Alpha Thalassemia

The alpha thalassemias are caused by a decrease in production of alpha globin chains due to a deletion or mutation of one or more of the four alpha globin genes located on chromosome 16. Alpha gene mapping can be obtained to determine the specific mutation. The alpha thalassemias can be generally categorized as: Silent Carrier, Alpha Thalassemia Trait, Hemoglobin H disease, Hemoglobin H-Constant Spring, and Alpha Thalassemia major. Frequently, the diagnosis of alpha thalassemia trait in a parent is discovered after the birth of an affected child.

The Silent Carrier status is characterized by three functional genes that code for the production of alpha globins (-a/aa). Outside the newborn period, it is not possible to make this diagnosis by conventional methods. There is overlap between the red blood cell indices of these individuals and normals, although the MCV may be slightly lower. The silent carrier will experience no health problems in his/her lifetime. This carrier state is diagnosed by deduction when a 'normal' individual has a child with Hgb H disease or with microcytic anemia consistent with alpha thalassemia trait. An unusual case of the silent carrier state is the individual who carries the Hemoglobin Constant Spring mutation [(a^csa/aa) or (aa^cs/aa)]. This is an elongated a-globin due to a termination codon mutation. Individuals who have this mutation have normal red blood cell indices, but can have children who have Hgb H-Constant Spring disease if the other parent has alpha thalassemia trait (--/aa). Generally, children with Hgb H-Constant Spring are more affected clinically than children who have classic Hgb H disease. Two Constant Spring carriers can also pass on their genes to have a child with Homozygous Constant Spring, a condition that has similar clinical implications as Hemoglobin H disease.

Alpha thalassemia trait is characterized by two functional genes that code for the production of alpha globins [(-a/-a) or (--/aa)]. The two genes can either occur on the same chromosome (cis-type) or on each of the pair (trans-type). Cis-type a-thalassemia trait tends to be found in individuals of Asian descent, while trans-type tends to run in individuals of African descent. Cis-type can be co-inherited with another cis-type or hemoglobin H disease to result in alpha thalassemia major, or hydrops fetalis. Individuals who have alpha thalassemia trait are identified by microcytosis, erythrocytosis, hypochromia, and mild anemia. The diagnosis is made by a combination of family studies and the ruling out of both iron deficiency anemia and beta thalassemia trait. In the neonatal period, when hemoglobin Bart's (g⁴) is present, the diagnosis can be strongly suspected. In children, there are no markers such as Hgb A₂ and Hgb F to make the diagnosis. (One exception is the case where both of the deletions occur on the same chromosome and zeta [z] globin is expressed in carriers. This is most common in Southeast Asians.) The diagnosis is one of exclusion. The clinician should be satisfied with the presumed diagnosis if the above criteria are met. During pregnancy, the microcytic anemia can be mistaken for anemia of pregnancy.

The individual with a thalassemia trait will experience no significant health problems except a possible slight anemia which cannot be treated with iron. <next>

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