Anemia refers to low red blood cell counts; common symptoms of anemia include fatigue and pallor. It is divided into three types: microcytic, normocytic, and macrocytic. Microcytic anemia means there are low numbers of red blood cells and they are small. They may also be described as hypochromic, meaning that they are a paler red than usual because of reduced hemoglobin concentration. Microcytic anemia has several possible causes including iron deficiency anemia, thalassemias, sideroblastic anemia, and anemia of chronic disease.
Hemoglobin
A commonality between the four causes of microcytic anemia is the inability of the body to synthesize hemoglobin properly. Red blood cells essentially are carriers of hemoglobin, which functions to carry oxygen, acts as a buffer, and carries some carbon dioxide. Hemoglobin is made up of four protein subunits, and a ring-like structure called a porphyrin. The porphyrin ring carries iron in its center, which is responsible for binding oxygen.
Iron Deficiency Anemia
The most common cause of microcytic anemia is iron deficiency anemia. Iron is required to carry oxygen in hemoglobin, the main component of red blood cells. According to Harrison's Principles of Internal Medicine, iron deficiency anemia is cause by increased demand for iron such as during rapid growth in infancy, adolescence, or pregnancy; increased iron loss from chronic GI bleeds or menses; and decreased iron intake or absorption due to an inadequate diet, or difficulty absorbing iron due to diseases such as sprue or Crohn's. Diagnosis includes blood counts, measures of the iron studies, and elucidating the cause to apply appropriate treatment. The iron may be restored by blood transfusion if the blood counts are too low, iron therapy by IV, or supplemental iron tablets.
Thalassemias
Thalassemias are defects of hemoglobin synthesis. Hemoglobin is composed of four protein chains; in adults these are two alpha chains and two beta chains. A defect in the alpha chain is called alpha-thalassemia, and a defect in the beta chain is called beta-thalasemia. There are four genes encoding alpha proteins, and the severity of alpha thalasemia depends on the number of genes deleted, with one deletion leading to an asymptomatic carrier state, and four deletions leading to fetal demise. Beta thalasemia's presentation is also highly variable; it commonly causes expansion of the bone marrow leading to bone defects. Treatment may include blood transfusions, splenectomy, and/or bone marrow transplantation.
Sideroblastic Anemia
Sideroblastic anemia is also a problem in hemoglobin synthesis. Whereas the thalassemias result from defects in the protein chain of hemoglobin, sideroblastic anemias result from defects incorporating iron into the hemoglobin molecule. The iron is available, but the defect prevents it from being utilized. Causes of sideroblastic anemia include toxins such as alcohol and lead, cancers, rheumatoid arthritis, or it may be genetically inherited. It is diagnosed by bone marrow biopsy. Treatment depends on the cause, such as removal of the toxic agent and vitamin B6 supplementation.
Anemia of Chronic Disease
Inflammatory conditions such as cancers, infections, and rheumatoid arthritis also cause microcytic anemia. Chemical mediators such as tumor necrosis factor and interleukin-1 released during chronic inflammation act to suppress red blood cell production. They do this by suppressing the activity of erythropoietin, a hormone made by the kidneys that signals the bone marrow to make red blood cells. In addition, during inflammatory conditions, the liver makes a protein called hepcidin, which suppresses iron utilization. Diagnosis includes blood counts and iron studies, and microscopy of the blood. Treatment is aimed at the underlying cause, and erythropoietin supplementation may help as well.
