MadSci Network: Medicine |
Sickle cell anemia (HbSS) is the most common variant of a group of inherited diseases that affect hemoglobin and results when an individual inherits a genetic mutation (substitution of valine for the normal glutamic acid in the 6th amino acid position of the beta globin chain of hemoglobin) from both parents. This substitution alters the hemoglobin molecule so it crystallizes and deforms the red cell into a sickle shape when the hemoglobin loses oxygen. Clinical manifestations of sickle cell anemia result from increased blood viscosity and vascular obstruction by deformed, sickled red cells. The most common problem for sickle cell patients is the pain episode, a self limiting and reversible pain in the extremities, back, chest, and abdomen. A key factor affecting life span in sickle cell anemics is bacterial infection (or "sepsis"): death resulting from bacterial infection (particularly in the spleen) is most common in infants but patients at any age are at great risk. In order to understand the advantages conferred by the sickle cell trait, it is necessary to understand a bit about malaria. Malaria is caused by a parasite usually spread by mosquitoes and leads to chills, sweating, fever, even death. After sustaining the bite of an infected mosquito, a person's red blood cells become like little factories in which the parasite can flourish. When the parasite enters a red blood cell containing sickle hemoglobin, that cell is more likely to become banana-(or sickle) shaped. Once it sickles, the red blood cell is recognized as abnormal and destroyed in the spleen. The malaria parasite is destroyed as well, and has less opportunity to make the person ill. In areas where malaria flourishes, people who lived long enough to have children tended to have the sickle cell trait: one normal beta-hemoglobin gene and one sickle beta-hemoglobin gene. They died of neither sickle cell disease (because both beta-hemoglobin genes were not sickle) nor malaria, and thus kept the sickle cell gene in the population. REFERENCES: Motulsky AG. Frequency of sickling disorders in US blacks. N Engl J Med. 1973;288:31 Sergeant G. Sickle cell disease. Oxford: Oxford University Press 1985. Vichinski EP, Johnson R, and Lubin, B. Multidisciplinary approach to pain management in sickle cell disease. Am J Ped Hematol Oncol 1982 4:328. Reid CD, Charache S, Lubin B, et al. Management and Therapy of Sickle Cell Disease. N.I.H. Publication No. 95-2117, 1995. Barrett-Connor E. Bacterial infection and sickle cell anemia. Medicine 1971;50:97-112. Embury SH, Hebble RP, Mohandas N, Steinbberg MH.(eds) Sickle Cell Disease: Basic Principles and Clinical Practice. Raven Press, Ltd., New York, 1991.
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