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Sickle cell anemia

Cause

The sickle cell syndromes are caused by a mutation in the b-globin gene that changes the sixth amino acid from glutamic acid to valine. HbS (a2b26Glu?Va1) polymerizes reversibly when deoxygenated to form a gelatinous network of fibrous polymers that stiffen the erythrocyte membrane, increase viscosity, and cause dehydration due to potassium leakage and calcium influx.

The prototype disease, sickle cell anemia, is the homozygous state for HbS. In this condition more than 80% of the red cell hemoglobin is HbS. In the deoxygenated state HbS moelcules link to form chains and these result in sickling and increased rigidity of the red cells. Sickling produces a shortening of red cell survival and an obstruction of the microcirculation.

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Precipitating factors

  • Infection
  • dehydration
  • cold
  • hypoxia

Another problem is that HbS releases its oxygen to the tissues more readily than does normal Hb and patients tend to feel well in comparision to their anemia except ofcourse, during crises or disease complications.

Symptoms and clinical picture

This varies from an asymptomatic disease detected only by blood counts to a severe hemolytic anemia with recurrent crises.

Types of sickle cell anemia crises

  • Aplastic crisis: Presents with a sudden fall of hemoglobin with low reticulocyte counts. This usually follows infection with parvovirus.
  • Hemolytic crisis
  • Sequestration crisis: presents with a rapid fall of Hb due to red cell trapping in the liver and spleen.
  • Painful crisis: These are due to infarctions as a result of obstruction of the microcirculation by HbS.

Complications

  • Osteomyelitis
  • Papillary necrosis of the kidney
  • Autosplenectomy
  • Susceptibility to infection
  • Pulmonary infarction
  • Priapism (painful erection)
  • Blindness due to retinal detachment or proliferative retinopathy

Diagnosis

Lab findings are those suggestive of hemolytic anemia (click here for details).

Sickling may be seen in blood film or induced by sodium metabisulphite. Diagnosis can be confirmed by hemoglobin electrophoresis which will show more than 80% of cellular hemoglobin to be of the Hb SS type.

Sickle solubility test using a reducing agent.

Treatment

Currently treatment rests on avoidance of the precipitating factors above and supportive therapy during a crisis. Supportive therapy is comprised of blood transfusion to reduce the concentration of HbS (by dilution) and IV fluids, analgesics and antibiotics. Bone marrow transplantation and gene therapy are currently under investigation.

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