Glucose-6-Phosphate Dehydrogenase Deficiency

Genetics and Epidemiology

• Most common red blood cell enzyme deficiency globally.
• Affects over 400 million individuals representing 4.9% global prevalence.
• High incidence correlates geographically with malaria endemicity.
• Represents balanced polymorphism conferring evolutionary resistance against falciparum malaria in heterozygous females.
• Inherited as x-linked recessive trait.
• Full disease expression occurs in hemizygous males and homozygous females.
• Heterozygous females exhibit variable intermediate expression due to random x-chromosome inactivation (Lyon hypothesis).
• More than 200 pathogenic variants identified.
• Complete gene deletions remain incompatible with embryonic life due to housekeeping essentiality.
• Missense point mutations cause amino acid substitutions and enzyme destabilization.
• Pathogenic variants near amino terminus associate with milder clinical disease.
• Pathogenic variants near carboxyl terminus affecting nadp-binding sites associate with severe chronic nonspherocytic hemolytic anemia.
• Variant a- affects approximately 13% of african-descent males.
• Variant mediterranean (b-) prevalent in 5-40% of greek, italian, middle eastern, and asian populations.
• Variant canton affects approximately 5% of chinese population.

Pathophysiology

• Hexose monophosphate shunt relies on glucose-6-phosphate dehydrogenase.
• Enzyme catalyzes conversion of glucose-6-phosphate to 6-phosphogluconic acid.
• Reaction generates reduced form of nicotinamide adenine dinucleotide phosphate (nadph).
• Nadph maintains cellular glutathione in reduced, functional state (gsh).
• Reduced glutathione physiologically inactivates generated oxidant compounds including hydrogen peroxide.
• Enzyme deficiency depletes nadph and reduced glutathione.
• Cellular inability to eliminate oxidants results in oxidative damage to red blood cell membrane sulfhydryl groups.
• Hemoglobin oxidizes to methemoglobin and denatures.
• Denatured hemoglobin precipitates forming intracellular inclusions called heinz bodies.
• Heinz bodies attach to red cell membrane band 3 proteins creating antigenic sites.
• Splenic macrophages identify abnormal membranes and physically pluck out heinz bodies.
• Phagocytosis produces fragmented erythrocytes, bite cells, and blister cells.
• Damaged red cells undergo rapid premature destruction via intravascular and extravascular hemolysis.
• Enzyme activity falls progressively as normal red cells age.
• Variant a- produces age-labile enzyme; older erythrocytes lyse preferentially while younger erythrocytes with residual enzyme survive.
• Age-labile enzyme kinetics produce compensated hemolytic process during continuous drug exposure.

World Health Organization Classification

[Source: 417, 637]

Clinical Manifestations

Asymptomatic State

• Majority of deficient individuals remain entirely asymptomatic.
• Clinical illness requires exposure to triggering external oxidants.

Episodic Acute Hemolytic Anemia

• Triggered by infections (most common), specific medications, or chemical exposures.
• Hemolysis onset typically 24-96 hours post-exposure.
• Features sudden onset pallor, dark urine (hemoglobinuria), fatigue, and jaundice.
• Associated chemical triggers include henna cosmetics and naphthalene (mothballs).

Favism

• Acute life-threatening hemolytic syndrome caused by fava bean ingestion or pollen exposure.
• Beans contain highly oxidative divicine, isouramil, and convicine producing hydrogen peroxide.
• Associated primarily with mediterranean and canton variants.
• Can progress rapidly to acute renal failure requiring urgent blood transfusion.

Neonatal Jaundice

• Spontaneous hemolysis and hyperbilirubinemia occur in preterm infants with a- variant.
• Severe hyperbilirubinemia progressing to kernicterus occurs in mediterranean and canton variants.
• Pathogenesis involves concurrent defective enzyme activity in hepatocytes reducing bilirubin glucuronidation.
• Transplacental maternal drug exposure or breast milk transmission can trigger neonatal hemolysis.
• Co-inheritance with ugt1a1 promoter mutations (gilbert syndrome) drastically worsens neonatal jaundice.

Chronic Nonspherocytic Hemolytic Anemia

• Typically associated with sporadic genetic variants.
• Features chronic continuous red cell destruction without external triggers.
• Manifests with severe transfusion-dependent anemia, reticulocytosis, and mild splenomegaly.
• Slight jaundice and gallstones commonly present.

Oxidant Drug Triggers

[Source: 40, 41, 639, 644] Note: standard doses of aspirin and trimethoprim-sulfamethoxazole typically do not cause clinically relevant hemolysis in the a- variant, though high-dose aspirin may produce severe episodes.

Laboratory Investigations

Primary Hematology

• Hemoglobin and hematocrit demonstrate precipitous fall during acute crises.
• Reticulocyte count heavily elevated indicating compensatory marrow hyperplasia.
• Direct antiglobulin test (coombs) strictly negative.

Peripheral Blood Smear

• Hallmark bite cells prominently visible.
• Blister cells show hemoglobin puddled to one side.
• Fragmented erythrocytes (schistocytes) and spherocytes present.
• Polychromasia indicates elevated reticulocyte presence.

Supravital Staining

• Crystal violet or brilliant cresyl blue stains reveal intracellular heinz bodies.
• Heinz bodies visible exclusively within first 3-4 days of acute illness before reticuloendothelial clearance.
• Heinz bodies absent on standard wright-stained blood films.

Hemolysis Markers

• Haptoglobin severely reduced or absent.
• Hemopexin levels reduced.
• Plasma free hemoglobin elevated (hemoglobinemia).
• Unconjugated (indirect) bilirubin significantly elevated.
• Lactate dehydrogenase markedly increased.
• Urinalysis reveals hemoglobinuria and hemosiderinuria.

Diagnostic Enzyme Assays

• Screening tests: methylene blue decolorization, methemoglobin reduction test, nadph fluorescence.
• Confirmatory test: quantitative spectrophotometric assay measuring precise nadph formation rate.
• Enzyme activity less than 10% of normal establishes definitive diagnosis.
• Critical timing caveat: false normal results frequently occur if tested immediately following acute hemolytic episodes.
• Reticulocytes and young red cells possess maximal enzyme activity; preferential destruction of older cells leaves behind younger cells with falsely reassuring enzyme levels.
• Diagnosis must be suspected if enzyme activity appears low-normal amid profound reticulocytosis.
• Definitive quantitative testing must be deferred several weeks post-crisis until red cell population normalizes.
• Molecular and electrophoretic analysis available for precise variant identification.

Management

Preventive Strategies

• Avoidance of deleterious agents remains paramount therapeutic measure.
• Strict dietary prohibition of fava beans.
• Meticulous review of prescribed medications to avoid oxidant drugs.
• Screen high-risk ethnic populations prior to initiating high-risk therapies (e.g., primaquine, rasburicase).
• Educate patients and families to recognize dark urine, pallor, and jaundice during infections.

Acute Hemolytic Crisis

• Discontinue inciting oxidant agent immediately.
• Maintain hemodynamic stability and strict fluid balance.
• Provide packed red blood cell transfusions for severe symptomatic anemia.
• Specific transfusion indications: hemoglobin falls below 7 g/dl, or persistent hemoglobinuria with hemoglobin below 9 g/dl.
• Complete recovery expected upon removal of oxidant stress.
• Absolute contraindication: avoid methylene blue administration for drug-induced methemoglobinemia; causes lethal severe oxidative hemolysis in enzyme-deficient hosts.

Neonatal Jaundice

• Treat aggressively utilizing established phototherapy protocols.
• Perform exchange transfusion if bilirubin levels threaten kernicterus.

Chronic Nonspherocytic Hemolytic Anemia

• Maintain baseline hemoglobin 8-10 g/dl via chronic red cell transfusions.
• Implement intensive iron chelation therapy to combat transfusional hemosiderosis.
• Administer daily folic acid supplementation (1 mg/day) to support hyperactive erythropoiesis.
• Splenectomy provides minimal benefit generally; reserved strictly for severe hypersplenism, unmanageable chronic anemia, or massive splenomegaly causing physical impediment.
• Provide genetic counseling and prenatal diagnosis if mother is known heterozygote carrying severe class I variants.