Sickle cell anemia (HbSS) is caused by homozygosity for a point mutation in the β-globin gene (HBB) that leads to replacement of glutamic acid by valine at position six of the β-globin chain of hemoglobin (β6 Glu>Val) leading to the synthesis of abnormal β-globin chains. The HbS β-globin chain pairs with normal a-globin chains to produce sickle hemoglobin or HbS (a2βS2).
Clinical presentation of HbSS is heterogeneous among patients even though all cases have the identical HbS mutation suggesting modification of the disease phenotype by other genes and the environment.
Complications can be related to sickle vaso-occlusion, for example, acute painful episodes, osteonecrosis, and acute chest syndrome and also be associated with the degree of hemolysis, for example, gallbladder disease, stroke, priapism, leg ulcers, and pulmonary hypertension.
The major treatment modalities include blood transfusion for severe anemia, stem cell transplantation for selected cases and administration of hydroxyurea (hydroxycarbamide) to stimulate the production of fetal hemoglobin (HbF) that by virtue of its effects on HbS polymerization, can decrease most complications of disease and extend lifespan.
HbSS is inherited in an autosomal recessive fashion. If both parents are carriers, each child has a 25% risk of inheriting the disease.
HbSS can be seen in successive generations given the high carrier rates in some populations.
Table 31-1Types of Sickle Cell Disease ||Download (.pdf) Table 31-1 Types of Sickle Cell Disease
| ||Hemoglobin HPLC || |
|Type ||HbA ||HbA2 ||HbF ||HbS ||Other ||Severity |
|HbSS (sickle cell anemia) ||0 ||<3.6 ||<10 ||90-95 || ||Severe |
|HbSS-α thalassemia ||0 ||<3.6 ||<10 ||90-95 ||Low MCV ||Severe |
|HbS-β0 thalassemia ||0 ||≥3.6 ||<10 ||90-95 || ||Severe |
|HbS-β+ thalassemia ||5-30 ||≥3.6 ||<10 ||60-90 || ||Moderate |
|HbSC disease ||0 ||<3.6 ||<3 ||45-50 ||HbC 45-50 ||Moderate |
|HbS-OArab ||0 ||<3.6 ||<3 ||45-50 ||Hb OArab 45-50 ||Severe |
|HbS-HPFH ||0 ||<3.6 ||20-40 ||60-80 || ||Mild |
Diagnostic Criteria and Clinical Characteristics
Diagnostic Criteria for Sickle Cell Anemia
HbSS is a monogenic disease caused by homozygosity for a point mutation in the HBB that leads to replacement of glutamic acid by valine at position six of β-globin (β6 Glu>Val). Diagnosis of HbSS can be established based on family studies, history and physical examination, hematologic studies and by separating the hemoglobin fractions present in red blood cells by high-performance liquid chromatography (HPLC) or some other similarly sensitive and rapid method (Table 31-1). The definitive method of ascertaining the presence of HbS and/or identifying homozygosity or heterozygosity for the HbS (βs) mutation is based on DNA testing. The term sickle cell disease includes HbSS and other genotypes as shown ...