Thalassemia is an autosomal recessive hemoglobinopathy that originated in the Mediterranean region. The genetic defect causes a reduction in the rate of globin chain synthesis which causes the formation of abnormal hemoglobin molecules. The resultant microcytic anemia is the characteristic presenting symptom of the thalassemias.
Thalassemia is a quantitative problem of globin synthesis, whereas sickle cell disease is a qualitative problem of synthesis of an incorrectly functioning globin.
Normal adult hemoglobin is composed of HbA (98%) and HbA2 (2%). HbA contains two α globin chains and two β globin chains, and HbA2 contains two α globin chains and two δ globin chains. They are arranged into a heterotetramer. Thalassemia patients produce a deficiency of either α or β globin, unlike sickle cell disease, which produces a specific mutant form of β globin.
The thalassemias are classified according to which chain of the hemoglobin molecule is affected. In α thalassemias, production of the α globin chain is reduced, while in β thalassemia production of the β globin chain is reduced.
The β globin chains are encoded by a single gene on chromosome 11; α globin chains are encoded by two closely linked genes on chromosome 16. Thus, in a normal person with two copies of each chromosome, there are two loci encoding the β chain, and four loci encoding the α chain. Deletion of one of the α loci has a high prevalence in people of African or Asian descent, making them more likely to develop α thalassemias. β thalassemias are common in Africans, but also in Greeks and Italians.
The thalassemia trait may confer a degree of protection against malaria, which confers a selective survival advantage on carriers.
Marrow proliferation consists of an expansion of the medulla, thinning of cortical bone, and resorption of cancellous bone resulting in a generalized loss of bone density and yellow-to-red bone marrow reconversion.
- skull: the occipital bone is spared, due to lack of hemopoietic bone marrow 6
- facial bones
- rodent or chipmunk facies
- hypopneumatisation of the frontal, maxillary, and sphenoid sinuses, filled with marrow containing bone 6
- the ethmoid sinuses are spared due to lack of red bone marrow 6
- dental malocclusion
- ribs: rib-within-a-rib appearance, noted particularly in the middle and anterior portions of the ribs
- extremities: widening/expansion of the metacarpal and metatarsal shafts
- premature fusion of the epiphyses
- osteoporosis 4
- extramedullary hematopoiesis
- diffusion impairment
- air-trapping 7
History and etymology
Named for the Greek word for "sea" (θάλασσα - thálassa), as the condition is more prevalent in those living around the Mediterranean Sea e.g. Italians, Greeks, etc. Cooley and Lee described bone abnormalities and severe anemia with associated splenomegaly in 1921 5.
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