Makindo Medical Notes

Heparin-induced thrombocytopenia (HIT) with or without thrombosis (HITT) is thrombocytopenia (low platelet counts) due to the administration of Heparin which induces the formation of antibodies that stimulate and activate platelets. Fatal thrombosis, both venous and arterial, can arise if heparin is not stopped immediately

About

• Consider with thrombosis and there is a fall in platelets.
• Usually large vessel involved
• Has been on heparin 5-14 days before
• Has antiplatelet factor 4 antibodies
• Stop ALL Heparin immediately

Aetiology

• There is a propensity to arterial platelet-rich thrombosis 5-10 days after initiation of Heparin therapy
• Heparin-induced thrombocytopenia is commoner with the older use of unfractionated Heparin.
• It is much less common with the use of low molecular weight agents
• HIT is caused by Heparin-dependent IgG that activates platelets via their FcIIa receptors.
• The antigen is a complex between Heparin and PF4

Clinical

• Fatal thrombosis, venous and arterial
• Cold painful pulseless leg due to acute arterial thrombosis
• Acute Ischaemic stroke, DVT and PE

Complications

• Arterial thrombosis
• Skin necrosis especially at injection sites
• Adrenal failure due to vein thrombosis and haemorrhagic necrosis
• When DVT and HIT combine the thrombosis can worsen so much to cause venous limb gangrene and the clue is a raised INR

The 4T Score

TOTAL SCORE
• = 3 = low probability of HIT
• 4-5 = intermediate probability of HIT
• = 6 = high probability of HIT

Management

• All Heparin should be discontinued in patients strongly suspected to have HIT even Heparin line flushes. This can be easily overlooked. Urgent Haematology advice should be sought.
• Anticoagulation with Hirudin may be used but is renally excreted, and the drug must be avoided (or the dosage substantially reduced) in patients with renal failure.
• Other agents - Argatroban, Bivalirudin. Warfarin and other oral anticoagulants should be avoided.
• Platelets not needed unless bleeding.
• Give IV Vitamin K to reverse any Warfarin given.
• Warfarin only when Platelets >150 and overlap with another agent for 5 days.

The facts that make HIT more likely are

• Falling platelets
• Heparin started within 5-10 days
• Thromboses
• Exclude other cause of low platelets + thromboses

References

• Lo GK, Juhl D, Warkentin TE, Sigouin CS, Eichler P, Greinacher A. Evaluation of pretest clinical score (4 Ts) for the diagnosis of Heparin-induced thrombocytopenia in two clinical settings. J Thromb Haemost 2006; 4: 75965.