Haemolysis

Haemolysis is the destruction of red blood cells, leading to a decreased red cell mass and the release of haemoglobin and other intracellular contents. It can occur within blood vessels (intravascular haemolysis) or outside the circulation, mainly in the spleen and liver (extravascular haemolysis). Intravascular haemolysis often causes haemoglobinaemia and, when the filtered load exceeds renal capacity, haemoglobinuria; extravascular haemolysis more commonly results in raised bilirubin and reticulocytosis.

Pathophysiology features two broad categories. Intravascular haemolysis arises from mechanical injury, immune-mediated processes, infections, or toxins that disrupt the RBC membrane or integrity. Extravascular haemolysis occurs when affected red cells are recognized and cleared by macrophages, typically in the spleen, due to membrane defects or antibody coating.

Common causes include autoimmune haemolytic anaemia (often Coombs-positive), alloimmune reactions to transfused blood, mechanical destruction from prosthetic heart valves or extensive microangiopathy, hereditary RBC membrane disorders (such as hereditary spherocytosis or elliptocytosis), enzymopathies like G6PD deficiency, and various infections or drugs. Congenital conditions and chronic diseases can lead to ongoing (chronic) haemolysis, while acute episodes may be triggered by transfusion reactions or drugs.

Clinical features range from pallor and fatigue to jaundice and dark urine. Laboratory findings typically show anaemia with reticulocytosis, raised lactate dehydrogenase, increased indirect bilirubin, and reduced haptoglobin in intravascular haemolysis. Peripheral smears may reveal schistocytes or spherocytes, depending on the cause. The direct antiglobulin test (Coombs test) helps distinguish immune from non-immune haemolysis.

Management targets the underlying cause and may include withholding offending drugs, immunosuppression for autoimmune disease, transfusion support, and in select cases splenectomy or plasma exchange. Long-term consequences can include pigmented gallstones and, in newborns, risk of hyperbilirubinaemia.