Heparin-Induced Thrombocytopenia

• Definition
  • HIT associated with thrombosis (paradoxical)
  • Mainly associated with unfractionated heparin but can occur with LMWH
    • Onset rare after 14 days
• Type 1 (MILD)
  • Clinically insignificant, mild, transient drop in platelet count
  • Incidence: 10-20%
  • Aetiology
    • Non-immune, Heparin induced platelet response to aggregating stimuli
  • Clinical
    • Insignificant - not related with thrombosis
    • Rapid onset, 1-5/7 after commencement of Rx
    • Platelet counts fall to 100-150 (NB: Platelet count could drop by 50% but still be in normal range)
  • Management
    • Continue Heparin and monitor
    • May find a relative heparin resistance (i.e. higher doses to achieve therapeutic APTT)
    • Return to normal within 1-5 /7 in spite of continued heparinisation
• Type II
  • Associated with limb & life threatening complications
  • HITTS (HIT + thrombosis syndrome)
  • Incidence 0-30%
  • 10% of Type II will develop thromboembolic complications (1% overall)
  • Aetiology
    • Immune related
      • Caused by autoantibodies to platelet factor 4 (РF4)
      • Antibody activates platelets and can cause catastrophic arterial and venous thrоmbοѕis
      • Activated platelets release microparticles which accelerates Thrombin generation ⇒ Thrombosis (venous & arterial)
    • Antiphospholipid syndrome potentially higher risk
  • Clinical
    • 6/7 after commencement of Rx, unless previous exposure
    • Features
      • Pain
      • Skin necrosis
      • Lower limb thrombosis
      • MI
      • Renal artery/vein thrombosis
      • Thrombosis often localised to area with pre-existing arterial disease
  • Diagnosis
    • Acute drop in platelets
      • Platelets < 50 x 109/L (if < 10 think of another process such as DIC)
    • 4 Ts score for estimating the pretest probability
    • Can confirmed by immunoassay assays
    • Count usually recovers in 5/7 after cessation of Rx
    • Platelet count related to chance of thromboembolic complications
  • Prognosis
    • HIT complicated by thromboembolism; Morbidity 22-80%; Mortality 12-30%
    • Platelets usually recover within 2 weeks of heparin stopping
    • Antibodies usually disappear within 3 months but recommended against further heparin
  • Management
    • Immediate
      • Stop Heparin/LMWH - substitute with Thrombin Inhibitor (lepirudin – derivative of hirudin the anticoagulant in the saliva of leeches) or factor 10a inhibitor (danaparoid)
      • If on, Stop Warfarin and reverse with vitamin K
      • Full-dose therapeutic anticoagulation with a non-hepаriո anticoagulant rather than discontinuing heparin alone or discontinuing hеpariո and providing a prophylactic dose of a non-hepаrin anticoagulant
    • Investigate other causes of decreased platelets
    • Surgical thrombectomy
    • Immunoglobulin aspirin, plasmapheresis

4 Ts score for estimating the pretest probability of heparin-induced thrombocytopenia (HIT)