroke, severe ACS, multi-organ failure. Can be performed manually (serial venesection and transfusion) or automated (erythrocytapheresis). Target HbS <30%.

Alloimmunisation: Patients with SCD are at high risk of developing red cell alloantibodies from repeated transfusion, making crossmatching difficult. Extended antigen phenotyping should guide blood selection. Autoimmune haemolytic anaemia can also complicate transfusion.

Iron overload: Chronic transfusion programmes cause iron accumulation (transfusion haemosiderosis). Managed with chelation therapy (desferrioxamine or deferasirox).

Chronic Complications

Long-term organ damage relevant to ICU care:

  • Pulmonary hypertension: From chronic haemolysis and hypoxic pulmonary vasoconstriction. Worsens prognosis; manage as per pulmonary hypertension.
  • CKD: From glomerular hyperfiltration, sickling in renal medulla, and repeated ischaemia. Affects dosing of renally cleared drugs.
  • Cardiomyopathy: High-output state from chronic anaemia leads to left ventricular dilatation and eventual dysfunction.
  • Functional asplenia: Lifelong susceptibility to encapsulated organisms; ensure prophylactic penicillin and appropriate vaccinations.

Viva Questions

What is acute chest syndrome in sickle cell disease and how is it managed in the ICU?

Acute chest syndrome is defined as a new pulmonary infiltrate on chest imaging in a patient with SCD, accompanied by fever, respiratory symptoms, or desaturation. It is the most common cause of ICU admission in SCD and the leading cause of premature death. Causes include pulmonary vaso-occlusion from sickling (often triggered by fat embolism from bone marrow infarction), infection with atypical or typical respiratory pathogens, and hypoventilation leading to atelectasis. Management involves maintaining SpO2 above 95% with supplemental oxygen or HFNO, adequate analgesia to allow deep breathing without causing respiratory depression, empirical antibiotics covering atypical organisms (macrolide or quinolone) plus a beta-lactam, careful IV hydration, and bronchodilators if bronchospasm is present. Transfusion is indicated for significant haemoglobin fall — exchange transfusion reducing HbS to below 30% is used for rapidly deteriorating ACS, or where the patient is on a chronic transfusion programme. Mechanical ventilation follows standard lung-protective principles if required.

Why are patients with sickle cell disease at high risk of sepsis and how is this managed?

Functional asplenia is the principal reason. The spleen filters encapsulated bacteria from the bloodstream and mounts a rapid IgM response to novel bacterial antigens. In SCD, repeated sickling within the splenic microvasculature causes progressive infarction of splenic parenchyma, beginning in early childhood. By the age of 5, most homozygous patients have a non-functioning spleen. This severely impairs immunity to encapsulated organisms: Streptococcus pneumoniae, Haemophilus influenzae type B, and Neisseria meningitidis. Overwhelming post-splenectomy infection (OPSI) can develop within hours, presenting as fever rapidly progressing to septic shock and is life-threatening. Prevention: patients should be on lifelong penicillin prophylaxis (phenoxymethylpenicillin 500 mg BD in adults), be vaccinated against all three encapsulated organisms and influenza, and receive booster vaccinations as per schedule. Any febrile episode in a patient with SCD must be taken seriously — broad-spectrum antibiotics should be started promptly after blood cultures, without waiting for a definitive source.

What are the principles of transfusion in sickle cell disease and when is exchange transfusion preferred over simple transfusion?

Simple transfusion increases overall haemoglobin and oxygen-carrying capacity. It is used when haemoglobin has fallen significantly (aplastic crisis, splenic sequestration, ACS with mild-moderate haemoglobin fall). However, simple transfusion in SCD carries risks: if haemoglobin is raised above 100–110 g/L, hyperviscosity may worsen sickling in the microcirculation. Exchange transfusion replaces HbS-containing red cells with donor HbA cells, simultaneously reducing the sickle cell burden without raising total haemoglobin. The target is to reduce HbS to below 30% of total haemoglobin. Exchange transfusion is preferred for acute stroke (to rapidly reduce HbS and prevent infarct extension), severe or rapidly deteriorating ACS, and multi-organ failure. It can be performed manually by sequential venesection and transfusion, or by automated erythrocytapheresis. All transfusion in SCD should use extended phenotype-matched blood where possible to reduce alloimmunisation risk, which affects a substantial proportion of patients and can make future crossmatching extremely difficult.