Paroxysmal Nocturnal Haemoglobinuria

  • Acquired membrane disorder (intrinsic) associated with intravascular haemolysis
  • Clone of platelet, RBC or neutrophil arising during or after a hypoplastic episode or appearing spontaneously (idiopathic), sensitive to lysis by complement (C')
  • Decay accelerating factor is a component of the RBC membrane which acts against C3b, stopping the C' cascade.
    • Other membrane components stop complement at the C1 or C8 stage.
  • In PNH, inhibitors of C' mediated lysis are missing, cells bind an abnormally large amount of C' and the number of terminally C' complexes required to penetrate the lipid bilayer of PNH cells is less
  • Usually, following sleep (during which the condition is "active"), bleeding into the urine (haematuria) can occur.
    • A slight drop in the pH of blood during sleep may be the cause of the haemolysis.
    • Haemoglobinuria and haemosiderinuria result from chronic intravascular haemolysis
  • May experience kidney failure due to haemoglobin toxicity
  • May suffer thrombosis due to clotting initiated by the release of thromboplastin from RBCs

Haematological Profile:

  • Anaemia
  • Normochromic macrocytic anaemia
  • No microspherocytes (complete lysis)
  • May also be a thrombocytopaenia and leucopaenia
  • May find nucleated RBCs
  • Iron stores are decreased or depleted, C' fixation test is normal
  • Osmotic Fragility Test (OFT) is normal
  • No real single diagnostic condition

Specific Diagnostics:

Sucrose Lysis Test
  • Fresh serum (from a normal patient pool) is put into a low ionic strength sucrose solution (promotes C' binding)
  • RBCs are taken from the patient, incubated for 30min at 37oC and examined for haemolysis
  • Because of the increased sensitivity to lysis, PNH cells are haemolysed before the control cells
  • If positive, follow with...
Ham (Acid Serum Lysis) Test
  • Patient's RBCs are incubated with acidified serum (pH 6.5-7.0)
  • Serum activates the alternate C' p'way
  • As C' components deposit onto RBCs, PNH cells lyse
  • Spherocytes (check on film) and Ab-sensitised cells are also lysed so repeat the test with heat inactivated serum - if lysis still occurs, not PNH
Osmotic Fragility Test (OFT)
  • A measure of the RBCs resistance to haemolysis by osmotic stress, depending primarily on the volume of the cell, the surface area and the cell's membrane function
  • RBCs are incubated in varying concentrations of a hypotonic solution of sodium chloride (NaCl)
  • As the concentration of NaCl decreases, cells take on water to produce osmotic equilibrium
  • Normal cells begin to haemolyse at NaCl concentrations of approximately 0.50%
  • Because of their Dec. surface area:volume, spherocytes can not expand as much as normal discoid cells, thus lysis is usually completed by 0.4-0.5%, starting at higher NaCl concentrations
  • Incubation of blood overnight at 37oC makes the OFT more sensitive
  • A shift to the left of the normal range (NR) is a decreased OF (eg. HS)
  • A shift to the right of the NR is an increased OF (eg. thalassaemia)
  • Dec. cell stability occurs in thalassaemia, sickle cell anaemia and conditions associated with target cells

Treatment:

  • Supportive
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