Contributor: Gordon K. Klintworth
Anemia is disorder in which their is a reduction in the concentration of hemoglobin below the normal range in the peripheral blood. It can result from a diminution in the number of erythrocytes or the in quanity of hemoglobin within the erythrocytes. Anemia is a manifestaion of numerous diseases (including abetalipoproteinemia, sickle cell disease, Hemoglobin C disease, thalassemia, renal disease, leukemia, polycythemia) and it has many causes. Anemia may follow blood loss from hemorrhage or blood destruction (as in hemolytic states). Defects in the synthesis of erythrocytes can also cause anemia (as in vitamin B deficiency, folic acid deficiency, and iron deficiency) and leukemia, and polycythemia. Numerous types of anemia are recognized: aplastic anemia [anemia - aplastic], megaloblastic anemia [anemia - megaloblastic], iron deficiency anemia [anemia - iron deficiency], hereditary spherocytosis, sideroblastic anemia [anemia - sideroblastic], autoimmune hemolytic anemia [anemia - autoimmune hemolytic], acute blood loss anemia [anemia - acute blood loss], and myelophthisic anemia [anemia - myelophthisic]. Anemia has been classified into three types based on the size of the circulating erythrocytes (macrocytic anemia [anemia - macrocytic], microcytic anemia [anemia - microcytic] and normocytic anemia [anemia - normocytic]). All types of anemia may produce edema of the optic nerve head [edema - optic disc], retinal hemorrhages [hemorrhage - retina], and exudates. The frequency of retinal hemorrhages depends partly on the coexistence of anemia and thrombocytopenia. When anemia is only present retinal hemorrhages occur in ~ 10% of patients, but when anemia is associated with thrombocytopenia 40% of patients develop retinal hemorrhages. When anemia and thrombocytopenia are both present and severe (hemoglobin < 3 g/dL and platelet count < 50,000/mm3) 70% of individuals have retinal hemorrhages. Retinal hemorrhages  are not a feature of thrombocytopenia alone.