Paroxysmal Nocturnal Hemoglobinuria (PNH)

Summary

• PNH = acquired stem-cell disorder causing intravascular hemolysis, thrombosis, and bone marrow failure.
• Caused by an acquired somatic mutation in the PIGA gene → no GPI anchor → loss of CD55 and CD59 on blood cells.
• Without CD55/CD59, complement attacks RBCs → chronic intravascular hemolysis with episodic dark (cola-colored) morning urine.
• Classic triad: Coombs-negative hemolytic anemia + pancytopenia + venous thrombosis at unusual sites (hepatic, portal, cerebral veins).
• Diagnosis: flow cytometry showing absent CD55/CD59 (and FLAER negativity) on blood cells.
• Treatment: eculizumab (anti-C5 monoclonal antibody) ± allogeneic stem-cell transplant (curative).

Definition

• Paroxysmal Nocturnal Hemoglobinuria (PNH) is a rare acquired clonal disorder of the hematopoietic stem cell.
• Results in production of blood cells that lack GPI-anchored complement-regulatory proteins (CD55 & CD59).
• "Paroxysmal" = comes in attacks. "Nocturnal" = hemolysis worsens at night (mild respiratory acidosis during sleep activates complement). "Hemoglobinuria" = free hemoglobin in urine.
• It is the only acquired intrinsic (membrane) cause of hemolytic anemia — all others (HS, G6PD, sickle cell) are inherited.

Epidemiology

• Rare: ~1–5 cases per million per year.
• Affects young adults (median age 30–40 years).
• No clear sex or ethnic predominance.
• Strong association with aplastic anemia (up to 25% of aplastic anemia patients have a detectable PNH clone) and myelodysplastic syndrome (MDS).

Etiology

• Acquired somatic mutation in the PIGA gene (Phosphatidylinositol Glycan class A) on the X chromosome in a hematopoietic stem cell.
• PIGA is required for the first step of GPI (glycosylphosphatidylinositol) anchor biosynthesis.
• Mutation is not inherited — it is acquired in a single bone-marrow stem cell, which then expands as a clone.
• The mutant clone gains a survival advantage when the marrow is under immune attack (explains the link with aplastic anemia).

Pathophysiology

The whole disease comes from one missing anchor:

1. PIGA mutation → cannot make GPI anchor.
2. Without the anchor, CD55 (DAF) and CD59 (MIRL) cannot attach to the RBC surface.
3. CD55 and CD59 normally inhibit complement (CD59 blocks formation of the MAC, C5b–9).
4. Without them, complement is freely activated on the patient's own RBCs → intravascular hemolysis → free hemoglobin in plasma and urine.

The same defect on platelets and WBCs explains the other two pillars:

• Thrombosis – complement-activated platelets release procoagulant microparticles; free hemoglobin scavenges nitric oxide → smooth-muscle constriction and platelet activation.
• Pancytopenia – the underlying stem-cell injury (often linked to aplastic anemia) reduces production of all lineages.

Hemolysis worsens at night because mild respiratory acidosis during sleep activates the alternative complement pathway → classic dark morning urine.

Clinical Features

Think of PNH as a triad:

• 1. Hemolytic anemia (intravascular)
  • Fatigue, pallor, jaundice.
  • Hemoglobinuria – dark/red/cola-colored urine, classically in the first morning sample.
  • Chronic hemolysis → iron deficiency (iron lost in urine as hemosiderin) and folate deficiency.
• 2. Thrombosis (most common cause of death)
  • Venous, often at unusual sites:
    • Hepatic veins → Budd-Chiari syndrome (very classic exam clue).
    • Portal, mesenteric, splenic, cerebral (sagittal sinus) veins.
  • Abdominal pain, hepatomegaly, ascites, headache.
• 3. Bone marrow failure (pancytopenia)
  • Anemia + thrombocytopenia (bleeding) + leukopenia (infections).
  • May evolve to aplastic anemia or rarely AML.

Other features: esophageal spasm/dysphagia, erectile dysfunction, and abdominal pain — all from NO depletion by free hemoglobin causing smooth-muscle dystonia.

Diagnosis

Initial labs – show intravascular hemolysis pattern:

• CBC: normocytic anemia (may be microcytic if iron deficient); often pancytopenia.
• ↑ LDH (very high), ↑ indirect bilirubin, ↓ haptoglobin, ↑ reticulocytes.
• Coombs (DAT): NEGATIVE – this distinguishes PNH from autoimmune hemolysis.
• Urine: hemoglobinuria (positive dipstick for blood, no RBCs on microscopy) and chronic hemosiderinuria (specific for chronic intravascular hemolysis).

Confirmatory test – flow cytometry:

• Gold standard: peripheral blood flow cytometry showing absent CD55 and CD59 on RBCs and WBCs.
• FLAER (fluorescent aerolysin) test is even more sensitive – binds GPI anchor directly; absent in PNH cells (especially used on granulocytes/monocytes).

Historical/obsolete tests (know the names — they may appear in older Step 1 questions):

• Ham test – acidified serum induces hemolysis of PNH cells.
• Sugar-water (sucrose) test – low-ionic solution activates complement and lyses PNH cells.

Differential Diagnosis

PNH overlaps with several conditions — separate them using the table below and one or two key clues:

Also consider when the picture is mainly pancytopenia:

• Aplastic anemia – hypocellular marrow; can coexist with PNH.
• MDS – dysplastic cells, often older patients.
• Acute leukemia – blasts on smear.

Also see the broader intravascular vs extravascular hemolysis categorization.

Management

Supportive care (every patient):

• Folate supplementation (ongoing high cell turnover).
• Iron replacement (lost in urine).
• Transfusion for symptomatic anemia.
• Anticoagulation for any thrombotic event (and long-term prophylaxis in high-risk patients).
• Vaccination against Neisseria meningitidis, pneumococcus, and H. influenzae at least 2 weeks before starting eculizumab.

Definitive/targeted therapy:

• Eculizumab – humanized monoclonal antibody against C5; blocks formation of MAC → dramatically reduces hemolysis, transfusion need, and thrombosis risk.
  • Main adverse effect: Neisseria meningitidis infection (terminal complement is needed to lyse Neisseria) → vaccinate + give penicillin prophylaxis.
• Ravulizumab – longer-acting anti-C5 (similar role).
• Allogeneic hematopoietic stem-cell transplant (HSCT) – the only curative option; reserved for severe disease (bone marrow failure, refractory thrombosis, transformation).

Complications

• Thrombosis – the leading cause of death in PNH. High-risk sites: hepatic (Budd-Chiari), portal, mesenteric, cerebral veins.
• Chronic kidney disease – from renal hemosiderin deposition (chronic hemoglobinuria).
• Pulmonary hypertension – chronic NO depletion.
• Iron-deficiency anemia – chronic urinary iron loss.
• Aplastic anemia – marrow failure may worsen over time.
• Transformation to MDS or AML (uncommon but recognized).
• Meningococcal sepsis – iatrogenic complication of eculizumab therapy.

Mnemonics

Key Points for Exams – نقاط مهمة للامتحانات

• PNH = the only acquired membrane defect causing hemolytic anemia.
• PIGA mutation (X-linked, but somatic — not inherited) → no GPI anchor → no CD55/CD59 → uncontrolled complement → intravascular hemolysis.
• Classic triad: Coombs-negative intravascular hemolysis + atypical thrombosis + pancytopenia.
• Most feared clinical scenario: Budd-Chiari syndrome in a young patient with cytopenias and dark morning urine.
• Labs: ↑ LDH, ↓ haptoglobin, ↑ retics, negative Coombs, urine hemosiderin positive.
• Diagnosis: flow cytometry (CD55/CD59 absent ± FLAER). Ham & sucrose tests are historical.
• Treatment: Eculizumab (anti-C5) — vaccinate against Neisseria meningitidis first. HSCT = cure.
• Main cause of death: thrombosis.