Hemolytic anemia pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Shyam Patel [2]

Pathophysiology

Drug-Induced Hemolysis

Drug induced hemolysis has large clinical relevance. It occurs when drugs actively provoke red cell destruction. Drug-induced hemolytic anemia can occur in an antibody-dependent or antibody-independent manner.

• Antibody-mediated hemolysis: This can occur via IgG or IgM binding to red blood cell membranes.
• Antibody-independent hemolysis: This occurs in the absence of IgG or IgM. It can occur via drug-induced protein adsorption on red blood cells.

Immune-Mediated Hemolysis

Immune-mediated hemolysis occurs when an antibody binds to red blood cell membranes. Causes of immune-mediated hemolysis include:

• Drugs: This is one of the most common causes of immune-mediated hemolysis.
  • Penicillin in high doses can induce immune-mediated hemolysis via the hapten mechanism in which antibodies are targeted against the combination of penicillin in association with red blood cells. Complement is activated by the attached antibody leading to the removal of red blood cells by the spleen.
  • Nivolumab: This is an antibody that binds to the PD-1 antigen found on lymphocytes. It is typically used to treat cancers like squamous cell carcinoma of the head and neck, melanoma, and lung cancer. Nivolumab can trigger significant autoimmune reactions. When the hematologic system is affected, hemolytic anemia can result.
  • Pembrolizumab: This is an antibody that binds to the PD-1 antigen found on lymphocytes. It is typically used to treat cancers like squamous cell carcinoma of the head and neck, melanoma, and lung cancer. Nivolumab can trigger significant autoimmune reactions. When the hematologic system is affected, hemolytic anemia can result.
  • Ipilimumab: This is an antibody that binds to cytotoxic T lymphocyte antigen-4 (CTLA-4) on T cells. It is commonly used to treat stage III melanoma in the adjuvant setting and stage IV melanoma.
• Infections: Amongst infectious agents, viruses are most likely to trigger hemolysis, compared to bacteria, parasites, or fungi.
• Autoimmune or rheumatologic disease: Activation of one's own immune system can result in destruction of red blood cells in an antibody-dependent manner. Females are more likely to develop autoimmune hemolytic anemia.
• Lymphoproliferative disorders: These represent a group of primary bone marrow disorders characterized by rapid proliferation of T cells or B cells. Chronic lymphocytic leukemia (CLL), for example, is a lymphoproliferative disorder that is a known etiology of hemolytic anemia.

The drug itself can be targeted by the immune system, e.g. by IgE in a Type I hypersensitivity reaction to penicillin, rarely leading to anaphylaxis.

Non-immune

Non-immune hemolysis is characterized by the absence of antibodies. Non-immune drug-induced hemolysis can also arise from drug-induced damage to cell volume control mechanisms; for example drugs can directly or indirectly impair regulatory volume decrease mechanisms, which become activated during hypotonic RBC swelling to return the cell to a normal volume. The consequence of the drugs actions are irreversible cell swelling and lysis (e.g. ouabain at very high doses).

• Glucose-6-phosphate dehydrogenase (G6PD) deficiency: Non-immune drug induced hemolysis can occur via oxidative mechanisms. This is particularly likely to occur when there is an enzyme deficiency in the antioxidant defence system of the red blood cells. An example is where antimalarial oxidant drugs like primaquine damages red blood cells in glucose-6-phosphate dehydrogenase deficiency in which the red blood cells are more susceptible to oxidative stress due to reduced NADPH production consequent to the enzyme deficiency. G6PD is the rate-limiting enzyme in the pentose phosphate pathway, or hexose monophosphate shunt. The normal function of G6PD is to confer reductive potential to erythrocytes via NADPH. Oxidation of NADPH to NADP+ in erythrocytes prevents oxidation of other molecules in these cells and thus prevents hemolysis. Intact G6PD allows for generation of reduced glutathione, which prevents oxidative stress and hemolysis. In the presence of G6PD deficiency, the stores of glutathione are depleted, and the sulfhydryl groups of hemoglobin and other proteins become oxidized. This creates precipitation of denatured hemoglobin known as Heinz bodies. This leads to irreversible membrane damage and thus hemolysis. Drugs that typically cause hemolysis in patients with G6PD deficiency include:
  • primiquine and other anti-malarial agents
  • fava beans
  • sulfa drugs like trimethoprim-sulfamethoxazole
  • dapsone

Some drugs cause RBC (red blood cell) lysis even in normal individuals. These include dapsone and sulfasalazine.

Compensatory response

Hemolytic anemia causes a compensatory increase in erythropoetin that in turn causes an increase in reticulocyte percentage and absolute reticulocyte count. This results in increased hemoglobin and RBC production.