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Hemoglobinopathy is a genetic defect that causes abnormal structure of one of the globin chains of the hemoglobin molecule. Thalassemia is a genetic defect resulting in the production of an abnormally low quantity of a given hemoglobin chain or chains. (Table H-2)

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Table Graphic Jump Location
Table H-2 Hemoglobin Disorders: An Overview
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At a Glance

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Genetically inherited hemolytic anemia, usually mild and often inapparent until adulthood. Mainly encountered in populations of African descent but also in populations of Sicilian and Hispanic descent.

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Synonyms

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CC Disease; Hemoglobinopathy C.

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Nature

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Hemolytic anemia with moderate reduction in red cell lifespan. In the oxyhemoglobin state, hemoglobin C is less soluble than normal hemoglobin, thus forming intraerythrocytic crystals (tactoids); deoxygenation further reduces solubility and increases blood viscosity. Patients with hemoglobin C disease have a survival advantage in areas of endemic Plasmodium falciparum malaria.

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Incidence

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Prevalence in the general population is 0.017% in African Americans but approximately 0.03% in Northern Africa. No sex prevalence.

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Genetic Inheritance

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Autosomal codominant (biparental inheritance). The beta gene cluster is on chromosome 11.

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Pathophysiology

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The double helix of hemoglobin C is composed of two normal alpha-chains and two variant beta-chains, in which glutamic acid is replaced by lysine at position 6. Hemoglobin C is unstable and tends to precipitate in erythrocytes where it crystallizes. This changes the physical properties of the erythrocytic membrane (decreased deformability and increased viscosity of blood). These crystal-containing red blood cells are removed by the spleen, which becomes enlarged with time (splenomegaly).

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Diagnosis

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Examination of the erythrocytes (blood smear) reveals several morphologic disorders (caused by crystallization of hemoglobin): target cells mainly, but also appearance of an off-center target, occasional pyknotic spherocytes, and polychromasia. Diagnosis is established by hemoglobin electrophoresis (100% hemoglobin C in homozygotes, up to 35% in heterozygotes). Reticulocyte count and lactic acid dehydrogenase levels determine the extent of hemolysis.

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Clinical Aspects

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Except for mild-to-moderate hemolytic anemia, most patients remain asymptomatic until adulthood. Symptoms usually consist of joint and musculoskeletal pain, visual disorders (retinopathy as a result of iron deposition in the Bruch membrane causing angioid streaks), cholelithiasis, ...

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