Anemia is defined as a reduction of the total circulating red cell mass below some defined limit. In practice this cannot be directly measured hence positive diagnoses of anemia are usually based of the ratio of packed red cells to the total blood volume: hematocirt—or hemoglobin concentration, both of which correlate with red cell mass except under conditions that significantly alter plasma volume such as fluid retention or dehydration. Reduction in red cell mass is often classified based on the diverse set of etiologies of anemia but is also classified by red cell morphology, which may be useful in the clinic. For example:
As a rule of thumb, microcytic and hypochromic anemias are a result of pathologic changes to hemoglobin synthesis while macrocytic anemias typically have etiology stems from erythroid precursors. Quantification of these these descriptors are:
Regardless of etiology, affected individuals present with: pallor; weakness; malaise; fatigability; dyspnea with mild exertion; headache, dimness of vision, or faintness; or fatty changes to liver, kidney, or myocardium due to hypoxia. Fatty changes of the myocardium can cause heart failure or angina pectoralis. Acute blood loss can cause oliguria or anuria due to renal hypoperfusion.
Pathology due to acute blood loss is due primarily to loss of intravascular volume though patient presentations vary widely depending on the rate of hemorrhage including: cardivascular collapse, shock, and death. If the hemorrhage is stopped in-time to avoid death, blood volume is restored by an influx of interstitial fluid to the vasculature causing hemodilution or a lowered hemoatocrit and reduce oxygen carrying capacity. This stimulates the kidneys to secrete erythroprotein, which in turn stimulates proliferation of commited erythroid progenitors (CFU-E) in the marrow. It take 5-days for the CFU-Es to mature into reticulocytes: new, macrocytic red blood cells with blue-red polychromatophilic cytoplasms—in peripheral blood.
Iron loss is not a concern if red blood cells extravasate into tissues other than the gut as it can be recaptured. If blood loss is significant enough to cause hypotension, a release of androgenic hormones results in leukocytosis, reticulocytosis, and thromocytosis
Chronic blood loss occurs when the rate of blood loss only mildly/moderately greater than the rate of red blood cell regeneration. A deficiency available iron can further inhibit the regenerative capacity of the marrow. As mentioned earlier, gastrointestinal bleed can cause a depletion of iron reserves further exacerbating anemia.
All hemolytic anemias are characterized by: a red cell life span < 120 days, elevated serum erythropoietin (and erythropoiesis), and an accumulation of products from red cell hemolysis. Most hemolytic anemias are a result of premature extravascular hemolysis: phagocytosis of red cells likely due to age-dependent changes to surface proteins. These changes typically cause red cells to be less malleable and unable to pass through the sinusoids of the spleen. Clinically, this causes: anemia, splenomegaly, and juandice as well as a potential decrease in haptoglobin, an α2-globulin that binds free hemoglobin to prevent its renal excretion. Because the pathologic destruction of red cells occurs in the spleen, extravascular hemolysis can be treated with a splenectomy.
The less prevalent, intravascular hemolysis is caused by mechanical injury to red cells (e.g trauma from cardiac valves, thrombotic narrowing, repetitive physical trauma), complement fixation, exogenous toxins, or intracellular parasites (e.g. falciparum malaria). Clinical features include: anemia, hemoglobinemia, hemoglobinuria, hemosiderinuria, and juandice. The increased rate of red cell lysis causes large amounts of free hemoglobin that is bound to hepatoglobin and cleared by mononuclear phagocytes or it is oxidised to methemoglobin. Renal tubular cells reabsorb the majority of free hemoglobin or methemoglobin but invariably some will escape in to the urine giving it a red-brown color. Accumulation of iron in the renal proximal tubular cells causes renal hemosiderosis. Splenomegaly is no observed as the spleen is not involved during intravascular hemolysis.
In both cases heme groups from hemoglobin and hepatoglobin complexes are catabolized into unconjugated bilirubin. If liver function is normal, jaundice is rarely observed as excessive bilirubin is excreted in to the GI tract. Regardless of the etiology, increased erythroid precursors and reticulocytosis can be seen in peripheral blood; hemosiderosis can be seed in the liver, spleen, and bone marrow; extramedullary hematopoiesis can be seen in the liver, spleen, and lymph during sever cases of anemia; and choleithiasis (pigment gallstones) can be see in chronic hemolysis.
Affected individuals have other symptoms of extravascular hemolysis however cholelithiasis is as common as 50%. Microscopy of a peripheral blood smear reveals smaller dark-appearing red cells with no central pallor with small dark nuclear remnants. Diagnosis is aided with family history. As many as two thirds of patients are susceptible to osmotic lysis during incubation of hypotonic solutions. Red cells also have an increased mean cell hemoglobin concentration.
Hereditary spherocytosis (HS) is an inherited disorder due to number of pathogenic variants in genes coding for ankyrin, band 3, spectrin, or band 4.2—cell membrane skeletal proteins—usually due to a frameshift or nonsense mutation. The loss of the α and β chains of the spectrin heterodimer scaffold or its association band 3 (a membrane bound ion channel) through band 4.2 and ankyrin results in a loss of membrane integrity and deformability.
Red cells maintain their normal shape but begin to shed fragments of the plasma membrane and gradually loss the ability to overcome the cohesive cytoplasmic forces after 10-20 days after maturity leading to the smallest possible surface area to volume ratio: a sphere. Cells are then trapped become trapped in the spleen—causing erythrostasis and splenomegaly—where they are deprived of glucose lyse and are phagocytosized primarily by macrophages.
Because the rate of erythropoiesis often outpaces the rate of hemolysis, 20-30% of individuals may be asymptomatic. Infections of parvovirus, which kills red cell progenitors, lasts about 1-2 weeks can cause aplastic crises: a sever and suddenly worsening anemia that may require transfusions until the infection is cleared. Hemolytic crises can be caused by an increased rate of destruction of red cells from an infectious mononucleosis, for example.
Affected individuals experience characteristic episodic events of hemolysis with exposure to oxidative stress, which are commonly: free radicals produced by activated leukocytes during an infection (e.g. viral hepatitis, pneumonia, and typhoid fever); drugs such antimalarials (e.g. primaquine and chloroquine), sulfanomides, nitrofurantoins, and others; and some foods such as fava beans.
There are to central cause of Glucose-6-phosphate dehydrogenase (G6PD) deficiency: defects in the monophospate shunt or glutathione metabolism. The resulting limited ability to reduce nicotinamide adenine dinucleotide phosphate (NADP) to NADPH by G6PDase and hence reduce the ability oferythrocytes to prevent oxidative injury. While there are several hundred variants on the X-chromosome leading to G6PD, two pathogenic variants are especially significant to clinicians. G6PD- is prevalent in about 10% of the black population in the United States while G6PD Mediterranean is relatively more prevalent among middle easterners. Compared to the most common variant for G6PD, the half-life of G6PD among G6PD- individuals is moderately reduced while those with G6PD Mediterranean are severely affected.
G6PD deficiency causes both extravascular and intravascular hemolysis. Oxidative stress can cause formation of large denatured membrane bound globin chains. In some cases these bodies can cause lysis of the cells while in circulation. The bodies can also prevent deformation of erythrocytes leading to formation of spherocytes or partially degraded cell and ultimately degradation in the spleen.
Signs and symptoms of acute intravascular hemolysis—anemia, hemoglobinemia, and hemoglobinuria—typically begin with 2-3 days of exposure to oxidative stress while symptoms of chronic hemolysis—such as splenomegaly and cholelithiasis—are absent. Episodes are often self-limited as older red cells lyse and individuals recover due to reticulocytosis.
Sickle cell disease is a autosomal-recessive disorder cause by a pathogenic point mutation in the gene for β-globin which leads to polymerization of deoxygenated hemoglobin causing deformation of red cells and ultimately hemolytic anemia, microvascular obstruction, and ischemia. There are hundreds of different pathogenic variates that cause erythrocyte deformities, the aforementioned causes a replacement of glutamate with valine and the only significant cause of sickle cell disease in the United States: about 8-10% of blacks are heterozygous for the variant and are largely asymptomatic. Nearly all of the hemoglobin content of homozygous individuals consists of HbS: hemoglobin tetromers of (α2βS2), which causes deformaties, as opposed to HbA (α2β2) with small amounts of HbA2 (α2δ2) and HbF (α2γ2).
HbS variants have been shown to emerge at least six time independently in areas wherein falciparum malaria is endemic. It imparts to genetic advantages to clear infections of malaria:
It as has also been suggested that Glucose-6-phosphate dehydrogenase deficiency and thalassemia syndromes are also protective against malaria due to their prevalence in areas with endemic malaria and etiology causing hemolysis.
Molecules of HbS favorably stack and form polymers when deoxygenated causing the cytosol to be highly viscous. HbS polymers eventually form long needle-like fibers distorting the shape of the red cell. If the polymers cause sever deformities, the plasma membrane allows the influx of Ca2+, which allows K+ and H2O to be released making the cell increasingly dehydrated, dense, rigid, and ultimately irreversibly sickled. 4 aspects of biochemical conditions affect the extent of erythrocyte deformation:
In contrast to the process pathogensis of hemolysis in this disorder, microvascular occultions, are not dependent on the degree of deformation of the red cells. Rather, sublet membrane damage triggers the presentation of adhesive molecules on the membrane surface. Additional factors such inflammation also promote hemostatsis. Once started, the process becomes a positive feedback loop: obstruction, hypoxia, increased HbS polymerization, more obstruction. Intravascular hemolysis also contributes to depletion of nitric oxide—a vasodilator and inhibitor of platelet aggregation—as free hemoglobin bind and inactive it. As one might expect thrombosis is much likely to occur as a result.
Affected individuals typically become symptomatic after the age of 5 months. While hypoxia, dehydration, acidosis, and infection all favor HbS polymerization most crises of sickle cell disease do not have a triggering event. Vaso-oclusive and pain crises typically involve the bones, lungs, liver, brain, penis, and spleen. Crises of the bone may be hard to distinguish form osteomyelitis. Infections may cause potentially fatal vaso-occlusive crises of the lungs and often presents with fever, couch, chest pain, and pulmonary infiltrates. Involvement of other organs can cause priapism (erectile dysfunction after puberty due to hypoxia), ischemia, retinopathy, acute chest syndrome, and blindness.
Chronic hypoxia is responsible for generalized impaired growth and development as well organ damage. Insidious damage to the spleen causes replacement of functional tissue with fibrous tissue termed autosplenectomy. Patients are also susceptible to P. pneumoniae, H. influenzae, septicemia, and meningitis due to defects in opsonization of bacteria.
Sequestration crises: rapid onset of splenomegaly, hypovolemia, and shock—can occur in children with intact spleens, which can be fatal and require exchange transfusions. Aplastic crises from infections of red cell progenitors can severely exacerbate hemolytic anemia.
Thalassmeias are a diverse set of disorders of either the α- or β-globin chains of hemoglobin. One gene on chromosome 11 is responsible for synthesis of β-globin while two identical genes on chromosome 16 are responsible for the synthesis of α-globin. Pathology is not only due to reduce expression of the chains but also relative excess of the non-pathologic globin chain. While the disorders cause anemia because of hemolysis, they also cause reduced red cell production.
Pathogensis. There are two classes of pathogenic variants of β-thalassemia: β0 and β+ mutations that cause absence of β-globin and reduce β-globin synthesis, respectively. mRNA splicing, DNA promoter region, and nonsense mutations are most often implicated. The resulting deficits cause hypochromic, microcytic red cells with decreased oxygen carrying capacity and dimished survival. Without β-globin to associate with α-globin form insoluble inclusion bodies and ultimately suffer extravascular hemolysis. However, in more severe cases of β-thalassemia, as much as 85% of red cell precursors succumb to apoptosis initiated by membrane damage.
Ineffective erythropoiesis to compensate for resulting anemia stimulates severe erythroid hyperplasia in the marrow at the expense of bony cortex, bone growth, and skeletal integrity. Extensive extramedullary hematopoiesis begins to take place in the liver, spleen, and lymph nodes. The metabolically active progenitors effectively starve other tissues exacerbating hypoxia and causing cachexia if left untreated. Yet another complication of ineffective erythropoiesis is unregulated iron absorption and secondary hemochromatosis.
Clinical symptoms. Individuals that homozygotes or compound heterozygotes for β0 or β+ are said to have β-thalasemmia major and are transfusion dependent. Anemia manifests within 6-9 months of age as HbF is replaced with the defective HbA. Without transfusion hemoglobin levels can be extremely low (3-6gm/dL), HbF will be markedly elevated with HbA2 > 3.5% (abnormally high). Patients with better survival have bony prominences due to bone marrow hyperplasia. α-globin inclusions cause extravascular hemolysis. Marked decrease in hemoglobin production causes secondary hemochromatosis and subsequent cardiac, liver, pancreas disease. Hepatosplenomegaly is due to extramedullary hematopoiesis. Peripheral blood smears show anisocytosis, poikilocytosis, microcytosis, and hypochromia along with target cells, schistocytes, and basophilic stippling.
Patients can live into their thirties with successful treatment with transfusions and iron chelation. Hematopoietic stem cell transplantation is becoming more common and effectively cures the disorder.
β-Thalassemia minor. Individuals that are heterozyotes of β0 or β+ are said to have β-thalassemia minor a much more common disorder especially in areas with endemic malaria. Individuals are generally asymptomatic or may have mild anemia. Blood spears will show some hypochromia, microcytosis, basophilic stippling, and target cells. Electrophoresis will show increased HbA2 (4-8%). A CBC with differential will show decreased mean corpuscular volume along with normal red cell distribution width.
There are 4 types of α-thalassemia with unique pathology due to the number of deletions of the α-globin gene. Individuals with one deletion, which causes an almost undetectable reduction in α-globin synthesis and is termed the silent carrier state.
α-thalassemia trait is caused by two ipsochromosal or contrachromosal deletions. The ipsochromosmal haplotype is relatively common among asians. While both types cause the same symptoms, they obviously have a different set of implications for affected individual’s progeny. Individuals with the α-thalassemia trait have the same set of symptoms as those with β-thalassemia minor: mild/no anemia, microcytosis, and hypochromaisa.
Deletions of three copies of the α-globin gene is termed Hemoglobin H (HbH) disease, which is more common among asians. Markedly unavailable α-globin results in formation β and γ globin tetramers. HbH has a very high affinity for oxygen making it ineffective for O2 delivery causing hypoxia. HbH is also prone to oxidation, which causes formation of intracellular inclusions and ultimately extravascular hemolysis.
Hydops fetalis is cause by deletion of all α-globin genes. It is typically incompatible with life though intruterine transfusion may allow the fetus to be brought to term and are then generally dependent on life long transfusions.
The general pathology of immunohemolytic anemias is caused by development of antibodies to erythrocytes. They are not necessarily autoimmune disorders as antibodies can develop due to exogenous sources such as drugs. The disorders are diagnosed with direct (Coombs) antiglobulin test (DAT): red cells are mixed with sera that contain human immunoglobulin antibodies or complement. The indirect (Coombs) antiglobulin test (IAT) is preformed by mixing an individual’s sera with standardized red cells with specific antigens.
Most etiology of warm antibody immunohemolytic anemia (WAIHA) is idiopathic; the remaining cases are secondary to autoimmune disorders such as systemic lupus erythematosis, lymphoid neoplasms, and antigenic drugs. Despite the cause, IgG (uncommonly IgA) antibodies coat red cells which then bind to the Fc receptor of phagocytes. Phagocytes then remove red cell membrane through “partial phagocytosis;” red cells become shperocytes as a result. As with hereditary shperocytosis, the sphereocytes are destroyed through extravascular hemolysis. While the cause of primary WAIHA is often illusive, there are two well defined causes of drug induced WAIHA:
Antigenic drugs: The most notable offending agents are cephalosporins and penicillin. Antibodies typically bind only to the drug in the latter example. While a drug-membrane complex is recognized by antibodies, in the former. Reactions begin 1-2 weeks after large intravenous infusions.
Tolerance-breaking drugs: α-methyldopa, an anti-hypertensive agent, is the prototypical example of a drug that induces antibody formation Rh groups as well as other antigens. 10% of patients will develop antibodies and 1% will develop clinically significant symptoms of hemolysis.
Cold agglutinin type immunohemolytic anemia (CAIHA) is mediated by IgM and can be caused by certain types of B-cell lymphonmas or can be idiopathic. It can also appear transiently with certain infections: Mycoplasma pneumoniae, Epstine Barr virus, cytomegalovirus, influenza virus, and HIV.
IgM bind and agglutinates and fixes complement to red cells in vascular beds that are exposed to temperatures ∈(0 − 5) e.g. digits and ears. Complement can act directly to induce intravascular hemolysis or act as opsonin to cause extravascular hemolysis in the spleen, liver, or bone marrow. Returning to physiologic body temperatures can released IgM before hemolysis occurs.
Also called paroxysmal cold hemoglobinuria, causes severe, sometimes fatal intravascular hemolysis and hemoglobinuria. IgG antibodies bind to P antigen on red cells in colder extremities. Complement mediated lysis the occurs in optimal, physiologic temperatures in proximal vasculature. Children with a recent history of a viral infection are commonly affected; most recover within a month.
While blood transfusion allows for otherwise life-incompatible medical procedures, it is not without its own dangers. Fever, chills, or mild dyspnea with the first 6 hours of red cell or platelet transfusion—termed febrile nonhemolytic reaction—is the most common reaction. Its likely caused by the inflammatory response of the donor leukocytes but is short lived and responsive to anipyretics. Other reactions are potentially fatal
Individuals that have received transfusion(s) in the past can may be sensitized antigens in donors’ blood. Urticarial allergic reactions occur in as many as 3% of transfusions. They mediated by recipient’s IgE but are typically mild, responsive to antihistamines, and do not require the transfusion to be halted.
The more severe and potentially fatal reactions are most likely to occur in individuals with an IgA deficiency and is mediated by the recipient’s IgG antibodies. While as many as 1 in 300 persons have an IgA deficiency, 1 in 20000 of the individuals will develop such a reaction.
Acute hemolytic reactions. Preformed IgM antibodies—usually A or B—against donor red cell fix complement causing intravascular hemolysis. Fever, shakes, chills, and flank pain appear readily with complement activation, while lysis of the red cell causes hemoglobinurea without any other symptoms of hemolysis. In sever cases patients may suffer DIC, shock, acute renal failure, or death. A DAT for IgM will be positive unless all the donor’s red cell have been eliminated.
Delayed hemolytic reactions. Delayed hemolytic reactions are mediated by IgG antibodies to donor antigens developed by the recipient through a prior sensitization event. The antibodies can be detected through DAT along with findings of hemolysis i.e. low haptoglobin, elevated LDH, etc. Antigens to Rh, Kell, and Kidd can produce potentially fatal reactions though complement fixation that are identical to those seen in ABO mismatches. Other antibodies can cause red cell opsonization leading to spherocytosis and ultimately extravascular hemolysis, which tends to be mild.
Activation of neutrophils in the microvasculature of the lung due to transfused factors can cause TRALI, though rare. Though not completely understood, it is suggested that a primary event leads to sequestration and sensitization of neutrophils in the lung and then factors in the transfused product activate the neutrophils. The antibody that is most commonly implicated in TRALI is directed toward class I MHC antigens. These are commonly found in multiparous women because of exposure to fetus class I MHC molecules. Protocol to exclude multiparous women was contemporaneous with halving in the rate of TRALI.
Affected individuals experience a sudden, dramatic respiratory faliure during or soon after transfusion with bilateral infiltrates unresponsive to diuretics. Fever, hypotension, and hypoemia are also associated with TRALI.
Infections from transfused products are most often caused by contamination of at the donation are bacterial in nature, most frequently skin flora. It is most often seen in transfusion of platelets as they are kept at room temperature. Symptoms of fever, chills, hypotension, etc. closely resemble hemolytic transfusion reactions. Infections may require broad-spectrum antibiotics. In rather rare cases when donors are acutely infected and the virus is undetectable, recipients can be infected with HIV, HCV, or HBV.