Heparin-induced thrombocytopenia (HIT) is a complex immune-mediated disorder that occurs in response to the administration of heparin, an anticoagulant medication commonly used to prevent blood clot formation. HIT is characterized by a significant decrease in platelet count (thrombocytopenia) and an increased risk of developing blood clots (thrombosis).

Mechanism:

HIT is primarily caused by an immune response to complexes formed between heparin molecules and platelet factor 4 (PF4), a protein released by activated platelets. The exact mechanism is not completely understood, but it involves the following steps:

1. Heparin-PF4 Complex Formation: When heparin is administered, it binds to PF4, a protein released from platelet granules upon activation. This complex can trigger an immune response in susceptible individuals.
2. Antibody Production: Some individuals, particularly those who have been previously exposed to heparin, can develop antibodies against the heparin-PF4 complex. These antibodies are of the IgG class and can activate platelets and enhance clotting.
3. Platelet Activation: The antibodies bind to the heparin-PF4 complexes on the surface of platelets. This leads to platelet activation and aggregation, promoting the formation of blood clots.
4. Thrombocytopenia and Thrombosis: As platelets are activated and consumed in clot formation, the platelet count decreases, leading to thrombocytopenia. Simultaneously, the activated platelets contribute to the formation of blood clots in various parts of the body, causing thrombosis.

Clinical Presentation:

The clinical presentation of HIT can vary, but common manifestations include:

1. Thrombocytopenia: A significant drop in platelet count, typically occurring 5 to 10 days after starting heparin therapy.
2. Thrombosis: Arterial and venous blood clots, including deep vein thrombosis (DVT), pulmonary embolism (PE), and arterial thrombosis (such as stroke or myocardial infarction).
3. Skin Lesions: Skin necrosis at injection sites, especially in patients on subcutaneous heparin.

Diagnosis:

The diagnosis of HIT involves a combination of clinical assessment and laboratory testing:

1. Platelet Count Monitoring: A rapid and substantial drop in platelet count from baseline is a key diagnostic indicator.
2. Functional Assays: Serotonin-release assay (SRA) and heparin-induced platelet activation assay (HIPAA) can detect heparin-dependent platelet-activating antibodies.
3. Enzyme Immunoassays: These tests detect antibodies against heparin-PF4 complexes. They are widely used for initial screening due to their high sensitivity.

Management:

Managing HIT involves two main aspects: discontinuing heparin therapy and preventing thrombosis:

1. Heparin Cessation: All forms of heparin, including unfractionated heparin and low molecular weight heparin, should be immediately stopped once HIT is suspected.
2. Alternative Anticoagulation: Direct thrombin inhibitors (e.g., argatroban, bivalirudin) are preferred for anticoagulation in HIT patients, as they do not interact with PF4.
3. Thrombosis Treatment: Existing thromboses require appropriate treatment with anticoagulation, sometimes with the addition of interventions like thrombectomy or thrombolysis.
4. Prevention: For patients with a history of HIT, alternative anticoagulation strategies should be considered in future procedures that require anticoagulation.

HIT is a serious and potentially life-threatening condition that requires prompt recognition and management to prevent complications. Healthcare providers should have a high index of suspicion in patients receiving heparin therapy and presenting with thrombocytopenia and/or thrombosis.