Topics: Red blood cell, Sickle-cell disease, Hemoglobin Pages: 8 (2028 words) Published: May 5, 2013
University of Belize
Faculty of Nursing and Allied Health
NURS 3051- Nursing Care of Infants and Children

Congenital Defects

Sickle Cell Anemia (SCA)

Sickle Cell Anemia (SCA) is one of a group of diseases collectively termed hemaglobinopathies, in which normal adult hemoglobin is partially or completely replaced by abnormal sickle hemoglobin(HgbS). Sickle Cell Anemia includes all of those hereditary disorder, the clinical, hematologic, and pathologic features of which are related to the presence of HbgS. Also know as SS and homozygous sickle cell disease. The most common form of SCD are:

Sickle Cell Anemia – The homozygous form of the disease (HgbSS) . Sickle Cell –C Disease a heterozygous variant of SCD, including both HgbS and hemoglobin C (HgbC). Sickle Cell- E (HgbE) Disease - A variant of SCD in which glutamic acid has been substituted for lysine in the number 26 position of the β-chain. Sickle Thalassemia Disease – A combination of sickle cell trait and β+ (beta plus) refers to the ability to still produce some normal adult hemoglobin. Βο (beta zero) indicates that there is no ability to produce normal adult hemoglobin.

Of the SCDs, SCA is the most common form in Blacks, followed by Sickle Cell C Disease and Sickle β-thalassemia. Another β-chain variant, HgbE, is found primarily in people of Southeast Asian origin. People who carry the trait for HgbE are completely asymptomatic, but those who are homozygous exhibit a disease similar to HgbC disease. SCA is found primarily in the Black race, occasionally in Hispanics and infrequently in Whites. Incidence of the disease vary in different geographic locations. Among American Blacks the incidence is 8%. In West Africa the incidence is reported to be as high as 40%. Believed to be a result of of selective protection of trait carriers against malaria caused by Plasmodium falciparium.

Mood of Transmission
The gene that determines the production of Hgbs is situated on an autosome. (autosomal recessive inheritance) The expected pattern of transmission from two parents who carry the heterozygous gene HgbAS is a 25% chance of their producing an offspring with SCA.

Characteristics of autosomal recessive inheritance
Males and females are affected with equal frequency
Affected individuals will have unaffected parents who are heterozygous for the trait There is a 1:4 (25%) chance that any child of two unaffected heterozygous parents will be affected. Unaffected siblings of an affected person have a ⅔ risk of being a carrier. Affected individuals mated to normal individuals will have normal children, all of whom will be carriers. There is usually no evidence of the trait in previous generations-a negative family history- unless consanguinity is a factor.

Basic Defect
The basic defect responsible for the sickling effect of erythrocytes is in the globin fraction of hemoglobin, which is composed of 574 amino acids. HgbS differs from HgbA in the substitution of only one amino acid (valine) for another (glutamine) at the sixth position of the β-polypeptide chain. Under conditions of dehydration, acidosis, hypoxia, and temperature elevations, the relatively insoluble HgbS changes its molecular structure to form long, slender crystals. These filamentous crystals cause distortion of the cell membrane from a pliable disk to a crescent- or sickle-shaped RBC. The filamentous forms are associated with much greater viscosity than the normal holly leaf structure of HgbA. In most instance the sickling response is reversible under conditions of adequate oxygenation and hydration. However after repeated cycles of sickling and unsicklng, the RBC becomes irreversibly sickled. Although the defect is inherited the sickling phenomenon is not apparent until later in infancy because of the presence of fetal hemoglobin (HgbF). As long as HgbF persists, sickling does not occur because there are no β-chains carrying the defect. The newborn has 60% to 80% fetal...
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