Sickle cell anemia
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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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.
- Papillary necrosis of the kidney
- Susceptibility to infection
- Pulmonary infarction
- Priapism (painful erection)
- Blindness due to retinal detachment or proliferative retinopathy
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.
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.